Resistor Types- Information, tutorial about the basics of the different types of resistors including fixed and variable resistors, carbon, film, carbon composition, metal film, etc

Resistor types include:

    •  Resistor types summary     •  Variable / adjustable resistor    •  Thermistor    •  LDR light dependent resistor

See also: SMT resistor     MELF resistor

There are many different types of resistor available for use within electronic circuits. These different resistor types have somewhat different properties dependent upon their construction and manufacture. This makes the different types of resistor suitable for different applications.

Over the years the resistor types used in mass electronics production have changed. Years ago, all the resistors used had leads and were relatively large, and by today's standards they offered a low level of performance. Today, the resistor types used are much smaller and offer much higher levels of performance.

Fixed & variable resistor types

The first major categories into which the different types of resistor can be fitted is into whether they are fixed or variable. These different resistor types are used for different applications:

Fixed resistors:   Fixed resistors are by far the most widely used type of resistor. They are used in electronics circuits to set the right conditions in a circuit. Their values are determined during the design phase of the circuit, and they should never need to be changed to "adjust" the circuit. There are many different types of resistor which can be used in different circumstances and these different types of resistor are described in further detail below.

Variable resistors:   These resistors consist of a fixed resistor element and a slider which taps onto the main resistor element. This gives three connections to the component: two connected to the fixed element, and the third is the slider. In this way the component acts as a variable potential divider if all three connections are used. It is possible to connect to the slider and one end to provide a resistor with variable resistance.

Fixed resistor types

There are a number of different types of fixed resistor:

Carbon composition:   These types were once very common, but are now seldom used. They are formed by mixing carbon granules with a binder which was then made into a small rod. This type of resistor was large by today's standards and suffered from a large negative temperature coefficient. The resistors also suffered from a large and erratic irreversible changes in resistance as a result of heat or age. In addition to this the granular nature of the carbon and binder lead to high levels of noise being generated when current flowed.

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Carbon film:   This resistor type is formed by "cracking" a hydrocarbon onto a ceramic former. The resulting deposited film had its resistance set by cutting a helix into the film. This made these resistors highly inductive and of little use for many RF applications. They exhibited a temperature coefficient of between -100 and -900 ppm / °Celcius. The carbon film is protected either by a conformal epoxy coating or a ceramic tube.

Metal film / metal oxide:   This type of resistor is now the most widely used form of resistor. Rather than using a carbon film, this resistor type uses a metal film deposited on a ceramic rod. Metals such as nickel alloy, or a metal oxide such as tin oxide are deposited onto the ceramic rod. The resistance of the component is adjusted in two ways. First the thickness of the deposited layer is controlled during the initial manufacturing stages. Then it can be more accurately adjusted by cutting a helical grove in the film. Again the film is protected using a conformal epoxy coating. This type of resistor has a temperature coefficient of around ±15 parts per million / °K, giving it a far superior performance to that of any carbon based resistor. Additionally this type of resistor can be supplied to a much closer tolerance, ±5%, ±2% being standard, and with ±1% versions available. They also exhibit a much lower noise level than carbon types of resistor.

Wire wound:   This resistor type is generally reserved for high power applications. These resistors are made by winding wire with a higher than normal resistance (resistance wire) on a former. The more expensive varieties are wound on a ceramic former and they may be covered by a vitreous or silicone enamel. This resistor type is suited to high powers and exhibits a high level of reliability at high powers along with a comparatively low level of temperature coefficient, although this will depend on a number of factors including the former, wire used, etc..

Thin film:   Thin film technology is used for most of the surface mount types of resistor. As these are used in their billions these days, this makes this form of resistor technology one of the most widely used.

The Wirewound Resistor Phil Ebbert, VP Engineering, Riedon Inc. reports on the wirewound resistor and the fact that it is widely used today

Like every component, the fabrication technology used in resistor manufacture has changed over time and resistive films have made a significant contribution in allowing the cost-effective mass production of devices that are increasingly miniaturized.

Yet the traditional wirewound resistor, despite a substantial decline in the number of manufacturers of this component in recent decades, remains the best solution for many specialized applications.

The wirewound resistor is still used in large quantities where particular elements of performance are required. In these areas, the wirewound resistor can greatly outperform many other resistor technologies.

Construction advantages

One reason for the survival of wirewound resistors is that all of the alternative fabrication techniques have drawbacks. For example, the use of conductive inks to produce carbon film or thick film resistors can produce very low-cost components but the resulting devices have limited pulse handling, no better than 0.1% initial tolerance and poor long-term stability, typically 500 to 1000ppm/year. The resistance is temperature dependent, with a temperature coefficient of

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resistance (TCR) of around 50 to 100ppm/°C. Moreover, relatively high current noise of -18 dB to -10 dB is typical, where:

dB = 20 x log (noise voltage [in μV]/DC voltage [in V])

 Figure 1: Easy customization is one of the major advantages of wirewound resistor technology

Carbon composition resistors, made by binding conductive carbon powder and an insulating material (usually ceramic) in a resin are some of the earliest resistor types. The proportions of carbon and insulating material determine the desired resistance value. It’s difficult to achieve accurate values so ±5% is often the best initial tolerance available and they exhibit poor temperature stability with a TCR of some 1000ppm/C. These resistors also have high current noise (-12 dB to +6 dB ) and suffer from poor stability over time.

Metal film types perform better. They deliver improved tolerance (as good as 0.01%), TCR of 10 to 200ppm/°C and stability of 200 to 600ppm/yr. But these figures still cannot match those of wirewound alternatives, and their pulse handling capability is significantly inferior.

As a result of the limitations of other technologies, wirewound components continue to be used in many applications. They can handle high level pulses and transients, can dissipate substantial amounts of power (some are rated at up to 2.5kW), and they can be made with great precision – some have initial tolerances down to 0.005%. Just as importantly, they are stable (15 to 50ppm/yr), maintaining their precision over time because they are made with stable materials. Wirewound resistors are also among the lowest current noise resistors available at -38dB.

The basic structure of a wirewound resistor has remained unchanged for many years. As the name suggests, a resistance wire is wound around a central core or former, usually made of ceramic. Metal end-caps are pressed onto the core, and the resistance wire welded to them. Finally, the assembly is encapsulated to protect it from moisture and physical damage.

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 Figure 2: The fundamental construction of a wirewound resistor has changed little over time.

Wirewound construction also produces devices that are easy to customize, so engineers have the freedom to specify exactly what they need, even if the final quantities required are in the hundreds, rather than tens of thousands. And although familiar, the technology has not stood still. For instance, advances in materials science allow the construction of devices with tightly controlled response across a range of temperatures, with TCR as low as 1ppm/°C.

The individual component’s resistance is determined by the length, cross-sectional area, and material (and hence resistivity) of the resistance wire. In terms of material choice, a small diameter copper wire 30m long may have a resistance of a few ohms. In contrast, the higher resistivity of a nickel-chrome alloy means that a small diameter wire only 30cm long made of this material may have a resistance of several thousand ohms.

Manufacturers of wirewound resistors offer a choice of metal alloys and sizes and the fabrication characteristics go a long way to explaining the advantages. When a high precision resistor is required, for example, a longer resistance wire can be used, allowing the value to be trimmed to great accuracy by removing a few centimetres (or even millimetres) of wire.

Temperature stability

The choice of material is also the major factor influencing the temperature characteristic of the resistor. For example, low-TCR “RO-800” alloy is formulated to have a TCR of 5 to 10ppm/°C. For comparison, pure nickel has a TCR of 6700ppm/°C, and copper a TCR of 3900ppm/°C.

The material choice therefore allows the manufacturer to tailor the resistor to desired characteristics. In general, low TCR is desirable. However, in some situations, such as temperature sensing and compensation applications, the opposite may be true, since the specific purpose of these components is to respond to changes in temperature.

Wirewound components are sometime chosen for their ability to continue operating in extreme temperatures. Devices such as Riedon’s UT series of axial resistors, for example, operate from -55°C to 275°C, and continue to function at even higher temperatures with de-rating. These capabilities make the technology well-suited for use in the aerospace industry, and in applications such as fire suppression systems.

Power handling and energy dissipation characteristics are similarly linked to the physical construction of the device. As a general rule, a resistor with a larger mass can safely absorb and dissipate more instantaneous power and more energy overall, and this is another strength of wirewound technology.

Pulse performance

One common use for wirewound resistors is in pulse handling. A device such as a medical defibrillator needs to dissipate a large amount of energy in a very short time, putting its electrical components under a high degree of stress.

To protect these components from failure, engineers typically design-in a resistor that can absorb the energy of a significant millisecond current surge. In another application, wirewound resistors are used to protect a metering module installed in a solid state electricity meter.

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Here, the resistor absorbs the high current generated when a metal oxide varistor (MOC) clamps in response to a voltage surge on the grid. Surges can have many causes, including lightning, inductive loads (motors), capacitor banks, switchgear, or even switching heating and ventilation systems in and out of circuit. For these types of application, resistors in the UT series mentioned above are sometimes used. They can withstand over 1000 Joules. Values range from 0.02 Ohms to 260kΩ with tolerances down to ±0.01% and TCR down ±20ppm/°C.

Determining the right pulse handling capabilities for a particular application is not always a straightforward task. Dealing with inrush current implies different requirements than transient suppression. It is not easy to capture within a datasheet all of the information required to make such a choice.

For pulses of up to five seconds, the industry standard specification is a withstand of five-times rated power for five seconds, so a 5W resistor must be able to handle 25W for 5 seconds (125 Joules), regardless of package size or resistance value.

For shorter pulses, the mass of the resistance wire determines the Joule rating, which is then dependent upon resistor value and package type, including its size and whether it’s an axial or surface mount component. Repetition rate and pulse shape - square, triangular or irregular – also have to be taken into account.

 Figure 3: Pulse shape, repetition rate and duration all need to be understood in order to calculate the required energy handling capabilities.

In current sensing applications, designers have different requirements. For example, monitoring battery life in a handheld device generally requires a small package, whereas measurements in industrial or medical equipment might necessitate high precision and high current withstand.

Wirewound devices excel where accuracy is important. For instance, four-terminal components are available in values from 0.01Ω to 1kΩ with tolerances down to 0.005% and current handling capability of up to 25A.

The most commonly cited disadvantage of wirewound resistors, particularly with respect to high frequency applications, is their self-inductance. However, this can overcome with a bifilar winding technique, shown in Figure 4, in which the turns are arranged so that two opposing magnetic fields are created ( one clockwise and the other counter-clockwise ), cancelling out the

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inductance, except the residual amount accounted for by terminations and connecting leads. Inductance is typically reduced by 90% compared to a standard part.

 Figure 4: Non-inductive winding can produce wirewound resistors with minimal self-inductance

Leaded and non-leaded resistor types

One of the key differentiators for resistors, and many other forms of component these days is the way in which they are connected. As a result of the mass production techniques sued and the widespread use of printed circuit boards, the form of connection used for components, especially those to be incorporated into mass produced items changed.

The two main forms of resistor type according to their connection method are:

Leaded resistors:   This type of resistor has been used since the very first electronic components have been in use. Typically components were connected to terminal posts of one form or another and leads from the resistor element were needed. As time progressed, printed circuit boards were used, and the leads were inserted through holes in the boards and typically soldered on the reverse side where the tracks were to be found.

Typical leaded carbon resistor

Surface mount resistors:   These resistor types have been used increasingly since the introduction of surface mount technology. Typically this type of resistor is manufactured using thin film technology. A full range of values can be obtained.

Typical SMD resistors on a PCB

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Variable / Adjustable Resistor or Potentiometernotes or overview about the adjustable resistor or variable resistor used as a trimmer resistor and the circuit symbol

Variable or adjustable resistors are often needed within electronic circuits to act as a preset control within the circuit. The variable resistor, is also widely referred to as a potentiometer as a result of its configuration.

While it is often considered poor design practice to include unnecessary adjustments in a circuit, the variations in circuit values may mean that a preset, or variable resistor may be required in order to set the circuit to function within its required limits.

Additionally, variable resistors are used for controls - a prime example is the volume control on a radio, television of hi-fi unit.

Variable resistor basics

The variable resistor comprises a fixed resistive element along which a slider passes. The variable or adjust able resistor forms a potential divider in which the overall resistance between the two end points remains the same, but the ratio of the two resistors in the legs changes.

In view of the fact that the variable resistor is effectively a potential divider, it is called a potentiometer.

Variable resistor is effectively a potential divider

Variable resistor symbol

The variable resistor symbol used in circuit diagrams indicates its construction. Effectively it is a fixed resistor with a slider that can move along the length of the resistive element. In this way it forms a potentiometer as described before.

Variable resistor symbol for circuit diagrams

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The variable resistor symbols depict the current version use din circuit diagrams today and the traditional format that may be seen on older circuit diagrams.

When a true variable resistor with only two connections is needed, it is common practice to connect the slider to the remote end of the variable resistor as shown below.

Variable resistor element with two connections

Types of adjustable or variable resistors

There are a number of different types of variable resistor that are available on the market. Each of these different types of variable resistor has slightly different properties and is suitable for different applications and situations.

Wirewound variable resistors:   Wirewound variable resistors are able to give a high level of performance and as a result they are often the variable resistor of choice for many applications such as audio, etc..

Wirewound variable resistors are manufactured using very fine resistance wire. This is wound around a former that is almost torroidal. The most commonly used form of resistance wire used is a nickel chrome alloy. Which has some further additives to improve its electrical characteristics.

Wirewound variable resistors offer a high level of linearity and close tolerance. Some very close tolerance versions may be able to offer linearity tolerances of ±0.1%. These variable resistors are also stable over a wide temperature range.

There are two main disadvantages with the wirewould variable resistor. The first is that often as the slider moves over the wires, the resistance changes have discrete steps. This may not be a problem in many applications, but it is a point to note. The second is that they are not suitable even for low frequency RF applications as the resistance wire forms a coil and has significant inductance.

Cermet variable resistors:   Cermet variable resistors are widely used, particularly for trimmer resistors. The name cermet is derived from the fact that the resistive element is made from CERamic and METal. The resistive element is made from a mixture of fine metal oxides or precious metal particles and glass in a viscous organic material. The resulting paste is applied to the substrate and fired to solidify the mixture.

Cermet variable resistors are ideal for trimmer resistors because they have a low to medium adjustment life, and they often have temperature coefficients of around ±100ppm/°C.

Carbon composition variable resistors:   For the carbon composition variable resistor, a mixture of carbon powder and a binder are moulded under heat into the required shape. In some manufacturing processes the carbon composition element is moulded at the same time as the plastic substrate.

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Carbon composition variable resistors are some of the least expensive types and they are widely used in many areas - they are a good all round general purpose variable resistor.

The carbon composition variable resistor element can be tailored to give approximately linear, logarithmic or even anti-logarithmic characteristics. The logarithmic forms are particularly useful for audio applications because the hearing characteristics of the ear mean that a logarithmic curve is more useful.

Carbon composition variable resistors can sometimes exhibit temporary resistance changes of up to around ±10% if operated at very high or low temperatures. Typically there operating temperature range might be expected to be from -55°C to +120°C.

These variable resistors can also become noisy after use as wear and dirt appear on the track. Often some switch cleaner improves the situation.

Conductive plastic variable resistors:   Conductive plastic variable resistors are made using a conductive plastic ink. This ink contains carbon, resin, solvent and other materials specific to the manufacturer. It is applied to the substrate, either by screening or co-moulding. As the ink has a relatively low curing temperature, this enables a variety of substrate materials to be used.

Conductive plastic variable resistors have a high rotational life as well as providing a low noise output. As a result, they are often used for position sensors in servo-controlled machines, etc.

Summary of characteristics of variable resistor types

The different types of variable resistor have different characteristics and this enables them to be used in different situations. The table below summarises some of the major characteristics of the most commonly used types of variable resistor.

Variable Resistor

Type

Typical Resistance

Ranges

Typical Tolerance

Typical Power

Handling

Typical Life

(Rotations)

Typical Temperature Coefficient(ppm/°K)

Wirewound 10Ω - 50kΩ ±5% up to 1 Watt 500 ±50Cermet 50Ω - 2MΩ ±10% 500mW 200 100Conductive Plastic

50Ω - 2MΩ ±10% 250mW 100 000 500

Carbon composition

50Ω - 2MΩ ±20% 250mW 1000 ~±10%/°K

By the very nature of this table the characteristics for the various forms of variable resistor can only be taken as a guide - with developments in technology occurring, along with the differing specifications of components from the variety of manufacturers, there is a large variation of performance of components from different sources.

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Resistor colour code table- resistor colour code chart / table for the four band, five band and six band systems used to provide value, tolerance and temperature coefficient details

Resistor colour codes are used to indicate the value of leaded resistors. These resistor colours have been widely used for many years.

The resistor colour code provides an easy and reliable method for value indication - often printing the values in figures can be obscured or erased during handling making identification difficult.

Resistor colour code basics

The resistor colour coding is carried on a number of coloured rings placed around the resistor. As virtually all leaded resistors are cylindrical, it is difficult to print numbers on them, and they can also become obscured during handling and use. As the resistor colour code system relies on rings around the resistor, even if part of the coding scheme is marked, other areas can be seen and the values and other information deciphered.

Dependent upon the tolerance and accuracy required for the resistor, there are a number of colour coding schemes that may be used. All the different resistor colour code systems are basically the same in outline, but there provide differing levels of information.

The main resistor colour coding schemes that are seen are:

Four band resistor colour code scheme Five band resistor colour code scheme Six band resistor colour code scheme

Dependent upon the number of rings used, the different resistor colour code schemes are able to provide

Four band resistor colour code system

The four band system is used for E6, E12, and E24 series values. It can accommodate values with up to two significant figures which is acceptable for the resistor value ranges up to E24 which normally accommodate tolerance values up to ±2%.

The resistor colour code bands give the value of the resistor as well as other information including the tolerance and sometimes the temperature coefficient. The band closest to the end of the resistor body is taken to be Band 1. The first two bands of the resistor colour code are the significant figures of the value, and the third of the resistor colour code is a multiplier.

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4 band resistor colour code

As an example the colours shown above are red, violet, and orange on the left - the forth band on the right is red. The value is given by the first three, red violet corresponds to the significant figures, 27, and then the orange corresponds to a multiplier of 1000. This gives the value 27kΩ. The fourth band gives a tolerance of ±2%.

Note:   If only three bands are present on the resistor, they will be the two significant figures, followed by the multiplier, i.e. no tolerance band.

Five band resistor colour code system

For resistors where higher tolerances are needed, i.e. ±1% and better and for the E48, E96 and E192 series resistors where three significant figures are required, an extra digit band is included. Otherwise this resistor colour coding system is the same as the four band colour code system.

5 band resistor colour code

Using the example in the diagram where the resistor colours are, orange; brown; blue; red; brown. From the first three resistor colour bands, it can be seen that the significant digits are 316, and the multiplier is 100. This gives 31600 or 31.6kΩ. The final band or ring indicates the tolerance is 1%

Six band resistor colour code system

The six band resistor colour code system provides the maximum amount of information about the resistor parameters. Like the Five band colour code system, this one is generally used with high tolerance values i.e. ±1% and better and E$*, E96, & E192 series resistor values.

6 band resistor colour code

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The example of the six band resistor colour code system shown in the diagram has colours of where the resistor colours are, orange; brown; blue; red; brown red. From the first three resistor colour bands, it can be seen that the significant digits are 316, and the multiplier is 100. This gives 31600 or 31.6kΩ. The fifth band or ring is brown and indicates the tolerance is 1%. The final red band indicates the temperature coefficient is 50 ppm/°K.

Resistor Colour Code Chart

The resistor colour code table or chart below summarises the different colour codes used for resistors.

Colour Digits(Sig Figs)

Multiplier Tolerance Temp Coefficientppm/°K

Black 0 100

Brown 1 101 ±1% 100Red 2 102 ±2% 50Orange 3 103 15Yellow 4 104 25Green 5 105 ±0.5%Blue 6 106 ±0.25%Violet 7 107 ±0.1%Grey 8 108 ±0.05%White 9 109

Gold ±5%Silver ±10%None ±20%

The resistor colour code is used in virtually all leaded resistors with power dissipation levels up to about a watt. Beyond this the resistors are generally large enough, and use a different form of construction allowing sufficient space for the values to be marked in figures. Nevertheless the resistor colour code is the most widely used system for leaded resistors. The same basic concept is also used on some capacitors.

Standard resistor values- standard resistor values given in the EIA E series - with explanations of the system & tables of these common resistor values.

The values given to resistors fall into a number of preferred or standard resistor values. These standard resistor values have a logarithmic sequence related to the component accuracy, enabling the standard resistor values to be spaced according to the tolerance on the component. These standard resistor values are also applicable for capacitors and other components as well as resistors.

Component values such as resistor values can never be manufactured exactly, each resistor has a tolerance associated with it. These may typically be ±20%, ±10% and ±5%. Other tolerances such as ±2% are also available.

In order to ensure that standard values can be chosen from one of a variety of manufacturers, list of preferred values or standard resistor values have been devised. By using these preferred resistor

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value lists, common resistor values can be chosen from available components. This not only makes manufacture easier, but it enables stock holdings of manufacturers to be reduced by having a preferred resistor value range. As most component values need not be high precision special values, this is a particularly attractive idea.

E series of standard resistor values

In order to enable the common resistor values to be spaced apart according to their tolerance, a series known as the E series for standard or preferred values is used. Resistors are spaced apart so that the top of the tolerance band of one value and the bottom of the tolerance band of the next one do not overlap. Take for example a resistor with a value of 1 ohm and a tolerance of ±20%. If the actual resistance of the component falls at the top of the tolerance band then it will have a value of 1.2 ohms. Take then a resistor with a value of 1.5 ohms. Again it is found that the value at the bottom of its tolerance band is 1.2 ohms. By calculating a range of values in this way a series can be built up. This is repeated for each decade.

The series generated in this way for standard resistor values is known as the E series and these are preferred values. The most basic series within the E range is the E3 series which has just three values: 1, 2.2 and 4.7. This is seldom used as such because the associated tolerance is too wide for most of today's applications, although the basic values themselves may be used more widely to reduce stock holding.

The next is the E6 series with six values in each decade for a ±20% tolerance, E12 series with 12 values in each decade for a ±10%, E24 series with 24 values in each decade for a ±5% tolerance. Values for resistors in these series are given below. Further series (E48 and E96) are available, but are not as common as the ones given below.

The E6 and E12 resistors are available in virtually all types of resistor. However the E24 series, being a much closer tolerance series is only available in the higher tolerance types. Metal oxide film resistors that are in common use today are available in the E24 series as are several other types. Carbon types are rarely available these days and in any case would only available in the lower tolerance ranges as their values cannot be guaranteed to such a close tolerance.

The E series preferred or standard resistor ranges are widely used and have been adopted by standards organisations. The EIA (Electrical Industries Association) based in North America has, for example adopted the system and as such the values included are often referred to as the EIA preferred values.

Summary of EIA Preferred or Standard Resistor SeriesE

SeriesTolerance(Sig Figs)

Number of values in each decade

E6 20% 6E12 10% 12E24 5%

[normally also available in 2% tolerance]

24

E48 2% 48E96 1% 96E192 0.5%, 0.25% and higher

tolerances192

Preferred and standard values of other components13

The system for adopting standard component values works very well for resistors. It is also equally applicable for other components. The same concept of using values in a standard list that are determined by the tolerance of the components is equally applicable.

Accordingly the E series preferred values are also widely used for capacitors, where some of the lower order series are used - E3, E6 as the values on many capacitors do not have a high tolerance. Electrolytic capacitors typically have a very wide tolerance, although others such as many ceramic types have a much tighter tolerance and many be available in ranges conforming to the E12 or even E24 values.

Another example of components that follow the EIA E series preferred values is Zener diodes for their breakdown voltages. The Zener diode standard voltages typically conform to the E12 values although E24 series voltage values are also available - especially 5.1 volt Zener diode for 5 volt rails.

Resistor E Series - E3, E6, E12, E24, E48, E96 Tables - chart and tables of standard resistor values in the E3, E6, E12, E24, E48, E96 ranges.

The EIA preferred values can be summarised in tabular form to give the different values within each decade.

When designing equipment, it is good practice to keep to the lowest E-series section, i.e. it is better to use E3 rather than E6. In this way the number of different parts in any equipment can be minimised. If decade values, i.e. 100R, 1K, 10, etc can be used so much the better.

For many digital designs where the resistor is used as a pull up or pull down, the resistor value is of little consequence and this is easy. For analogue designs it is a little more complicated, and E12, or E24 values are needed. E48, E96 or even E192 series values are needed for high accuracy and close tolerance requirements.

As the higher order series are used less, their costs are also normally higher.

Resistor E series tables of values

E6 Standard Resistor Series1.0 1.5 2.23.3 4.7 6.8

E12 Standard Resistor Series1.0 1.2 1.51.8 2.2 2.73.3 3.9 4.75.6 6.8 8.2

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E24 Standard Resistor Series1.0 1.1 1.21.3 1.5 1.61.8 2.0 2.22.4 2.7 3.03.3 3.6 3.94.3 4.7 5.15.6 6.2 6.87.5 8.2 9.1

E48 Standard Resistor Series1.00 1.05 1.101.15 1.21 1.271.33 1.40 1.471.54 1.62 1.691.78 1.87 1.962.05 2.15 2.262.37 2.49 2.612.74 2.87 3.013.16 3.32 3.483.65 3.83 4.024.22 4.42 4.644.87 5.11 5.365.62 5.90 6.196.49 6.81 7.157.50 7.87 8.258.66 9.09 9.53

E96 Standard Resistor Series1.00 1.02 1.051.07 1.10 1.131.15 1.18 1.211.24 1.27 1.301.33 1.37 1.401.43 1.47 1.501.54 1,58 1.621.65 1.69 1.741.78 1.82 1.871.91 1.96 2.002.05 2.10 2.162.21 2.36 2.322.37 2.43 2.492.55 2.61 2.672.74 2.80 2.872.94 3.01 3.093.16 3.24 3.323.40 3.48 3.573.65 3.74 3.833.92 4.02 4.124.22 4.32 4.42

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4.53 4.64 4.754.87 4.91 5.115.23 5.36 5.495.62 5.76 5.906.04 6.19 6.346.49 6.65 6,816.98 7.15 7.327.50 7.68 7.878.06 8.25 8.458.66 8.87 9.099.31 9.59 9.76

Tables of standard resistor values in the E series

Virtually all resistors that are available fall into the standard resistor values that are given in the table above. Although resistors are specified up to the E96 series, for most applications a comparatively few number of resistor values is needed. By choosing from E3 or E6 series of standard resistor values, and not going to some of the higher order series, it can reduce the stock holding as there is a greater chance the same values may be used elsewhere in a design. Only where close tolerance types are required should resistors from the E24, or even E48 or E96 series of standard resistors should be used.

SMD Resistor- an overview of surface mount technology SMT resistors, or SMD resistors, their packages and properties.

Surface mount technology, SMT includes:

    •  SMT overview    •  SMD component packages    •  SMD resistor    •  SMD resistor markings    •  MELF SMD resistor    •  SMD capacitor    •  Quad Flat Package, QFP    •  BGA, Ball Grid Array    •  SMD PLCC

Surface mount device , SMD, resistors are the most widely used electronic component. Every day many millions are used to produce the electronic equipment from cell phones to televisions and MP3 players, and commercial communications equipment to high technology research equipment.

Basic SMD resistor construction

SMD resistors are rectangular in shape. They have metallised areas at either end of the body of the SMD resistor and this enables them to make contact with the printed circuit board through the solder.

The resistor itself consists of a ceramic substrate and onto this is deposited a metal oxide film. The thickness, and the length of the actual film determines the resistance. In view of the fact that

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the SMD resistors are manufactured using metal oxide, means that they are quite stable and usually have a good tolerance.

SMD resistor packages

SMD resistors come in a variety of packages. As the technology has moved forward so the size of the resistor packages has fallen. The main packages with their sizes are summarised below:

Package style Size (mm) Size (inches)2512 6.30 x 3.10 0.25 x 0.122010 5.00 x 2.60 0.20 x 0.101812 4.6 x 3.0 0.18 x 0.121210 3.20 x 2.60 0.12 x 0.101206 3.0 x 1.5 0.12 x 0.060805 2.0 x 1.3 0.08 x 0.050603 1.5 x 0.08 0.06 x 0.030402 1 x 0.5 0.04 x 0.020201 0.6 x 0.3 0.02 x 0.01

It can be seen from the dimensions in Imperial measurements, that the package names correspond to the dimensions in hundredths of an inch. This an SMD resistor with an 0805 package measures 0.08 by 0.05 inches.

SMD resistor specifications

SMD resistors are manufactured by a number of different companies. Accordingly the specifications vary from one manufacturer to the next. It is therefore necessary to look at the manufacturers rating for a specific SMD resistor before deciding upon exactly what is required. However it is possible to make some generalisations about the ratings that might be anticipated.

Power rating:     The power rating needs careful consideration in any design. For designs using SMDs the levels of power that can be dissipated are smaller than those for circuits using wire ended components. As a guide typical power ratings for some of the more popular SMD resistor sizes are given below. These can only be taken as a guide because they may vary according to the manufacturer and type.

Typical Power RatingsPackage

styleTypical Power Rating

(W)2512 0.50 (1/2)2010 0.25 (1/4)1210 0.25 (1/4)1206 0.125 (1/8)0805 0.1 (1/10)0603 0.625 (1/16)0402 0.0625 - 0.031 (1/16 - 1/32)0201 0.05

Some manufacturers will quote higher power levels than these. The figures given here are typical.

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Tolerance:     In view of the fact that SMD resistors are manufactured using metal oxide film they available in relative close tolerance values. Normally 5%, 2%, and 1% are widely available. For specialist applications 0.5% and 0.1% values may be obtained.

Temperature coefficient:     Again the use of metal oxide film enables these SMD resistors to provide a good temperature coefficient. Values of 25, 50 and 100 ppm / C are available.

Applications

SMD resistors are used in many designs. Their size not only means that they are suitable for compact circuit boards, and for automatic assembly techniques, but it also ahs the advantage that they perform well at radio frequencies. Their size means that they have little spurious inductance and capacitance. Nevertheless care has to be taken when calculating their power dissipation as they can only dissipate small levels of power. ..............

SMD / SMT Resistor Markings- details of the SMD or SMT resistor markings and the resistor marking systems used to indicate the resistor values - including EIA SMD resistor marking scheme.

Although not all SMD resistors, or SMT resistors are marked with their values, some are, and in view of the lack of space the SMD resistor making systems may not always provide an obvious indication of the resistor value.

The SMT resistor marking systems provide are mainly used to enable service, repair and fault-finding. During manufacture the resistors are held either in tapes that are reeled, or in hoppers used for the surface mount machines. The SMD resistor markings can be used as a check to ensure the correct values are being fitted, but normally the reels or hoppers will be suitable marked and coded.

SMT resistor marking systems

Many SMD resistors do not have any markings on them to indicate their value. For these devices, once they are loose and out of their packaging it is very difficult to tell their value. Accordingly SMD resistors are typically used within reels or other packages where there is no chance of different values being mixed.

Many resistors do have markings on them. There are three systems that are used:

Three figure SMD resistor marking system Four figure SMD resistor marking system EIA96 SMD resistor marking system

3 figure SMT resistor marking system

A three figure SMT resistor marking system is the one that is normally used for standard tolerance resistors.

As the name indicates this SMD resistor marking system uses three figures. The first two indicate the significant figures, and the third is a multiplier. This is the same as the coloured rings used for wired resistors, except that actual numbers are used instead of colours.

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Therefore an SMD resistor with the figures 472 would have a resistance of 47 x 102 ohms, or 4.7kΩ. However beware of resistors marked with figures such as 100. This is not 100 ohms, but it follows the scheme exactly and it is 10 x 100 or 10 x 1 = 10 Ω.

Three figure resistor marking

Where resistance values less than ten ohms are used, the letter "R" is used to indicate the position of the decimal point. As an example, a resistor with the value 4R7 would be 4.7Ω.

4 figure SMT resistor marking system

the four digit or four figure SMT resistor marking scheme is used for marking high tolerance SMD resistors. Its format is very similar to the three figure SMT resistor making scheme, but expanded to give the higher number of significant figures needed for higher tolerance resistors.

In this scheme, the first three numbers will indicate the significant digits, and the fourth is the multiplier.

Therefore an SMD resistor with the figures 4702 would have a resistance of 470 x 102 ohms, or 47kΩ.

Four figure SMT resistor marking

Resistors with values of less than 100 ohms are marked utilise the letter 'R', as before, to indicate the position of the decimal point.

EIA96 SMD resistor marking system

A further SMD resistor marking scheme or SMD resistor coding scheme has started to be used, and it is aimed at 1% tolerance SMD resistors, i.e. those using the EIA96 or E-96 resistor series. As higher tolerance resistors are used, further figures are needed. However the small size of SMT resistors makes the figures difficult to read. Accordingly the new system seeks to address this. Using only three figures, the actual characters can be made larger than those of the four figure system that would otherwise be needed.

The EIA SMD resistor marking scheme uses a three character code: the first 2 numbers indicate the 3 significant digits of the resistor value. The third character is a letter which indicates the multiplier. In this way this SMD resistor marking scheme will not be confused with the 3 figure markings scheme as the letters will differentiate it, although the letter R can be used in both systems.

To generate the system the E-96 resistor series has been taken and each value or significant figure set has been numbered sequentially. As there are only 96 values in the E-96 series, only two figures are needed to number each value, and as a result this is a smart way of reducing the number of characters required.

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EIA SMD resistor marking

The details for the EIA SMT resistor marking scheme are tabulated below:

Code MultiplierZ 0.001Y or R 0.01X or S 0.1A 1B or H 10C 100D 1 000E 10 000F 100 000EIA SMT resistor marking scheme multipliers

Code Sig Figs Code Sig Figs Code Sig Figs Code Sig Figs01 100 25 178 49 316 73 56202 102 26 182 50 324 74 57603 105 27 187 51 332 75 59004 107 28 191 52 340 76 60405 110 29 196 53 348 77 61906 113 30 200 54 357 78 63407 115 31 205 55 365 79 64908 118 32 210 56 374 80 66509 121 33 215 57 383 81 68110 124 34 221 58 392 82 69811 127 35 226 59 402 83 71512 130 36 232 60 412 84 73213 133 37 237 61 422 85 75014 137 38 243 62 432 86 76815 140 39 249 63 422 87 78716 143 40 255 64 453 88 80617 147 41 261 65 464 89 82518 150 42 267 66 475 90 84519 154 43 274 67 487 91 86620 158 44 280 68 499 92 88721 162 45 287 69 511 93 90922 165 46 294 70 523 94 93123 169 47 301 71 536 95 95324 174 48 309 72 549 96 976

EIA SMT resistor marking scheme significant figures

For example a resistor that is marked 68X can be split into two elements. 68 refers to the significant figures 499, and X refers to a multiplier of 0.1. Therefore the value indicated is 499 x 0.1 = 49.9Ω.

MELF Resistor- details of the MELF resistor a surface mount device, SMD used to provide superior performance over SMT resistors in certain applications.

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Another form of SMD resistor that can be used is known as the MELF resistor - Metal Electrode Leadless Face. These resistors are not nearly as widely used as the standard SMD resistors, but in some instances they provide advantages and can be used.

MELF resistor basics & construction

The MELF resistor is cylindrical in shape and have metallisation on both ends. Land pattern sizes for MELF resistors are the same as SMD chip resistors.

The manufacture of MELF resistors is more complicated than the more standard thick film SMD resistors. A metal film is deposited onto a high dissipation ceramic former. To make the terminations tin plated terminating caps are fitted. The resistor is then adjusted to the correct value by producing a helical cut in the film. The body of the MELF resistor is finally protected by a lacquer coating.

MELF Resistor Outline

The MELF SMD resistors are used for a number of reasons:

MELF resistors provide a high level of reliability. A MELF resistor has a more predictable pulse handling capacity than other SMD resistors MELF resistors can be manufactured with tolerances as tight as 0.1% They can be manufactured with very low levels of temperature coefficient, sometimes as low as 5

ppm/°C

Although the standard flat chip resistors are cheaper and much easier to handle during manufacture, the performance of MELF resistor can be an overriding factor making them a cost effective solution

MELF resistors in electronics manufacture

While MELF resistors provide some significant and compelling technical advantages for use in certain applications, they are not always the easiest to handle in manufacture.

The most common form of SMD resistor by far is the flat or cuboid format. These require one form of nozzle on a pick and place machine, however MELF resistors require a different one that allows the cylindrical shape of the MELF resistor to be accommodated. They also require a higher level of vacuum on the pick and place machine.

MELF SMD resistor markings

MELF SMD resistors are used on occasions in some designs. These resistors are cylindrical and do not lend themselves to characters being printed on the surface, although coloured bands are easy to use. As such the MELF SMD resistor marking code is effectively the same as that used for leaded resistors.

There are three variations used:

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Four band code:   This system is used for resistors with tolerances up to 5% using the E24 resistor series. The first two bands provide the significant digits. The third band provides the multiplier and the fourth, normally wider, provides the tolerance.

MELF Resistor 4 band code

Sometimes an alternative colour banding system may be used where the bands are all grouped towards one end of the MELF resistor rather than having a wider band at the far end.

Alternative MELF Resistor 4 band code

Five band code:   This system is used for higher tolerance resistors typically better than 1% that use the E48, E96 or E192 series values. The first three bands provide the significant figures. The fourth band gives the multiplier and the fifth band gives the tolerance.

MELF Resistor 5 band code

Six band code:   This code provides space for a temperature coefficient marking. As with the four band code the first three bands give the significant figures. Next is the tolerance band, and finally the fifth band provides the tolerance. .

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MELF Resistor 6 band code

Tables showing the various colours and figures are given below:

Colour CodeColour Digit Multiplier Tolerance

None ±20%Silver 10-2 ±10%Gold 10-1 ±5%Black 0 100

Brown 1 101 ±1%Red 2 102 ±2%Orange 3 103

Yellow 4 104

Green 5 105 ±0.5%Blue 6 106 ±0.25%Violet 7 107 ±0.1%Grey 8 108

White 9 109

Temperature Coefficient MarkingColour Code(6th Band)

TCR ppm/°K

Brown ± 100Red ± 50Yellow ± 25Orange ± 15Blue ± 10Violet ± 5

Thermistor23

- overview or tutorial about what is a thermistor & the basics its operation as well as the NTC thermistor and the PTC thermistor.

The name thermistor is a shortening of the words thermally sensitive resistor. This describes the action of the thermistor particularly well.

Today, thermistors are used in a wide variety of devices from temperature sensors through to providing temperature compensation in electronic circuits.

As such thermistors are widely used in electronic, although they are obviously not as commonly used as ordinary resistors, capacitors and transistors.

Thermistor categories

There are a number of ways in which thermistors can be categorised. The first is dependent upon the way they react to heat. Some increase their resistance with increasing temperature, while others exhibit a fall in resistance. Accordingly it is possible categorise them accordingly:

Positive temperature coefficient (PTC)   The PTC thermistor has the property where the resistance increases with increasing temperature

Negative temperature coefficient (NTC)   The NTC thermistor has the property where the resistance decreases with increasing temperature

In addition to the nature of the resistance change, thermistors can also be categorised according to the type of material used. Typically they use one of two materials:

Metallic compounds including oxides etc. Single-crystal semiconductors

History & development of the thermistor

As early as the nineteenth century people have been able to demonstrate the variation of a resistor with temperature. These have been used in a variety of ways, but many suffer from a comparatively small variation over even a large temperature range. Themistors generally imply the use of semiconductors, and these provide a much larger resistance variation for a given temperature change.

Of the two types of material used for thermistors, the metallic compounds were the first to be discovered. The negative temperature co-efficient was observed by Faraday in 1833 when he measured the resistance variation with temperature of silver sulphide. However it took until the 1940s before metallic oxides became available commercially.

With the work that was undertaken into semiconductor materials after the Second World War, crystal germanium thermistors were studied, and later silicon themistors were investigated.

Although there are two types of themistor, the metallic oxides and the semiconductor varieties, they cover different temperature ranges and in this way they do not compete.

Thermistor structure & composition

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Thermistors come in a variety of shapes and sizes, and they are made from a variety of materials dependent upon their intended application and the temperature range over which they need to operate. In terms of their physical shape they can come as flat discs for applications where they need to be in contact with a flat surface. However they can also be made in the form of beads or even rods for use in temperature probes. In fact the actual shape of a thermistor is very dependent upon the requirements for the application.

Metallic oxide thermistors are generally used for temperatures in the range 200 - 700 K. These thermistors are made from a fine powder version of the material that is compressed and sintered at high temperature. The most common materials to be used for these thermistors are Manganese oxide, nickel oxide, cobalt oxide, copper oxide and ferric oxide.

Semiconductor thermistors are used for much lower temperatures. Germanium thermistors are more widely used than their silicon counterparts and are used for temperatures below 100 K, i.e. within 100 degrees of absolute zero. Silicon thermistors can be used at temperatures up to 250 K. Above this temperature a positive temperature coefficient sets in. The thermistor itself is made from a single crystal which has been doped to a level of 10^16 - 10^17 per cubic centimetre.

Thermistor applications

Thermistors are found in many applications. They provide very cheap, yet effective elements in circuits and as such they are very attractive to use. The actual applications depend upon whether the thermistor is a positive (PTC) or negative (NTC) temperature co-efficient.

1. Applications for negative temperature coefficient (NTC) thermistors:o Very low temperature thermometers:   NTC thermistors are used as resistance

thermometers in very low-temperature measurements.o Digital thermostats:   Thermistors are also commonly used in modern digital thermostats.o Battery pack monitors:   Thermistors are also used to monitor the temperature of battery

packs while charging. As modern batteries such as Li-ion batteries are very sensitive to overcharging, the temperature provides a very good indication of the charging state, and when to terminate the charge cycle.

o In-rush protection devices:   NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purpose designed for this application.

2. Applications for Positive temperature coefficient (PTC) thermistors:o Current limiting devices:   PTC thermistors can be used as current limiting devices in

electronic circuits, where they can be used as an alternative to a fuse. Current flowing through the device under normal conditions causes a small amount of heating which does not give rise to any undue effects. However if the current is large, then it gives rise to more heat which the device may not be able to loose to the surroundings and the resistance goes up. In turn this gives rise to more heat generation in a positive feedback effect. As the resistance increases, so the current falls, thereby protecting the device.

Thermistors can be used in a wide variety of applications. They provide a simple, reliable and inexpensive method of sensing temperatures. As such they may be found in a wide variety of

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devices from fire alarms to thermostats. Although they may be used on their own, they may also be used as part of a Wheatstone bridge to provide higher degrees of accuracy. Another used for thermistors is as temperature compensation devices. Most resistors have a positive temperature co-efficient, their resistance increasing with increasing temperature. In applications where stability is required, a thermistor with a negative temperature co-efficient can be incorporated into the circuit to counteract the effect of the components with a positive temperature co-efficient.

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Light dependent resistor, photo resistor, or photocell- Notes on the basics of a photoresistor, light dependent resistor or photocell including its construction, operation, circuit symbol, and circuit applications

The light dependent resistor, LDR, is known by many names including the photoresistor, photo resistor, photoconductor, photoconductive cell, or simply the photocell. It is probably the term photocell that is most widely used in data and instruction sheets for domestic equipment.

The photo resistor, or light dependent resistor, LDR, finds many uses as a low cost photo sensitive element and was used for many years in photographic light meters as well as in other applications such as flame, smoke and burglar detectors, card readers and lighting controls for street lamps. Often within the literature the photoresistor is called the photocell as a more generic term.

Photoresistor discovery

Photo-resistors, or light dependent resistors have been in use for very many years. Photoresistors have been seen in early forms since the nineteenth century when photoconductivity in selenium was discovered by Smith in 1873. Since then many variants of photoconductive devices have been made.

Much useful work was conducted by T. W. Case in 1920 when he published a paper entitled "Thalofide Cell - a new photo-electric cell".

Other substances including PbS, PbSe and PbTe were studied in the 1930s and 1940s, and then in 1952, Rollin and Simmons developed their photoconductors using silicon and germanium.

Light dependent resistor symbol

The circuit symbol used for the light dependent resistor or photoresistor combines its resistor action while indicating that it is sensitive to light. The basic light dependent resistor symbol has the rectangle used to indicate its resistor action, and then has two incoming arrows - the same as those used for photodiodes and phototransistors to indicate its light sensitivity.

Light dependent resistor symbol used in circuit diagrams

For most applications, the light dependent resistor symbol used will be that with the resistor with arrows, but in some instances those drawing circuit diagrams prefer to encase the resistor in a circle. The more commonly used photoresistor symbol is the resistor without the circle around it.

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Photoresistor mechanism

A photoresistor or photocell is a component that uses a photconductor between two contacts. When this is exposed to light a change in resistance is noted.

Photoconductivity - the mechanism behind the photoresistor - results from the generation of mobile carriers when photons are absorbed by the semiconductor material used for the photoconductor. While the different types of material used for light dependent resistors are semiconductors, when used as a photo-resistor, they are used only as a resistive element and there are no PN junctions. Accordingly the device is purely passive.

There are two types of photoconductor and hence photoresistor:

Intrinsic photoresistor:   This type of photoresistor uses a photoconductive material that involves excitation of charge carriers from the valence bands to the conduction band.

Extrinsic photoresistor:   This type of photoresistor uses a photoconductive material that involves excitation of charge carriers between an impurity and the valence band or conduction band. It requires shallow impurity dopants that are not ionised in the presence of light.

Extrinisc photoresistors or photocells are generally designed for long wavelength radiation - often infra-red, but to avoid thermal generation they need to be operated at low temperatures.

Basic photoresistor structure

Although there are many ways in which light dependent resistors, or photo resistors can be manufactured, there are naturally a few more common methods that are seen. Essentially the photoresisitor or photocell consists of a resistive material sensitive to light that is exposed to light. The photo resistive element comprises section of the material with contacts at either end.

A typical structure for a light dependent or photo resistor uses an active semiconductor layer that is deposited on an insulating substrate. The semiconductor is normally lightly doped to enable it to have the required level of conductivity. Contacts are then placed either side of the exposed area.

One form of photoresistor structure

Within the basic photoresistor or photocell structure, the resistance of the material itself is a key issue. To ensure the resistance changes resulting from the light dominate, contact resistance is minimised. To achieve this, the area around the contacts is normally heavily doped to reduce the resistance in this region.

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In many instances the area between the contacts is in the form of a zig zag, or interdigital pattern. This maximises the exposed area and by keeping the distance between the contacts small it reduces the spurious resistance levels and enhances the gain.

Photoresistor or photocell with interdigital contact pattern

It is also possible to use a polycrystalline semiconductor that is deposited onto a substrate such as ceramic. This makes for a very low cost light dependent resistor

Photoresistor applications

The photoresistor or light dependent resistor is attractive in many electronic circuit designs because of its low cost, simple structure and rugged features. While it may not have some of the features of the photo-diode and photo-transistor, it is ideal for many applications. As a result the photo-resistor is widely used in circuits such as photographic meters, flame or smoke detectors, burglar alarms, card readers, controls for street lighting and many others.

The properties of photoresistors can vary quite widely dependent upon the type of material used. Some have very long time constants, for example. It is therefore necessary to carefully choose the type of photoresistor for any given circuit or application.

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Capacitor types- an overview, information or tutorial about the different capacitor types looking at different types of capacitor and their construction, specifications and parameters.

Capacitor types includes:

    •  Capacitor types overview    •  Uses and applications    •  Electrolytic capacitor    •  Ceramic capacitor    •  Tantalum capacitor    •  Polycarbonate capacitor    •  Silver mica capacitor    •  Glass dielectric capacitor    •  Polystyrene capacitor    •  Capacitor markings

Electronic capacitors are one of the most widely used forms of electronics components. However there are many different types of capacitor including electrolytic, ceramic, tantalum, plastic, sliver mica, and many more. Each capacitor type has its own advantages and disadvantages can be used in different applications.

The choice of the correct capacitor type is of great importance because it can have a major impact on any circuit. The differences between the different types of capacitor can mean that the circuit may not work correctly if the correct type of capacitor is not used.

Accordingly a summary of the different types of capacitor is given below, and further descriptions of a variety of capacitor types can be reached through the related articles menu on the left hand side of the page below the main menu.

Capacitor basics

there are many different types of capacitor, but they all conform to the same basic physical laws. These determine the basic way the capacitor operates, its value, i.e. the amount of charge it will hold and hence its capacitance.

In order to understand some of the reasons why various forms of capacitor are used, it is necessary to look at the basic theory behind capacitance.

Note on Capacitance:

All capacitors conform to the same basic laws. Regardless of the dielectrics and many other enw developments made, the same laws apply.

Click on the link for further information about Capacitance

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Capacitor types & dielectrics

Although all capacitors work in essentially the same way, key differences in the construction of different capacitor types makes an enormous difference in their properties.

The main element of the capacitor that gives rise to the different properties of the different types of capacitor is the dielectric - the material between the two plates. Its dielectric constant will alter the level of capacitance that can be achieved within a certain volume.

Some types of capacitor may be polarised, i.e. they only tolerate voltages across them in one direction. Other capacitor types are non-polarised and can have voltages of either polarity across them.

Typically the different types of capacitor are named after the type of dielectric they contain. This gives a good indication of the general properties they will exhibit and for what circuit functions they can be used.

Overview of different capacitor types

There are many different types of capacitor that can be used - most of the major types are outlined below:

Ceramic capacitor:   The ceramic capacitor is a type of capacitor that is used in many applications from audio to RF. Values range from a few picofarads to around 0.1 microfarads. Ceramic capacitors are by far the most commonly used type of capacitor being cheap and reliable and their loss factor is particularly low although this is dependent on the exact dielectric in use. In view of their constructional properties, these capacitors are widely used both in leaded and surface mount formats Read more about the ceramic capacitor

Electrolytic capacitor:   Electrolytic capacitors are a type of capacitor that is polarised. They are able to offer high capacitance values - typically above 1μF, and are most widely used for low frequency applications - power supplies, decoupling and audio coupling applications as they have a frequency limit if around 100 kHz. Read more about the electrolytic capacitor

Tantalum capacitor:   Like electrolytic capacitors, tantalum capacitors are also polarised and offer a very high capacitance level for their volume. However this type of capacitor is very intolerant of being reverse biased, often exploding when placed under stress. They must also not be subject to high ripple currents or voltages above their working voltage. They are available in both leaded and surface mount formats. Read more about the tantalum capacitor

Silver Mica Capacitor:   Silver mica capacitors are not as widely used these days, but they still offer very high levels of stability, low loss and accuracy where space is not an issue. They are primarily used for RF applications and and they are limited to maximum values of 1000 pF or so. Read more about the silver mica capacitor

Polystyrene Film Capacitor:   Polystyrene capacitors are a relatively cheap form of capacitor but offer a close tolerance capacitor where needed. They are tubular in shape resulting from the fact that the plate / dielectric sandwich is rolled together, but this adds inductance limiting their frequency response to a few hundred kHz. They are generally only available as leaded electronics components.

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Polyester Film Capacitor:   Polyester film capacitors are used where cost is a consideration as they do not offer a high tolerance. Many polyester film capacitors have a tolerance of 5% or 10%, which is adequate for many applications. They are generally only available as leaded electronics components.

Metallised Polyester Film Capacitor:   This type of capacitor is a essentially a form of polyester film capacitor where the polyester films themselves are metallised. The advantage of using this process is that because their electrodes are thin, the overall capacitor can be contained within a relatively small package. The metallised polyester film capacitors are generally only available as leaded electronics components.

Polycarbonate capacitor:   The polycarbonate capacitors has been used in applications where reliability and performance are critical. The polycarbonate film is very stable and enables high tolerance capacitors to be made which will hold their capacitance value over time. In addition they have a low dissipation factor, and they remain stable over a wide temperature range, many being specified from -55°C to +125°C. However the manufacture of polycarbonate dielectric has ceased and their production is now very limited. Read more about the polycarbonate capacitor

Polypropylene Capacitor:   The polypropylene is sometimes used when a higher tolerance is necessary than polyester capacitors offer. As the name implies, this capacitor uses a polypropylene film for the dielectric. One of the advantages of the capacitor is that there is very little change of capacitance with time and voltage applied. They are also used for low frequencies, with 100 kHz or so being the upper limit. They are generally only available as leaded electronics components.

Glass capacitors:   As the name implies, this type of capacitor uses glass as the dielectric. Although expensive, these capacitors offer very high levels or performance in terms of extremely low loss, high RF current capability, no piezo-electric noise and other features making them ideal for many performance RF applications. Read more about the glass dielectric capacitor

These capacitors include some of the main types of capacitor, although there are other types that are used for more specialist applications.

Capacitor applications, uses and usage- notes on capacitor applications, uses and usage, capacitor choice - which type to use in a particular application, circuit or function.

The choice of capacitor for a particular application or use is of paramount importance. Even if the correct value is chosen for a particular capacitor application or capacitor use, the selection of the correct type is of equal importance.

In some instances one form of capacitor may work very well, but another capacitor type may cause the circuit to not work at all. It is therefore critical that the capacitor use or capacitor application is matched to the type or form of capacitor used.

Table of capacitor uses and applications

The most suitable way to summarise the various types of capacitor and the applications for which these electronic capacitors are suited is in a table.

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Application Suitable types with reasons details & comments

Power supply smoothing

Aluminium electrolytic   High capacity and high ripple current capability **

Audio frequency coupling

Aluminium Electrolytic:   High capacitance Tantalum:   High capacitance and small size Polyester / polycarbonate :   Cheap, but values not as

high as those available with electrolytics

RF coupling Ceramic COG:   Small, cheap and low loss Ceramic X7R:   Small and cheap but higher loss than

COG, although high capacitance per volume Polystyrene:   Very low loss, but larger and more

expensive than ceramic

RF decoupling Ceramic COG:   Small, low loss, but values limited to

around 1000 pF max. Ceramic X7R:   Small, low loss, higher values available

than for COG types

Tuned circuits Silver mica:   Close tolerance, low loss and stable, but

high cost Ceramic COG:   Close tolerance, low loss, although not

as good as silver mica

** Care must be taken to ensure that the ripple current rating of the capacitor meets the requirements of the capacitor application.

This table gives the typical capacitor applications or capacitor uses for areas where particular capacitors be used. However it is necessary to look at the exact requirements for any capacitor application in a circuit, and choose the capacitor according to the needs and specifications available.

Aluminium Electrolytic capacitors- an overview, information or tutorial about the basics of the aluminium electrolytic capacitor: its construction, properties and the uses of the electrolytic capacitor.

Today electrolytic capacitors or as they are more correctly termed, aluminium electrolytic capacitors are used in huge quantities. They are very cost effective and able to provide a larger capacitance per volume than other types of capacitor. This gives them very many uses in circuits where high currents or low frequencies are involved. Aluminium electrolytic capacitors are typically used most in applications such as audio amplifiers of all types (hi-fi to mobile phones) and in power supply circuits.

Like any other capacitor, it is necessary to understand the advantages and limitations of these capacitors to enable them to be used most effectively.

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Electrolytic capacitor development

The electrolytic capacitor has been in use for many years. Its history can be traced back to the very early days or radio around the time when the first broadcasts of entertainment were being made. At the time, valve wireless sets were very expensive, and they had to run from batteries. However with the development of the indirectly heated valve or vacuum tube it became possible to use AC mains power. While it was fine for the heaters to run from an AC supply, the anode supply needed to be rectified and smoothed to prevent mains hum appearing on the audio. In order to be able to use a capacitor that was not too large Julius Lilienfield who was heavily involved in developing wireless sets for domestic use was able to develop the electrolytic capacitor, allowing a component with sufficiently high capacitance but reasonable size to be used in the wireless sets of the day.

Construction of electrolytic capacitors

The plates of an electrolytic capacitor are constructed from conducting aluminium foil. As a result they can be made very thin and they are also flexible so that they can be packaged easily at the end of the production process. The two plates, or foils are slightly different. One is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte is placed between them. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode.

In order to package them the two aluminium foils with the electrolyte soaked paper are rolled together to form a cylinder, and they are placed into an aluminium can. In this way the electrolytic capacitor is compact while being robust as a result of the protection afforded by the can.

There are two geometries that are used for the connection leads or tags. One is to use axial leads, one coming from each circular face of the cylinder. The other alternative is to use two radial leads or tags, both of which come from the same face of the cylinder.

The lead styles give rise to the descriptions used for the overall capacitors. Descriptions of axial and radial will be seen in the component references.

Electrolytic capacitor properties

There are a number of parameters of importance beyond the basic capacitance and capacitive reactance when using electrolytic capacitors. When designing circuits using electrolytic capacitors it is necessary to take these additional parameters into consideration for some designs, and to be aware of them when using electrolytic capacitors.

1. ESR Equivalent series resistance:   Electrolytic capacitors are often used in circuits where current levels are relatively high. Also under some circumstances and current sourced from them needs to have a low source impedance, for example when the capacitor is being used in a power supply circuit as a reservoir capacitor. Under these conditions it is necessary to consult the manufacturers datasheets to discover whether the electrolytic capacitor chosen will meet the requirements for the circuit. If the ESR is high, then it will not be able to deliver the required amount of current in the circuit, without a voltage drop resulting from the ESR which will be seen as a source resistance

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2. Frequency response:   One of the problems with electrolytic capacitors is that they have a limited frequency response. It is found that their ESR rises with frequency and this generally limits their use to frequencies below about 100 kHz. This is particularly true for large capacitors, and even the smaller electrolytic capacitors should not be relied upon at high frequencies. To gain exact details it is necessary to consult the manufacturers data for a given part.

3. Leakage:   Although electrolytic capacitors have much higher levels of capacitance for a given volume than most other capacitor technologies, they can also have a higher level of leakage. This is not a problem for most applications, such as when they are used in power supplies. However under some circumstances they are not suitable. For example they should not be used around the input circuitry of an operational amplifier. Here even a small amount of leakage can cause problems because of the high input impedance levels of the op-amp. It is also worth noting that the levels of leakage are considerably higher in the reverse direction.

4. Ripple current:   When using electrolytic capacitors in high current applications such as the reservoir capacitor of a power supply, it is necessary to consider the ripple current it is likely to experience. Capacitors have a maximum ripple current they can supply. Above this they can become too hot which will reduce their life. In extreme cases it can cause the capacitor to fail. Accordingly it is necessary to calculate the expected ripple current and check that it is within the manufacturers maximum ratings.

5. Tolerance:   Electrolytic capacitors have a very wide tolerance. Typically this may be -50% + 100%. This is not normally a problem in applications such as decoupling or power supply smoothing, etc. However they should not be used in circuits where the exact value is of importance.

Polarisation

Unlike many other types of capacitor, electrolytic capacitors are polarised and must be connected within a circuit so that they only see a voltage across them in a particular way. The capacitors themselves are marked so that polarity can easily be seen. In addition to this it is common for the can of the capacitor to be connected to the negative terminal.

It is absolutely necessary to ensure that any electrolytic capacitors are connected within a circuit with the correct polarity. A reverse bias voltage will cause the centre oxide layer forming the dielectric to be destroyed as a result of electrochemical reduction. If this occurs a short circuit will appear and excessive current can cause the capacitor to become very hot. If this occurs the component may leak the electrolyte, but under some circumstances they can explode. As this is not uncommon, it is very wise to take precautions and ensure the capacitor is fitted correctly, especially in applications where high current capability exists.

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Electrolytic capacitors rating and anticipated life

Great care should be taken not to exceed the rated working voltage of an electrolytic capacitor. Normally they should be operated well below their stated working value. Also in power supply applications significant amounts of current may be drawn from them. Accordingly electrolytic capacitors intended for these applications have a ripple current rating which should also not be exceeded. If it is, then the electronic component may become excessively hot and fail. It is also worth noting that these components have a limited life. It can be as little as 1000 hours at the maximum rating. This may be considerably extended if the component is run well below its maximum rating.

Electrolytic SMD capacitors

Electrolytic capacitors are now being used increasingly in SMD designs. Their very high levels of capacitance combined with their low cost make them particularly useful in many areas. Originally they were not used in particularly high quantities because they were not able to withstand some of the soldering processes. Now improved capacitor design along with the use of reflow techniques instead of wave soldering enables electrolytic capacitors to be used more widely in surface mount format.

Often SMD electrolytic capacitors are marked with the value and working voltage. There are two basic methods used. One is to include their value in microfarads (m F), and another is to use a code. Using the first method a marking of 33 6V would indicate a 33 F capacitor with a working voltage of 6 volts. An alternative code system employs a letter followed by three figures. The letter indicates the working voltage as defined in the table below and the three figures indicate the capacitance on picofarads. As with many other marking systems the first two figures give the significant figures and the third, the multiplier. In this case a marking of G106 would indicate a working voltage of 4 volts and a capacitance of 10 times 10^6 picofarads. This works out to be 10 F

Letter Voltagee 2.5G 4J 6.3A 10C 16D 20E 25V 35H 50

Voltage codes for SMD electrolytic capacitors

Reforming aluminium electrolytic capacitors

It may be necessary to re-form electrolytic capacitors that have not been sued for six months or more. The electrolytic action tends to remove the oxide layer from the anode and this needs to be re-formed. Under these circumstances it is not wise to apply the full voltage as the leakage current will be high and may lead to large amounts of heat being dissipated in the capacitor which can in some instances bring about its destruction.

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To reform the capacitor, the normal method is to apply the working voltage for the capacitor through a resistor of around 1.5 k ohms, or possibly less for lower voltage capacitors. (NB ensure that it has sufficient power rating to handle the capacitor in question). This should be applied for an hour or more until the leakage current drops to an acceptable value and the voltage directly on the capacitor reaches the applied value, i.e. virtually no current is flowing through the resistor. This voltage should then be continued to be applied for a further hour. The capacitor can then be slowly discharged through a suitable resistor so that the retained charge does not cause damage

Ceramic capacitors- an overview, information or tutorial about the basics of the ceramic capacitor: its construction, technical information, properties and the uses of the ceramic capacitor.

Ceramic capacitors are one of the most widely used forms of capacitor used in electronics equipment these days. Ceramic capacitors have also been used for many years, being found in valve or tube circuits dating from the 1930s.

Today ceramic capacitors area available in a variety of formats ranging from leaded components to surface mount technology, SMT varieties. As leaded versions disc ceramic capacitors are widely available, and as SMT devices, ceramic capacitors are available in all the common formats. As such these ceramic capacitors are used in virtually every type of electronics equipment.

The actual performance of the ceramic capacitors is highly dependent upon the dielectric used. Using modern dielectrics, very high values are available, but it is also necessary to check parameters such as the temperature coefficient and tolerance. Different levels of performance are often governed by the dielectric used, and therefore it is necessary to choose the type of dielectric in the ceramic capacitor.

Ceramic capacitors range in value from figures as low as a few picofarads to around 0.1 microfarads. In view of the wide range and suitability for RF applications they are used for coupling and decoupling applications in particular. Here they are by far the most commonly used type being cheap and reliable and the loss factor is particularly low although this is dependent on the exact dielectric in use.

Ceramic capacitor basics

Ceramic capacitors are the workhorses of the capacitor world these days. Ceramic capacitors are used in millions as a result of a combination of their cost and performance. There is a wide variety of dielectrics that can be used as described below, but as the name of the ceramic capacitor suggests, they are all ceramic in nature.

In order to ensure that sufficient levels of capacitance can be obtained within a single capacitor package, ceramic capacitors, like types of capacitor have multiple layers. This increases the level of capacitance to enable the required values of capacitance to be achieved.

Ceramic capacitors are available now in three main types although other styles are available:

leaded disc ceramic capacitors for through hole mounting which are resin coated multilayer surface mount chip ceramic capacitors

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Specialist microwave bare leadless disc ceramic capacitors that are designed to sit in a slot in the PCB and are soldered in place

Although it is possible to obtain other types of ceramic capacitor, these are the main types that can be found today. Of these the surface mount variety is used in the greatest quantities by far because of the manufacturing methods used these days for electronic equipment.

Ceramic dielectrics

Ceramic capacitors have a variety of different ceramic dielectrics as the basis of the capacitor. Ceramic dielectrics are made from a variety of forms of ceramic dielectric. The exact formulas of the different ceramics used in ceramic capacitors vary from one manufacturer to another but common compounds include titanium dioxide, strontium titanate, and barium titanate.

In view of the wide variation of ceramics used in capacitors the EIA (Electronic Industries Alliance) classifies ceramics into groups. In general the lower the group or class the better the overall characteristics, but this is usually at the expense of size. Types within each class define the working temperature range, temperature drift, tolerance, etc.

1. Class 1:   Class 1 ceramic capacitors are the most stable forms of ceramic capacitor with respect to temperature. They have an almost linear characteristic and their properties are almost independent of frequency within normal bounds.

The common compounds used as the dielectrics are magnesium titanate for a positive temperature coefficient, or calcium titanate for capacitors with a negative temperature coefficient. Using combinations of these and other compounds it is possible to obtain a dielectric constant of between 5 and 150. Also temperature coefficients of between +40 and -5000 ppm/C may be obtained.

Class 1 capacitors also offer the best performance with respect to dissipation factor. This can be important in many applications. A typical figure may be 0.15%. It is also possible to obtain very high accuracy (~1%) class 1 capacitors rather than the more usual 5% or 10% tolerance versions. The highest accuracy class 1 capacitors are designated C0G or NP0.

2. Class 2:   Class 2 capacitors offer better performance with respect to volumetric efficiency, but this is at the cost of lower accuracy and stability. As a result they are normally used for decoupling, coupling and bypass applications where accuracy is not of prime importance. A typical class 2 capacitor may change capacitance by 15% or so over a -50C to +85C temperature range and it may have a dissipation factor of 2.5%. It will have average to poor accuracy (from 10% down to +20/-80%). Howeer for many applications these figures would not present a problem.

3. Class 3:   Class 3 ceramic capacitors offer a still high volumetric efficiency, but again this is at the expense of poor accuracy and stability and a low dissipation factor. They are also not normally able to withstand high voltages. The dielectric used is often barium titanate that has a dielectric constant of up to about 1250.A typical class 3 capacitor will change its capacitance by -22% to +50% over a temperature range of +10C to +55C. It may also have a dissipation factor of around 3 to 5%. It will have a fairly poor accuracy (commonly, 20%, or -20/+80%). As a result, class 3 ceramic capacitors are typically used as decoupling or in other power supply applications where accuracy is not an issue. However they must not be used in

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applications where spikes are present as these may damage the capacitor if they exceed the rated voltage.

EIA temperature coefficient codes

In order that the performance of ceramic capacitors can be standardised and easily defined, a set of codes has been defined by the EIA (Electrical Industries Association). These codes enable ceramic capacitor performance to be defined in an easily managed way. The codes are different, though for class 1 and class 2 ceramic capacitors.

Class 1 capacitor codes:Less common is the EIA code for temperature compensated capacitors. This comprises a three character code:

1. The first character is a letter which gives the significant figure of the change in capacitance over temperature in ppm/C

2. The second character is numeric and gives the multiplier

3. The third character is a letter and gives the maximum error in ppm/C

The table below details what each of the EAI codes means.

First character(letter)

significant figures

Second character(digit)

Multiplier

Third character(letter)

toleranceC 0.0 0 -1 G +/-30B 0.3 1 -10 H +/-60L 0.8 2 -100 J +/-120A 0.9 3 -1000 K +/-250M 1.0 4 +1 L +/-500P 1.5 6 +10 M +/-1000R 2.2 7 +100 N +/-2500S 3.3 8 +1000T 4.7V 5.6U 7.5

As an example, one common type of class 1 capacitor is a C0G and this will have 0 drift, with an error of ±30PPM/C.

Class 2 capacitor codesIn order to define the class of temperature coefficient of a particular capacitor, a three letter code designated by the EIA is used. For non-temperature-compensating capacitors this EIA code comprises of three characters:

1. The first character is a letter. This gives the low-end operating temperature.

2. The second is numeric and this provides the high-end operating temperature.

3. The third character is a letter which gives capacitance change over that temperature range.

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The table below details what each of the EAI codes means.

First character(letter)

low temperature

Second character(digit)

high temperature

Third character(letter)change

X -55C (-67F) 2 +45C (+113F) D +/-3.3%Y -30C (-22F) 4 +65 (+149F) E +/-4.7%Z +10C (+50F) 5 +85 (+185F) F +/-7.5%

6 +105 (+221F) P +/-10%7 +125 (+257F) R +/-15%

S +/-22%T +22% / -33%U +22% / -56%V +22% / -82%

Two very common examples of class 2 ceramic capacitors are the X7R capacitor which will operate from -55°C to +125°C with a capacitance change of up to ±15%, and the Z5U capacitor which will operate from +10°C to +85°C with a capacitance change of up to +22% to -56%.

SMD / SMT ceramic capacitors

The vast majority of ceramic capacitors that are used today are in the form of surface mount technology devices. Millions of these ceramic capacitors are used every day in every form of mass produced electronics equipment.

SMD / SMT ceramic capacitors are shaped in the form of a rectangular block or cuboid. The capacitor itself consists of the ceramic dielectric in which a number of interleaved precious metal electrodes are contained. This structure gives rise to a high capacitance per unit volume. The inner electrodes are connected to the two terminations, either by silver palladium (AgPd) alloy in the ratio 65 : 35, or silver dipped with a barrier layer of plated nickel and finally covered with a layer of plated tin (NiSn).

Care must be taken when soldering these capacitors. If heat is applied for too long, then the terminations can be damaged. Fortunately modern versions are far more robust than much older capacitors which used to suffer from metalisation if heat was applied for too long. Despite this care should be taken, especially if these components are being soldered manually. Normally production methods using infra-red reflow with carefully controlled heat profiles is to be recommended.

SMT / SMC ceramic capacitors are normally contained within standard package sizes. These have various designations as described in the table below:

Package designation Size(mm)

Size(inches)

1812 4.6 x 3.0 0.18 x 0.121206 3.0 x 1.5 0.12 x 0.060805 2.0 x 1.3 0.08 x 0.05)0603 1.5 x 0.8 0.06 x 0.030402 1.0 x 0.5 0.04 x 0.020201 0.6 x 0.3 0.02 x 0.01

Capacitor package designations

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It can be noted that the package designation is derived from the package size in 0.01 inch increments.

Wired ceramic capacitors

While the majority of ceramic capacitors that are used are in the form of SMT or SMD components, large quantities of wired components are still used. A large proportion of the wired ceramic capacitors are in the form of disc ceramic capacitors. As the name suggests these electronic components are shaped in the form of a disc.

Tantalum capacitors- an overview, information or tutorial about the basics of the tantalum capacitor: its construction, properties and the uses of tantalum capacitors.

Tantalum capacitors are widely used in electronics design these days. Tantalum capacitors offer a form of capacitor that provides a very high capacity density. As a result this form of capacitor has found widespread use in many areas of electronics. In view of its size and the attainable levels of capacitance, these capacitors are widely used in many mass produced items of electronics equipment.

The tantalum capacitor is similar to the electrolytic capacitor, but using tantalum within the construction of the capacitor it is able to offer extremely high levels of capacitance for any given volume. As such tantalum capacitors are widely used in electronics equipment where there is a need for small size and a high level of capacitance. In view of its advantages, the tantalum capacitor is used in large volumes within the electronics manufacturing industry.

Types of tantalum capacitor

While tantalum capacitors are widely used, it is not so well known that there are three types of tantalum capacitor that are available:

Tantalum foil electrolytic capacitor:   The tantalum foil capacitor was introduced around 1950. It was developed to provide a more reliable form of electrolytic capacitor without the shelf life limitations of the aluminium electrolytic capacitor. It was able to be developed as a result of the availability of high purity tantalum foils and wires. Initially plain foil variants were introduced, but this was quickly followed by etched variants.

The purity of the materials used plays a major part in determining the leakage current of this type of tantalum capacitor.

These tantalum capacitors have a higher capacitance density than their aluminium electrolytic counterparts. They can often operate at temperatures up to about 120C and therefore they are often used in equipment used in extreme conditions.

Tantalum capacitors with porous anode and liquid electrolyte:   This form of tantalum capacitor is also known as the wet tantalum capacitor and it was the first form to be introduced. It still offers the best space factor.

A variety of electrolytes can be used within this form of tantalum capacitor. Those using sulphuric acid as the electrolyte have excellent electrical characteristics and the maximum

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working voltages that are manufactured tend to be a maximum of about 70 volts.

Basically this type of capacitor consists of a sintered porous anode of tantalum power. This is housed in a silver or silver plated container. The porous anode is made by pressing high purity tantalum power into a cylindrical body and then sintering in a vacuum at about 2000C.

These wet tantalum capacitors are very much more expensive than their newer brothers and as a result they are not as widely used.

Tantalum capacitors with porous anode and solid electrolyte:   This variant of the tantalum capacitor family is also known as the solid tantalum, and it is the variety that is most commonly used. Many millions of them are sued each day, and they can be found in many items of consumer and commercial electronic equipment.

The capacitor was developed by the Bell Telephone Laboratories by using a porous anode and then replacing the liquid electrolyte with a solid semiconductor. This overcome the problem od requiring a vent that is common to all other forms of electrolytic capacitor.

These capacitors are superior to electrolytic capacitors in many ways exhibiting excellent temperature and frequency characteristics. They are also smaller than their aluminium electrolytic counterparts. However they are not able to handle high levels of current or voltage spikes. They are also damaged almost instantaneously by reverse polarity - usually exploding quite nicely.

Leaded tantalum capacitors

The most common form of leaded tantalum capacitors in use today are the "solid" tantalum capacitors. They offer particularly small package sizes and as a result they have been widely used in many areas of electronics.

Leaded tantalum capacitors (solid tantalum variety) are generally small and encapsulated in epoxy to prevent damage. The capacitor marking may be written directly onto the encapsulation as figures, although many used a colour coding system.

Warning: - In view of the nature of these capacitors, great care should be taken not to stress these capacitors. The polarity should not be reversed, nor should they be exposed to over-voltage

conditions - even spikes. If they are exposed to these conditions then they may fail, sometimes exploding.

Tantalum SMD capacitors

Tantalum SMD capacitors are widely used to provide levels of capacitance that are higher than those that can be achieved when using ceramic capacitors. The capacitor technology that is used within SMD tantalum capacitors is based on the solid tantalum capacitor technology. This is robust and enables very small capacitors to be made.

For many years tantalum capacitors were used in SMD applications because electrolytic capacitors were not able to survive the high temperatures of the soldering process. Now that electrolytic capacitor technology has been developed to withstand the soldering process, these capacitors are now also widely used. Despite this, the other advantages of tantalum capacitors are employed in many circuits, and they are still used in vast quantities.

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As a result of the different construction and requirements for tantalum SMT capacitors, there are some different packages that are used for them. These conform to EIA specifications.

Package designation Size(mm)

EIA designation

Size A 3.2 x 1.6 x 1.6 EIA 3216-18Size B 3.5 x 2.8 x 1.9 EIA 3528-21Size C 6.0 x 3.2 x 2.2 EIA 6032-28Size D 7.3 x 4.3 x 2.4 EIA 7343-31Size D 7.3 x 4.3 x 4.1 EIA 7343-43

Tantalum capacitor advantages and disadvantages

tantalum capacitors offer many advantages over other types of capacitor. This has meant that their use has risen considerably over the years, and now they are widely used in all forms of electronics equipment. The advantages of tantalum capacitors can be summarised:

Volumetric efficiency:   Tantalum capacitors offer a very high level of volumetric efficiency - much greater than many other types. In particular they are better than electrolytic capacitors which are their main rival.

Good frequency characteristics:   The frequency response of tantalum capacitors is superior to that of electrolytic capacitors. This means that they are more suitable for use in a number of applications where electrolytics could not be used.

High reliability:   Tantalum capacitors are more reliable than many other forms of capacitor. Provided they are operated within their ratings they are able to provide an almost unlimited life. Their use is not time limited as in the case of electrolytic capacitors.

Wide operating temperature range:   Tantalum capacitors are able to operate over a very wide temperature range. They are often specified for operating over the range -55C to +125C. This makes them an ideal choice for use in equipment for use in harsh environmental conditions.

Compatibility with modern production methods:   Modern production techniques often expose components to high temperatures during soldering as the whole assembly is heated by infra-red heat. Using conventional leaded components only the board surface was heated and the amount of heat conducted by the leads was usually insufficient to damage the components. Tantalum capacitors are able to withstand the temperatures of SMT production and are there fore ideal for use in many new electronics designs.

Tantalum capacitors have a number of disadvantages, and these need to be considered when using them in new designs.

Low ripple current ratings:   It is hardly surprising in view of their size, that tantalum capacitors do not have a high ripple current rating. They should not normally be used in areas that require any levels of current to be passed.

Not tolerant to reverse or excess voltage:   Tantalum capacitors do not like reverse or excess voltage. Even spikes can destroy them. If they are exposed to excess or reverse voltages then they can explode.

More expensive than other types:   Tantalum capacitors are more expensive than many other forms of capacitor. As a result their cost should be considered during the design phase as the other benefits may outweigh any increased costs.

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Polycarbonate capacitors- an overview, tutorial about the basics of the polycarbonate capacitor or polycarbonate film capacitor: its construction, properties and general data and information.

The polycarbonate capacitor has been available for many years. The polycarbonate dielectric material is very stable having a high tolerance and can operate over a temperature of range of typically -55°C to +125°C without de-rating. Additionally the insulation resistance and dissipation factor are good and the dielectric constant means that polycarbonate capacitors are a reasonable size for their capacitance.

While polycarbonate capacitors have been widely used within many electronics circuits and found favour with many electronics design centres, they are not as widely used these days. The Bayer Corporation which manufactures the majority of polycarbonate announced in 2000 that it was to discontinue production of the dielectric film used in these capacitors. Although many saw this as the end of polycarbonate capacitors, there are still some smaller sources of the dielectric material and some capacitors are still made. However many are cautious about using polycarbonate capacitors in new electronics designs as there are fewer suppliers, and relying on a single source for the long term supply of an electronics component is not wise.

Polycarbonate dielectric

Polycarbonates are a group of thermoplastic polymers which find uses in many areas of industry as they are easily moulded and thermoformed. They also posses a number of useful features in that they are temperature resistant impact resistant (virtually bullet-roof). They can also be used for vandal-proof glazing.

Polycarbonate is also used in capacitors as a dielectric. Polycarbonate is very stable, offering the possibility of high tolerance capacitors that can be used over a wide temperature range, and shows little sign of ageing.

The basic electrical properties of polycarbonate are summarised below:

Parameter ValueDielectric constant 3.2Dissipative factor 0.0007 @ 50Hz

0.001 at 1MHzVolume resistivity 10-17 ohm cmDielectric strength 38 kV / mmWater absorption 0.16%

Polycarbonate capacitor construction

Polycarbonate dielectric capacitors are typically manufactured in an extended foil format. Metallized electrodes are then used to make the connections. This dielectric is made from a solvent casting process and performs best as a metallized construction. Metallized types feature vapour deposited metal electrodes and give significant size savings, a definite plus in precision applications. In addition, they feature self-healing. Self-healing removes a fault or short circuit by vaporizing the electrode in the region of the short and restores the capacitor to useful life, thereby greatly extending the lifetime of the capacitor.

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Polycarbonate capacitor encapsulation

The encapsulation of the polycarbonate capacitor is important, and a variety of different types can be used. Typically the capacitor may be contained within an epoxy moulded encapsulation, but other popular alternatives include a metal enclosure or preformed box assembly.

It is important to choose the encapsulation required for the particular environment in which the capacitor will be used because the polycarbonate dielectric is sensitive to moisture which can be absorbed as seen by the figures given in the electrical properties section. This water absorption in the polycarbonate dielectric will naturally change some of the electrical properties.

Polycarbonate capacitor applications and use

Polycarbonate capacitors have been used in a wide variety of applications because of the superior performance offered. Typically they are used in applications where precision capacitors are needed (less than ±5%). They are generally used in electronics circuits such as filters, as well as for timing and precision coupling applications.

Polycarbonate capacitors can also be used for AC applications. They are sometimes found in switching power supplies. Care must be taken when using them in these applications. Although the dissipation factor is low, the current must be restricted to prevent them from overheating, although they can tolerate temperature better than many other types of capacitor.

Polycarbonate capacitor replacements

With polycarbonate capacitors being less widely available these days since the Bayer Corporation ceased production of polycarbonate in a form suitable for use as a dielectric, a number of alternative types of capacitor have been sought, especially for use in some military applications where capacitors to a given standard need to be used. A variety of types can be used as almost direct replacements:

Polyethylene napthalate (PEN) Polyphenylene sulphide (PPS) Polyimide (PI) Polytetrafluoroethylene (PTFE

Of these polyphenylene sulphide, PPS is being widely used in many areas as an almost direct replacement.

Polyphenylene sulphide, PPS has many of the same characteristics of polycarbonate and can be often be used as a direct replacement. It has gaining a variety of MIL standards and as such it is being used in many high specification applications. PPS has been found to have a superior temperature performance both in terms of the temperature range applicable and the temperature coefficient.

It is found that polyphenylene sulphide, PPS and polycarbonate have the almost the same dielectric constant. This means that the size of equivalent capacitors will be virtually the same, making replacement in existing designs much easier. Unfortunately not all capacitors will be able to be made exactly the same size because PPS and polycarbonate are not available in the same thicknesses.

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Silver Mica Capacitor- an overview or tutorial about the basics of the silver mica capacitor, its construction, properties and the uses of silver mica capacitors particularly in RF circuits.

Silver mica capacitors have been widely used as high performance capacitors over the years. Although silver mica capacitors are not as widely used, these days, nevertheless they are still available and used in a variety of applications where their particular properties are needed.

Silver mica capacitors are able to provide very high levels of accuracy, stability and low loss. As a result silver mica capacitors found many uses in radio frequency applications, particular for oscillator and filter circuits where their stability, accuracy and low loss (leading to high Q) were needed. Although not as widely used these days, they can still be obtained and are used where stability of value is of the utmost importance and where low loss is required.

Two main reason for the decline in the use of silver mica capacitors is their size, resulting from the materials used and their construction. The cost of silver mica capacitors is higher than many other types that can often be used these days.

Silver mica capacitor properties

The reason for the continued use of silver mica capacitors is the fact that they can offer very high levels of performance, better in many areas than any other type of capacitor. However in many applications, other more modern technologies provide levels of performance that meets the needs for that particular requirement.

The particular properties of the silver mica capacitor are summarised below:

High accuracy:   Silver mica capacitors can be obtained with tolerance figures of +/- 1%. This is much better than virtually every other form of capacitor available today.

Temperature co-efficient:   The temperature co-efficient of silver mica capacitors is much better than most other types of capacitor. The temperature coefficient is positive and is normally in the region 35 to 75 ppm / C, with +50 ppm / C being an average value

Value range:   Values for silver mica capacitors are normally in the range between a few picofarads up to two or possibly three thousand picofarads.

Low capacitance variation with voltage :   Silver mica capacitors exhibit very little voltage dependence.

High Q :   Silver mica capacitors have very high levels of Q and conversely small power factors. These are both almost independent of frequency.

Although silver mica capacitors have a high tolerance and low temperature co-efficient they are known to jump in value on occasions.

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Mica dielectric

The mica dielectric obviously forms the basis for silver mica capacitors. Its properties govern the performance of the silver mica capacitor. It was also one of the first dielectric materials to be used for capacitors in the early days or wireless because of its combination of stability and general physical and mechanical attributes.

Although there are several different forms of mica, they all have very similar properties. They are fundamentally very stable both mechanically and chemically, enabling the capacitors manufactured with mica to exhibit similar properties. The material has a dielectric constant ranging from around 5 to 7.

It is also found that the crystalline structure of mica has binding forces that are different in different planes. In one plane they are

strong, but weak in the perpendicular plane. This gives it a layered structure and enables it to be spilt along the lines of the weak bond into very thin flat sheets. The sheets used in capacitor manufacture are from less than about 0.025 to 0.1 mm.

Natural mica has to be carefully selected because some samples do contain impurities including, iron, sodium, ferric oxide, and lithium. This introduces some variability into any mica that might be used for capacitor manufacture and therefore it must be carefully inspected and classified. This is one of the reasons why silver mica capacitors are more expensive than other types which have less manual intervention.

Mica is chemically very stable and chemically inert. Mica does not react with oil, water, many acids alkalis, and solvents. As a result of this, ageing does not occur to any major degree, and the variations of water vapour in the atmosphere do not cause undue variations in the overall capacitor performance.

Although more costly than other dielectrics, mica is an ideal form of dielectric for very high performance capacitors such as silver mica capacitors. A summary of the properties of mica are given below:

Parameter ValueDielectric constant 6Dielectric strength 10 000 volts per mil

Construction

For silver mica capacitors the silver electrodes are now plated directly on to the mica dielectric, although originally thin sheets of silver foil were placed between the mica dielectric. Again several layers are used to achieve the required capacitance. Wires for the connections are added and then the whole silver mica capacitor assembly is encapsulated to provide protection.

Today a ceramic encapsulation is used, although early versions, used in some valve or vacuum tube radios can be seen to have a form of wax encapsulation. This was effective for the day in protecting the capacitor from moisture, but when warmed, the wax melted, and often these capacitors had little wax on them from the warm environment of a vacuum tube or valve radio

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Glass capacitors- an overview or tutorial about the basics of the glass capacitor, its construction, properties and the uses of glass dielectric capacitors particularly in RF circuits.

Glass capacitors are used where the ultimate performance is required for RF circuits. Glass dielectric capacitors offer very high levels of performance, although their cost is high when compared to many other forms of capacitor. Typically a glass capacitor will have a relatively low capacitance value. The values of glass capacitors may range between a fraction of a picofarad up to two to here thousand picofarads. As such these capacitors are used mainly in radio frequency circuit design.

While the performance of glass capacitors is exceedingly high, this is also usually reflected in the cost - it can run into many pounds or dollars for each component. As such glass dielectric capacitors are reserved only for the most exacting RF requirements, often on low volume products where cost is not such an issues as it is in high volume products. The supply of glass capacitors is also limited to a small number of manufacturers and suppliers, and the capacitors may not be available ex-stock.

Glass capacitor advantages and characteristics

Glass capacitors offer several advantages over types of capacitor. In particular glass capacitors are applicable for very high performance RF applications:

Low temperature coefficient:   Glass capacitors have a low temperature coefficient. Figures of just over 100 ppm / C are often obtained for these capacitors.

No hysteresis:   Some forms of capacitor exhibit hysteresis in their temperature characteristic. This is not the case for glass capacitors which follow the same temperature / capacitance when the temperature is rising and falling.

Zero ageing rate:   Many electronics components change their value with age as chemical reactions take place within the component. Glass capacitors do not exhibit this effect and retain their original value over long periods of time.

No piezo-electric noise :   Some capacitors exhibit the piezo-electric effect to a small degree. This can result in effects such as microphony on oscillators. Where this could be a problem, the use of glass capacitors could help solve the problem.

Extremely low loss / High Q:   Glass capacitors are very low loss as there is virtually no dielectric loss. This enables very high Q circuits to be built using them. provided the other components (e.g. inductors) are not lossy.

Large RF current capability:   Some capacitors are not able to withstand large values of current. This is not the case for glass capacitors which are suitable for use in RF high power amplifiers, etc.

High operating temperature capability :   Glass dielectric capacitors are able to operate at very high temperatures. Many are able to operate at temperatures up to about 200C without fear of damage or performance shortfall.

Glass capacitor construction

The construction of glass dielectric capacitors is relatively straightforward to understand. The capacitor consists of three basic elements: the glass dielectric, aluminium electrodes and the encapsulation. However the assembly of the glass capacitors is undertaken in a manner that ensures the required performance is obtained.

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As the capacitance between two plates is not always sufficient to provide the required level of performance, the majority of capacitors use a multiplayer construction to provide several layers of plates with interspersed dielectric to give the required capacitance.

Although the glass plates are always flat, and tubular forms of construction are not applicable, the glass capacitors are usually available with leads emanating in either a radial or axial form. Essentially the leads either exit the encapsulation at the side or the end.

Glass capacitor applications

Glass capacitors can find applications in many areas as a result of their performance characteristics. They do tend to be specialist components and are normally fairly costly.

Circuits exposed to temperature extremes:   With the tolerance to a wide range of temperatures, both high and low, some circuits that may be exposed to very harsh environmental conditions may choose to use glass capacitors. Not only can they withstand high and low temperatures, but they do not change value at these extremes by a great amount. Accordingly remote sensors may choose to use glass capacitors.

Applications requiring a high Q circuit:   Many circuits including oscillators and filters may require high Q components to give the required performance. Filters will be able to attain their required bandwidth, and for oscillators there are a number advantages including improvement of phase noise performance, reduction in drift and reduction of spurious oscillations.

Low microphony requirements:   It may be expedient to use glass capacitors in circuits where microphony may be a problem. RF oscillators including those found in phase locked loops and PLL synthesizers may benefit from their use.

High power amplifiers:   The high current capability of glass capacitors may enable their use in RF power amplifiers where other forms of capacitor would not be suitable.

High tolerance areas:   In many areas such as filters or free running oscillators the high tolerance and precision accompanied by the low temperature coefficient may be required to maintain the tolerances within a precision circuit.

Polystyrene capacitor- summary and notes on the polystyrene capacitor or polystyrene capacitor detailing its properties, advantages and disadvantages.

Polystyrene capacitors are used within a limited number of applications. Polystyrene capacitor construction does not lend itself to surface mount technology and accordingly polystyrene capacitors tend to be used for leaded applications.

As a result of their construction and limited use, polystyrene capacitors are not widely sued these days and can be difficult to source.

Polystyrene capacitor properties

Polystyrene capacitors are not widely available these days, however they still find applications within audio circles. This is as a result of their electrical characteristics which lend them to these applications.

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The polystyrene capacitors provide a number of electrical characteristics which make them suitable for a number of applications. These capacitors provide high insulation, low leakage, low dielectric absorption, low distortion and excellent temperature stability.

In view of their properties, polystyrene capacitors can often be used in place of silver mica or ceramic disc capacitors.

Polystyrene capacitor advantages and disadvantages

While the polystyrene capacitor has many advantages it also has a number of disadvantages as well.

Polystyrene capacitor advantages Polystyrene capacitor disadvantages

High insulation Low leakage Low dielectric absorption Low distortion (audio enthusiasts

like them because of this) Good temperature stability

Technology does not lend itself to SMT

Not heat resistance - polystyrene melts

Very limited availability

Polystyrene dielectric properties

The dielectric used within polystyrene capacitors has a number of properties which enable them to have excellent properties for many applications.

Property DetailsDielectric constant 2.5 - 2.6Dielectric strength 19.7 MV/mLoss tangent 0.0001 @ 100 MHz

0.00033 @ 3 GHz

Capacitor numeric marking codesCapacitor marking or code systems are often used to indicate the value and other parameters on a capacitor. Large capacitors are able to have their values marked on the case, but on smaller ones there is insufficient space to give the data and capacitor code systems are required. Some use a colour code, but increasingly capacitors used a figure code is used on the smaller capacitors.

On a large electrolytic capacitor there is sufficient space to mark the value, the tolerance, working voltage, and often other data such as the ripple voltage. Smaller capacitors may only have room for a few figures printed as a code for the value.

On one scheme there may just be two figures. These refer to the capacitance in picofarads, i.e. 10 is 10 pF.

Another scheme uses three or four characters. It bears many similarities to the colour code system adopted for resistors, but without the colour part of the coding scheme. The first two figures refer to the significant figures, whereas the third one acts as a multiplier. The value of the capacitor being denoted in picofarads.

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Third Figure Multiplier0 11 102 1003 10004 10 0005 100 0006 --7 --89

Multiplier used on Capacitor Marking Code

SMT / SMD Capacitor- an overview of the Surface Mount Device, SMD capacitor, its performance, construction, mechanical details and other useful information.

SMD capacitors are used in vast quantities. After SMD resistors they are the most widely used type of component. There are many different types of SMD capacitor ranging from ceramic types, through tantalum varieties to electrolytics and more. Of these, the ceramic SMD capacitors are the most widely used.

Ceramic SMD capacitors

The ceramic SMD capacitors form the majority of SMD capacitors that are used and manufactured. They are normally contained in the same type of packages used for resistors.

1812 - 4.6 mm x 3.0 mm (0.18" x 0.12") 1206 - 3.0 mm x 1.5 mm (0.12" x 0.06") 0805 - 2.0 mm x 1.3 mm (0.08" x 0.05") 0603 - 1.5 mm x 0.8 mm (0.06" x 0.03") 0402 - 1.0 mm x 0.5 mm (0.04" x 0.02") 0201 - 0.6 mm x 0.3 mm (0.02" x 0.01")

Construction:   The SMD capacitor consists of a rectangular block of ceramic dielectric in which a number of interleaved precious metal electrodes are contained. This structure gives rise to a high capacitance per unit volume. The inner electrodes are connected to the two terminations, either by silver palladium (AgPd) alloy in the ratio 65 : 35, or silver dipped with a barrier layer of plated nickel and finally covered with a layer of plated tin (NiSn).

Ceramic capacitor manufacture:   The raw materials for the dielectric are finely milled and carefully mixed. Then they are heated to temperatures between 1100 and 1300�C to achieve the required chemical composition. The resultant mass is reground and additional materials added to provide the required electric properties.

The next stage in the process is to mix the finely ground material with a solvent and binding additive. This enables thin sheets to be made by casting or rolling.

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For multilayer capacitors electrode material is printed on the sheets and after stacking and pressing of the sheets co-fired with the ceramic compact at temperatures between 1000 and 1400�C. The totally enclosed electrodes of a multilayer capacitor guarantee good life test behaviour as well.

Tantalum SMD capacitors

Tantalum SMD capacitors are widely used to provide levels of capacitance that are higher than those that can be achieved when using ceramic capacitors. As a result of the different construction and requirements for tantalum SMT capacitors, there are some different packages that are used for them. These conform to EIA specifications.

Size A 3.2 mm x 1.6 mm x 1.6 mm (EIA 3216-18) Size B 3.5 mm x 2.8 mm x 1.9 mm (EIA 3528-21) Size C 6.0 mm x 3.2 mm x 2.2 mm (EIA 6032-28) Size D 7.3 mm x 4.3 mm x 2.4 mm (EIA 7343-31) Size E 7.3 mm x 4.3 mm x 4.1 mm (EIA 7343-43)

Electrolytic SMD capacitors

Electrolytic capacitors are now being used increasingly in SMD designs. Their very high levels of capacitance combined with their low cost make them particularly useful in many areas.

Often SMD electrolytic capacitors are marked with the value and working voltage. There are two basic methods used. One is to include their value in microfarads (m F), and another is to use a code. Using the first method a marking of 33 6V would indicate a 33 F capacitor with a working voltage of 6 volts. An alternative code system employs a letter followed by three figures. The letter indicates the working voltage as defined in the table below and the three figures indicate the capacitance on picofarads. As with many other marking systems the first two figures give the significant figures and the third, the multiplier. In this case a marking of G106 would indicate a working voltage of 4 volts and a capacitance 0f 10 times 10^6 picofarads. This works out to be 10 F

Letter Voltagee 2.5G 4J 6.3A 10C 16D 20E 25V 35H 50

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Inductor Types- Summary of the different types of inductor with descriptions of their attributes and applications.

Inductors tutorial includes:

    •  Inductor types    •  Inductor parameters & specifications    •  Inductor ferrites    •  Ferrite bead inductors

Inductors of many types and styles are used throughout the electronics industry.

Inductors perform a number of different styles of function within a circuit. Some types can be used for filtering and removing spikes on power lines, others are used within high performance filters. Others may be used within oscillators, and there are many other areas where inductors can be used.

As a result of this, there are many different types of inductor that can be obtained. Size, frequency, current, value, and many other factors means that there is a whole host of different types and forms of inductor.

Inductor basics

Although there are many different types of inductor, they all comply with the same basic laws of nature. Each inductor sets up a magnetic field around the conductor and also has a certain reactance.

The basic parameters are used within the inductor, whatever type it is.

Note on Inductance:

Inductance is one of the basic factors that affect electrical circuits. Any wire or coil has a certain inductance associated with it which is caused by the magnetic field that is set up when the current flows. Energy is stored in the field, and the action of the coil is to exhibit a resistance to change of the current flow within the conductor or coil.

Click on the link for further information about Inductance

Inductor cores

Inductors are normally made in the form of a coil. The reason for this is that the magnetic field is linked between the windings and builds up. As such an inductor with a sufficiently large inductance can be built up more easily.

As the permeability of the medium in which the coil is located has a major effect on the inductance, a core running down the centre of the coil is often used.

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Cores such as iron, ferrite and other magnetic materials are used. These all significantly increase the level of inductance that can be obtained, but care has to be taken in the choice of core to ensure its performance is suitable for the power level, frequency and general application of the inductor.

Different inductor core types

Like other types of component such as the capacitor, there are very many different types of inductor. However it can be a little more difficult to exactly define the different types of inductor because the variety of inductor applications is so wide.

Although it is possible to define an inductor by its core material, this is not the only way in which they can be categorised. However for the basic definitions, this approach is used.

Air cored inductor:   This type of inductor is normally used for RF applications where the level of inductance required is smaller. The fact that no core is used has several advantages: there is no loss within the core as air is lossless, and this results in a high level of Q, assuming the inductor or coil resistance is low. Against this the number of turns on the coil is larger to gain the same level of inductance and this may result in a physical increase in size.

Iron cored inductor:   Iron cores are normally used for high power and high inductance types of inductor. Some audio coils or chokes may use iron laminate. They are generally not widely used.

Ferrite cored inductor :   Ferrite is one of the most widely used cores for a variety of types of inductor. Ferrite is a metal oxide ceramic based around a mixture of Ferric Oxide Fe2O3 and either manganese-zinc or nickel-zinc oxides which are extruded or pressed into the required shape.

Iron power inductor:   Another core that can be used in a variety of types of inductor is iron oxide. Like ferrite, this provides a considerable increase in the permeability, thereby enabling much higher inductance coils or inductors to be manufactured in a small space.

Different mechanical inductor types and applications

Inductors may also be categorised in terms of their mechanical construction. There are a number of different standard types by which inductors may be categorised:

Bobbin based inductor:   This type of inductor is would on a cylindrical bobbin. They may be designed for printed circuit board mounting, even surface mount of they may be much larger and mounted via some other mechanical means. Some older versions of these inductors may even be in a similar format to normal leaded resistors.

Toroidal inductor:   This form of inductor is wound on a toroid - a circular former. Ferrite is often used as the former as this increases the permeability of the core. The advantage of a toroid is that the toroid enables the magnetic flux to travel in a circle around the toroid and as a result the flux leakage is very low. The disadvantage with a toroidal inductor is that it requires a special winding machine is required to perform the manufacture as the wire has to be passed thought the toroid for each turn required.

Multilayer ceramic inductor:   This type of inductor is widely used for surface mount technology. The inductor is manufactured within a ferrite or more commonly a magnetic ceramic material. The coil is contained within the body of the ceramic and is presented to the external circuit on end caps in the same way as chip capacitors, etc.

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Film inductor:   This form of inductor uses a film of conductor on a base material. The film is then etched or shaped to give the required conductor profile.

As it can be seen, there are a number of ways to classify the different types of inductor. Each has its own advantages, and it is therefore necessary to make a decision about the various options available when choosing an inductor for a particular application. Modern materials and technology has meant that the performance of inductors has increased and many more options are open to the circuit designer whether for RF applications, combatting EMI, or for power applications.

Inductor Parameters & Specifications- important parameters and specifications associated with inductors used in electronic circuits.

Like any electronic component, there are several parameters and specifications that are associated with inductors.

The inductor parameters and specifications enable the component to be satisfactorily described and correctly used within the circuit.

The various parameters that can be used enable the inductor performance to be fully specified so that it can confidently used within the required circuit.

Inductance

The key parameter for any inductor is its inductance. The inductance is the property of the inductor that tends to oppose any change in the current flowing.

The SI unit of inductance is the henry, H. The inductance of a circuit is one henry if the rate of change of current in a circuit is one ampere per second and this results in an electromotive force of one volt.

The actual level of the inductance is influenced by many factors including the number of turns on the coil, coil diameter and in particular the core used within the coil.

As a one henry coil would be very large and only used in very low frequency applications, inductor parameters are more normally specified in terms of microhenries,, µH. Other values may also be used and can be converted according to the table below:

Value Value in terms of microhenries

1 henry 1 000 000 µH1 millihenry 1 000 µH1 nanohenry 0.001µH

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DC Resistance

Another important inductor parameter is the DC resistance it exhibits. As inductors are often manufactured from very thin wire, the DC resistance can sometimes be significant. Even when thicker wire is used, it is still an important factor because it can significantly affect the overall performance as an inductor.

The DC resistance can be considered to be in series with the pure indictor for the sake of most circuit simulations, although in reality it is distributed throughout the inductor.

The DC resistance, measured with a steady current is normally specified in Ohms, Ω and typically given as a maximum value as it is sometimes difficult to control accurately.

Saturation Current

The saturation current is another parameter or specification which is of importance for an inductor.

In an inductor it is possible to saturate the core because there is a limit to the level of magnetic flux a magnetic core such as iron, ferrite or another compound can take. When this occurs the relative permeability falls and in turn this causes the level of inductance falls.

The saturation current is generally taken to be the current at which the level of inductance falls by a specified amount. Figures of 10% are often used for inductors with ferrite cores and 20% for those with iron powdered cores.

Incremental current

Often inductors run with a bias current passing through them. For example, this may be the quiescent current for a transistor collector where the inductor is in the collector circuit itself. There is a drop in inductance that is caused by this current and it is necessary to understand this so that the circuit will be able to operate satisfactorily even when the DC bias current is flowing.

The incremental current inductor parameter is generally taken as the DC bias current flowing through the inductor that causes the inductance to fall by 5% from its initial value with zero bias.

The value for the incremental current parameter or specification indicates the level where a further increase in current would cause the inductance to fall by a significant value.

The incremental current value for an inductor is most important when using ferrite cores as they exhibit a much faster reduction in inductance with increasing current than other forms of core such as a powered iron core.

Rated current

Another important inductor parameter is the rated current. This specification is the maximum continuous current that the inductor can withstand. Generally the limiting factor for this parameter is the temperature rise of the inductor.

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With thin wire being used in many inductors to keep the size to a minimum, the current flow can result in power being dissipated in the inductor with the result that the temperature rises. Undue temperature rises can reduce the reliability or even cause catastrophic failure in some circumstances.

Core permeability

The permeability of the inductor core is a key parameter. It governs the inductance of the inductor for a given inductor geometry. Higher permeability core materials result in the inductor providing a higher level of inductance.

The core material as well as the core shape, size and geometry affect the overall effective permeability, and therefore these factors also need to be taken into consideration as well.

Winding self-capacitance

The inductor self-capacitance or distributed capacitance is a particularly important parameter in many applications. It arises from the fact that apart from adding inductance, the wires also have a small but appreciable level of capacitance between each other.

The diagram shows individual capacitors within the inductor as this a simplified way of showing the self-capacitance. However the capacitance is distributed throughout the whole inductor and it is not separate capacitance.

The level of capacitance depends on the area of the wire, the distance between the two wires and the permittivity of the material between them. Normally the level is relatively low, but it manifests itself to an external circuit as a small amount of capacitance across the inductor. This gives rise to what is termed the self-resonant frequency of the inductor.

Self resonant frequency

In view of the self-capacitance or distributed capacitance, the inductor forms a parallel resonant circuit as shown. At the point where the inductor resonates the inductive reactance and the capacitive reactance will cancel each other out, and the overall impedance of the circuit will fall to a value governed by the DC resistance of the circuit..Below the resonant frequency the inductive reactance will dominate, whereas above the self-resonant frequency the capacitive reactance will dominate.As a result inductors are normally used below their self-resonant frequency to ensure that the effects of self-resonance are not experienced.

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Quartz crystals and the quartz crystal resonator- overview, information or tutorial about the basics of quartz crystals, and they may be used as a quartz crystal resonator in a radio frequency oscillator or filter.

Quartz crystals, crystal technology includes:

    •  Quartz crystals    •  Quartz crystal specifications    •  Quartz crystal ageing    •  VCXO    •  TCXO    •  OCXO    •  Crystal bandpass fitlers    •  Monolithic crystal filter

Quartz crystals are widely used in today's electronics circuits as high quality tuned circuits or resonators.

Despite their high performance quartz crystals are cheap to produce and they find many uses in applications from oscillator clock circuits in microprocessor boards, the timing element in digital watches as well as their more traditional applications in radio frequency applications where they may be used as the resonators in highly stable quartz crysal oscillators of high performance crystal filters.

As the name implies quartz crystal resonators are made from quartz, a naturally occurring form of silicon, although most of that used for electronics applications is manufactured synthetically these days.

Quartz crystal resonators rely on the remarkable properties of quartz for their operation. When placed into an electronic circuit a quartz crystal acts as a tuned circuit. However it has an exceptionally high Q. Ordinary LC tuned circuits may exhibit values of a few hundred if carefully designed and constructed, but quartz crystals exhibit values of up to 100 000. Apart from their Q, crystals also have a number of other advantages. Their stability is remarkably good with respect to temperature and time. In fact most crystals will have these figures specified and they might typically be ±5 ppm (parts per million) per year for the ageing and ±30 ppm over a temperature range of 0 to 60 degrees Celsius.

A crystal of naturally occurring quartz

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How quartz crystal resonators work

A quartz crystal resonator depends on the piezo-electric effect to work. This effect converts a mechanical stress in a crystal to a voltage and vice versa. In this way the piezo-electric effect converts the electrical impulses to mechanical stress which is subject to the very high Q mechanical resonances of the crystal, and this is in turn linked back into the electrical circuit.

The quartz crystal can vibrate in several different ways, and this means that it has several resonances, all on different frequencies. Fortunately the way in which the quartz crystal blank is cut from the original crystal itself can very significantly reduce this. In fact the angle of the faces relative to the original crystal axes determines many of its properties from the way it vibrates to its activity, Q, and its temperature co-efficient. There are three main ways in which a crystal can vibrate: longitudinal mode, low frequency face shear mode, and high frequency shear. A cut known as the AT cut used for most crystals used in traditional radio and electronics circuits uses the high frequency shear mode.

 Vibrational modes of a quartz crystal resonator

(For the sake of clarity, the movements have been greatly exaggerated)

Equivalent circuit of a quartz crystal resonator

To analyse the electrical response of a quartz crystal resonator, it is very often useful to depict it as the equivalent electrical components that would be needed to replace it. This equivalent circuit is can then be used to analyse its response and predict its performance. The basic equivalent circuit of a crystal is shown below. In this circuit C1 represents the capacitance between the electrodes. L, C, and R represent the vibrational characteristics of the crystal. The inductance results from the mass of the material, C from the compliance, and R arises from the losses of which the greatest contributor is frictional losses.

The equivalent circuit of a quartz crystal resonator

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Looking at this circuit it can be seen that there are two ways in which the circuit can resonate. One is from the resonance of L and C which provides a series resonance, giving a very low value of impedance at resonance. This is determined by the value of the resistance R. In this mode the external circuit has very little effect on the crystal resonance.

The other is a parallel resonance providing a very high impedance. This occurs when the combination of L, and C has an inductive reactance that equals C1 together with any value of capacitance provided by the external circuit. It is for this reason that crystals designed to operate in this mode have a value of load capacitance specified. This value of capacitance must be provided by the external circuit if the crystal is to operate at its specified frequency.

Quartz crystal resonators can operate in either mode, and in fact the difference between the parallel and series resonant frequencies is quite small. Typically they are only about 1% apart. Of the two modes, the parallel mode is more commonly used, but either may be found. Oscillator circuits for using the different modes are naturally different, as one oscillates when the crystal reaches its maximum impedance whilst the other operates when the crystal reaches its minimum impedance.

Impedance characteristics of a quartz crystal resonator

Apart from their use in oscillators, quartz crystals find uses in filters. Here they offer levels of performance that cannot be achieved by other forms of filter. Often several crystals may be used in one filter to provide the correct shape.

How quartz crystal resonators are made

The individual quartz crystal resonators are manufactured from large man-made crystals that are generally several inches long. They are around two inches in diameter and have a hexagonal cross section. The individual quartz crystals are cut from the large crystal using diamond wheels. These are required in view of the hardness of the material. The angle of the cut to the axes of the original crystal is determined very accurately to ensure the final crystal has the right properties. The blanks that are created from the cutting process are in the form of discs, often about the size of a small coin, although this varies according to their final frequency of operation. Once the blanks have been cut they are lapped using a very fine paste to bring them to nearly the right size. The lapping paste normally consists of very fine silicon carbide or aluminium oxide. The final stage of preparation usually involves chemical etching, because this process enables the required very fine finish to be obtained.

The next stage in manufacture involves mounting the quartz crystal. Silver or gold contacts are chemically deposited onto both sides of the blank. The amount of metal that is used in this process

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can be used to trim the operating frequency of the crystal to its final value. Finally the crystal is mounted into its can or glass envelope. This is either evacuated or filled with an inert gas to minimise ageing.

Specifying quartz crystal resonators

When choosing a quartz crystal resonator there are many parameters that need to be selected. Many are fairly simple like the tolerance figures. However a few of the others need a little extra explanation. One is the type of resonance. Like any tuned circuit a crystal can have a parallel or series form of resonance as shown. This will have to be specified. If the crystal is to have a parallel resonance then a load capacitance will have to be chosen. This is required because any capacitance across the crystal will alter its resonance slightly. Typically this might be 30 pF, but it will be dependent upon the circuit to be used. Also the tolerance required must be specified. The closer the tolerance, the more expensive the crystal will be, so it is wise not to over-specify the item.

Quartz crystal resonators are widely used within the electronics industry. They can be sued in quartz crystal oscillators and crystal filters where they provide exceptionally high levels of performance. In addition to this, low cost elements with lower tolerance specifications are widely used in crystal oscillators for microprocessor board clocks where they are used as cheap resonator elements. Whatever its use a quartz crystal resonator provides an exceptionally high level of performance for the cost of its production.

Quartz Crystal Resonator Specification- summary, overview or tutorial about the basics of a crystal specification and specifying a quartz crystal resonator for radio and electronics applications.

Quartz crystals used in radio and electronics circuits are precision electronic components and when buying them it is necessary to be able to specify them precisely. There are normally several elements to a crystal specification, many of which are specific to quartz crystals and not widely used elsewhere in radio applications. Also there are a number of elements to a crystal specification that may be set down by the manufacturer for a given range of crystals and when ordering a component it is necessary to be aware of them.

Frequency specification

The frequency of the quartz crystal is obviously a fundamental specification. It is normally expressed to as many significant figures as demanded by the frequency tolerance, although seven figures is normally the maximum. It is wise to express the frequency to the right number of significant figures to avoid misunderstandings in this area of the quartz crystal specification.

Crystal resonator mode

Quartz crystals may either operate in a fundamental mode or in an overtone mode. Below frequencies of around 25 MHz crystals are normally designed to operate in their fundamental mode, whereas above this they will normally be designed for overtone operation, although with manufacturing techniques improving higher frequency crystals are becoming available. The mode is therefore an important element of the crystal specification.

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When ordering an overtone crystal quote the exact frequency of operation and not what is expected to be the fundamental frequency as confusion may arise over the frequency required, and as the overtone frequency of the crystal is not an exactly the same as the harmonic of the fundamental frequency this may result in an incorrect frequency being supplied. The frequency of overtone crystals is normally expressed in MHz, whereas one operating at its fundamental frequency is normally expressed in kHz.

Resonance type

There are two types of resonance that are applicable to quartz crystals. One is parallel resonance and the other is series resonance. The actual type required will depend on the circuit in use. Although crystals will operate in either mode, the frequency of resonance for each type of resonance is slightly different. For some applications such as microprocessor clock generators the small difference between the two frequencies may not be a problem, but for many others it is. Accordingly the crystal specification should clearly include the type of resonance required.

Parallel resonance is the more commonly used type. However when specifying this type a load capacitance is required because the external capacitance forms part of the resonant circuit. One common value of load capacitance is 30pF, although 20pF is also becoming common.

Holder style

Crystals come in a variety of packages. There are a number of standard varieties used with through-hole mounting and sockets. Styles such as HC43, etc are still widely available, but there are also many new packages for use with surface mount soldering. It is necessary to consult the manufacturers datasheets to make the final choice.

Calibration tolerance

This is the final frequency of the crystal at manufacture at a temperature of 25C which is normally assumed to the operating temperature of electronic equipment. However if the crystal is to be used in an oven then the temperature of the oven should be stated instead. The calibration tolerance itself is expressed in ppm (parts per million).

Temperature stability

The temperature stability is another important area of the crystal specification and it is the allowable frequency deviation as the temperature varies. Again normally expressed in ppm, from the frequency at the reference temperature per degree Celsius. Sometimes the crystal specification may use a frequency tolerance consisting of the sum of the calibration and temperature stability tolerances is quoted.

Quartz crystal ageing

Ageing of quartz crystals will depend upon a number of factors and in particular the encapsulation. It is generally greatest in the first few weeks of operation, and as a result crystals to be used in high quality oven oscillators are run in before use. Figures for ageing are expressed in a certain number of ppm over a given time, often a day and/or a year.

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Activity

A measure of the activity of a crystal is the resistive component that is seen in the motional arm of its equivalent circuit. As would be expected the resistance and Q are inversely proportional to each other.

Spurious responses

In some applications the spurious responses may be of importance, but there are some responses that may be within a few hundred kilohertz of the main frequency. These are normally low, and rarely cause problems in oscillator circuits except if the tuned circuit used in the oscillator resonates on the same frequency as a nearby response. They may be more important in filter applications, and it may be necessary to specify maxim response levels relative to the main response. It is likely that in a filter several crystals will be used, and they will not all use the same frequency. This will result in the responses also appearing on different frequencies, making the problem less severe.

When ordering a quartz crystal resonator for any application it is necessary to ensure that the crystal specification is correct and expressed in the right format. As there are many elements to a crystal specification, it is necessary that it is well checked before issuing it to the manufacturer, and in this way the item that is delivered should perform as needed.

Quartz crystal ageingQuartz crystals used in filters and oscillators in electronic circuits are renowned for their performance, stability, frequency tolerance and their high Q. Yet they do change their frequency very slightly with time in a process known as ageing. Although the frequency variations are small by many standards, they are permanent and may have an effect in some applications where the frequency is of great importance. As a result manufacturing techniques take account of this to reduce the effects of ageing in these crystals as far as possible.

Ageing is caused by a number of interrelated factors. These include internal contamination, excessive drive level, surface change of the crystal, various thermal effects, wire fatigue and frictional wear. The level of ageing can be minimised in a number of ways. During manufacture they should be encapsulated in an inert gas environment, the ensuring should have a good seal so that other gases do not enter. Also the final stages of the preparation of the crystal blank must be prepared as finely as possible. Rather than lapping the blank to bring it to the right dimensions, chemical etching is used. In this way the minimum disruption is caused to the crystal lattice, and this reduces the ingress of contaminants over time that will cause ageing.

The design of the circuit in which the crystal will be used also has an effect. By keeping the drive levels low again the crystal ageing will be less.

As expected the rates of change of the crystal frequency vary with the time after manufacture. The maximum rate of change of frequency occurs immediately after manufacture and decays thereafter. As a guide it is found that it is fastest within the first 45 days of operation. Even so there is always some degree of ageing throughout the life of the crystal. In view of the fact that the greatest rate of change is immediately after manufacture, high tolerance items are run for some time before being shipped. In very high tolerance items this may extend to a few months of operation.

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Once the ageing rate has settled it is found that typical figures can be quoted for many types. It is found that one of the main variations is the type of encapsulation that is used. The two most common methods of encapsulation for through-hole crystals are resistance weld and cold weld. These will typically give figures of around 5 parts per million (ppm) for a resistance weld sealed encapsulation, and 2 ppm for a cold weld sealed encapsulation using an HC43/U holder. These both move in a downward direction. Glass encapsulated crystals may also be found on some occasions. These tend to move in an upward direction, and may have a tolerance or slightly less than 5 ppm. Also there is a wide variety of surface mount crystals. A typical plastic package or a glass seam weld package may give around �5 ppm while a metal seam weld package may give less than 3 ppm.

If the crystal is maintained in the same circuit and at the same temperature then the effects of ageing may stabilse after some years of operation. However if these are changed they may cause the ageing rate to change. It may even alter direction.

Cleanliness of the environment around the crystal is one of the main ways of reducing ageing. It is therefore esential to ensure that the crystal package or encapsulation is not damaged in anyway. The seal should not be damaged, nor the pins bent as this may break the seal.

While the effects of ageing may not be of importance in applications such as clock oscillators for running many digital circuits, they are important where high frequency stability is required. By choosing the right crystal, these effects can be kept within reasonable limits so that they do not cause any problems.

VCXO, Voltage Controlled Crystal Oscillator- an overview giving information about the basics of voltage controlled crystal oscillator, VCXO, which is used for generating controllable, or slightly variable but stable frequency reference signals.

Voltage controlled crystal oscillators are used in a number of applications where a stable frequency source is required, but with the additional capability of fine tuning it using an electronic voltage. By using a crystal oscillator as the basis of the circuit, high levels of frequency stability and low levels of phase noise can be maintained while still being able to control the frequency over a small range.

VCXO circuit

The basic circuit for an VCXO comprises a standard crystal oscillator but with an electronic means of tuning or "pulling" the frequency slightly. This is almost invariably achieved using varactor or varicap diodes. In most VCXOs a pair of back to back diodes are placed across the crystal. A reverse bias is applied to anodes of the diodes which then act as a variable capacitor across the crystal. In most cases a Colpitts oscillator circuit is used.

The amount by which the crystal frequency can be pulled depends upon a variety of factors including the level of capacitance applied, the circuit conditions themselves and the crystal. However the frequency cannot be pulled too far, because the activity of the crystal reduces as the level of capacitance across the crystal increases. If it is necessary for the VCXO to be pulled over a large range, then an inductor can be incorporated into the circuit.

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VCXO performance

Using this approach, VCXO figures of frequency variation of around 35 to 50 ppm/volt are reasonably easy to achieve and VCXOs with these figures are quite common.

Naturally the fact that the frequency of the VCXO can be pulled reduces the overall performance of the oscillator circuit. The phase noise performance of the oscillator is degraded because the effective Q of the resonator is considerably reduced. Additionally the frequency stability is not as good.

One of the major problems with VCXOs is that of temperature drift. As this varies over the voltage control range, it cannot be optimised for all levels of control voltage, the final design being a compromise. This when used without other forms of temperature compensation they may drift more than other forms of crystal oscillator.

VCXO applications

VCXOs are used in many applications. They are used in TCXOs where the temperature compensation voltage is applied to a control terminal of the VCXO. In this way the drift can be considerably reduced, although the performance is still not as good as a full oven controlled crystal oscillator.

In another application, VCXOs are often found in narrow band phase locked loops where only a small amount of frequency variation is required.

TCXO, Temperature Compensated Crystal Oscillator- an overview of the TCXO, used for providing a much higher levels of temperature stability than are possible with a normal crystal oscillator

The temperature controlled xtal or crystal oscillator, TCXO, is a form of crystal oscillator used where a high precision frequency source is required within a small space and at reasonable cost. By compensating within the oscillator for temperature changes, it is possible to considerably improve on the basic performance of the oscillator.

Effect of temperature

Although crystal oscillators offer a highly stable form of oscillator, they are nevertheless affected by temperature. The cut of the actual crystal element from the overall grown crystal can help to minimize the effects of temperature, but they are still affected to some degree. For a crystal cut known as the AT cut, the drift with temperature can be minimized around normal ambient temperature, but the rate of drift will rise above and below this.

The effects o temperature are, to a large degree, repeatable and definable. Therefore it is possible to compensate for many of the effects using a temperature compensated crystal oscillator, TCXO.

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TCXO solution

A TCXO adjusts the frequency of the oscillator to compensate for the changes that will occur as a result of temperature changes. To achieve this, the main element within a TCXO is a Voltage Controlled Crystal Oscillator (VCXO). This is connected to a circuit that senses the temperature and applies a small correction voltage to the oscillator.

The temperature sensing and compensating network may take a variety of forms these days. The traditional method was to use a thermistor network. More recently digital techniques such as DSP have been used to enable highly accurate compensation to be achieved.

The problem with applying the temperature compensation in a TCXO is that the temperature coefficient of the crystal changes with temperature, and it is not linear. Accordingly the design of the compensation network is complicated.

Additionally TCXOs normally have an external adjustment to enable the frequency to be reset periodically. This enables the effects of the ageing of the crystal to be removed. The period between calibration adjustments will depend upon the accuracy required, but may typically be six months or a year. Shorter periods may be sued if very high levels of accuracy are required.

TCXO performance

The performance of a TCXO is generally considerably better than that of a normal crystal oscillator. Figures within the range 1 to 5 ppm are often achieved. However it should be mentioned that some of this figure comes from a frequency / temperature hysteresis, dependent upon whether the temperature was increasing or decreasing.

The power dissipation of a TCXO will be greater than an ordinary oscillator in view of the additional circuitry required. Additionally the cost is greater. It should also be remembered that it will take a short while after start up for the oscillator to stabilize. This may be of the order of 100 ms, or possibly longer, dependent upon the design.

TCXO packages

TCXOs can be supplied in a variety of packages dependent upon the way they have been designed and the requirements of the end user. The most common form of construction is to construct the circuit on a small printed circuit board that can be house in a plat metal package. This is then suitable for mounting onto the main circuit board of the overall equipment. As the crystal itself is sealed, this means that sealing of the overall TCXO package is not critical, or even required for most applications.

When TCXOs utilize ASIC technology this often enables much smaller packages to be used. These may even be in surface mount packages that are far more suitable for today's manufacturing techniques

Summary

TCXOs are widely used where accurate frequency sources are needed. They are less expensive and smaller than oven controlled oscillators, as such they offer an ideal solution for many portable units requiring a reasonably accurate source.

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OCXO, Oven Controlled Crystal Oscillator- an overview of the basics of the oven controlled crystal oscillator, OCXO, which is used for generating stable frequency reference signals.

Oven Controlled crystal (Xtal) Oscillators, OCXOs, are used in applications where a very high degree of frequency stability is required. While crystal oscillators show a high degree of stability even when the outside temperature is varied over a significant range, for some applications even higher levels of temperature stability are required. In these applications oven controlled crystal oscillators (OCXO) are often used.

Stability of a crystal

Like any physical item, quartz crystals are subject to slight changes as a result of temperature variations. These changes reflect back into the resonant frequency of the crystal causing slight variations. The degree of variation is highly dependent upon the way the crystal is cut during manufacture. The angles of the plane of the blank with reference to the axes of the original crystal determine many of its properties. These include the mode of vibration, the degree of the piezo-electric effect - i.e. its activity, and of course the temperature stability.

The type of crystal cut most used for general RF applications is known as the AT cut. This provides a crystal with very good all round properties as required for RF applications. For the temperature stability it is found that the change of frequency measured Δf/f in ppm (parts per million) reaches a minimum at around 25�C rising at temperatures above this and typically falling at temperatures below this figure. Some variation is found dependent upon the exact angle the crystal blank is cut with respect to the crystal axes.

OCXO

Despite this it is still sometimes necessary to ensure a better degree of stability. This can be achieved by placing the crystal in a thermally insulated container with a thermostatically controlled heater. By heating the crystal to a temperature above that which would normally be encountered within the electronic equipment the temperature of the crystal can be maintained at a constant temperature. This results in a far greater degree of temperature stability. Additionally the crystal in the OCXO will be cut to ensure that its temperature stability is optimised for the internal operating temperature of the OCXO.

The typical specification for an OCXO might be �5 x 10-8 per degree Celsius (0.05 ppm), whereas a non-oven controlled oscillator may be between 10 and 100 times poorer. As the oscillator assembly will also contain buffering circuitry as well as supply voltage regulation the other characteristics of the oscillator should also be good. Typically it might be expected that frequency stability would be around �5 x 10-9 (0.005 ppm) per day and �5 x 10-7 (0.5 ppm) per year and 1 x 10-7 for a 5% change in supply voltage. All of these are far better than would be expected from a simple crystal oscillator.

In order to ensure that the optimum overall accuracy is maintained, combating elements such as ageing of the crystal itself, a periodic calibration of the OCXO may be required. Typical calibration periods for OCXOs may be of the order of six months to a year, but the actual period will depend upon the OCXO itself and the requirements of the application in which it is being used.

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OCXO physical considerations

OCXOs are physically much larger than a simple crystal oscillator. Not only do they need to incorporate the crystal oscillator itself, but also the heater, control circuitry and the thermal insulation around the crystal oscillator.

Typically the heater will be run from a different supply to the oscillator. It does not need the same level of regulation, and indeed the oscillator is most likely to have its own regulator to remove any stray noise and RF that may appear on the supply line and thereby degrade the performance of the OCXO.

The supply for the heater in the OCXO may be quite current hungry. Some OCXO heaters may require an Amp or so on warm up. This figure will reduce as the temperature inside the OCXO rises and less heat is needed. As will be imagined the temperature of the OCXO is thermostatically controlled.

Summary

These OCXO units are naturally more expensive than crystals on their own, but the performance of an OCXO is considerably enhanced on that of a simple crystal in an unregulated electrical and physical environment.

Quartz crystal filter- summary, overview or tutorial about the basics of the quartz crystal filter describing its operation, use, design and specification.

Quartz crystal filters provide an effective means of realising filter solutions for many high performance radio frequency filter applications. The high Q values that quartz crystals possess can be utilised in bandpass filters for use in areas such as radio receivers. These quartz crystal filters are far superior to those that could be manufactured using LC components. Although they are more costly than LC filters, the performance of a quartz crystal filter is still superior and in terms of cost they actually provide excellent value for money.

Today, quartz crystal filters can be designed with pass bands ranging from frequencies in the kilohertz region up to many Megahertz - with the latest technology this can rise to 100 MHz and more. However for the best performance and lowest costs the passband of the filter is generally kept to below about 30 MHz or so.

Quartz crystal overview

Quartz crystals use the piezo electric effect to convert the incoming electrical impulses into mechanical vibrations. These vibrations are affected by the mechanical resonances of the crystal, and as the piezo electric effect operates in both directions, the mechanical resonances affect the electrical stimuli, being reflected back into the electrical circuit.

The levels of Q that can be achieved using quartz crystals range into figures well over 10 000. Values of 100 000 are widely used in filters and values can sometimes reach 500 000. By utilising this level of performance, quartz crystal filters can achieve very high levels of performance. This can be reflected in the crystal filters very narrow filter bandwidths and sharp cut-off curves.

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Quartz crystal cuts

When manufacturing the quartz crystal blanks used to make the electronic components used in filters, the angle at which these blanks are cut from the unprepared crystal, have a major bearing on the properties. A form of cut known as the AT cut is used for most radio applications. This provides the optimum set of parameters for most radio applications. The size of the crystal blank using this cut is such that it is sufficiently robust to withstand the manufacturing process without a high level of failures and rejects, and to withstand the vibration that is likely to be expected in use. Additionally the level of spurious responses is low. A further advantage is that the temperature stability is high. The final angle of the cut can be adjusted to ensure that the temperature characteristic is optimum for the particular application for which it is intended. Even a difference of 2 minutes of arc can be detected, although the normal manufacturing spread is around 3 minutes of arc.

In addition to this the cut of the quartz crystal governs the way in which it vibrates. As there are several modes in which a crystal can vibrate it is necessary to choose a cut in which unwanted modes are not easy to excite. If they are present then they will be seen as spurious responses in the crystal filter.

Quartz crystal filter parameters

There are two main areas of interest for a filter, the pass band where it accepts signals and allows them through, and the stop band where it rejects them. In an ideal world a filter would have a response something like that shown below. Here it can be seen that there is an immediate transition between the pass band and the stop band. Also in the pass band the filter does not introduce any loss and in the stop band no signal is allowed through.

The response of an ideal filter

In reality it is not possible to realise a filter with these characteristics and a typical response more like that shown in Figure 3. It is fairly obvious from the diagram that there are a number of differences. The first is that there is some loss in the pass band. Secondly the response does not fall away infinitely fast. Thirdly the stop band attenuation is not infinite, even though it is very large. Finally it will be noticed that there is some in band ripple.

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Typical response of a real filter

In most filters the attenuation in the pass band is normally relatively small. For a typical crystal filter figures of 2 - 3 dB are fairly typical. However it is found that very narrow band filters like those used for Morse reception may be higher than this. Fortunately it is quite easy to counteract this loss simply by adding a little extra amplification in the intermediate frequency stages and this factor is not quoted as part of the receiver specification.

It can be seen that the filter response does not fall away infinitely fast, and it is necessary to define the points between which the pass band lies. For receivers the pass band is taken to be the bandwidth between the points where the response has fallen by 6 dB, i.e. where it is 6 dB down or -6 dB.

A stop band is also defined. For most receiver filters this is taken to start at the point where the response has fallen by 60 dB, although the specification for the filter should be checked this as some filters may not be as good. Sometimes a filter may have the stop band defined for a 50 dB attenuation rather than 60 dB.

Shape factor

It can be seen that it is very important for the filter to achieve its final level of rejection as quickly as possible once outside the pass band. In other words the response should fall as quickly as possible. To put a measure on this, a figure known as the shape factor is used. This is simply a ratio of the bandwidths of the pass band and the stop band. Thus a filter with a pass band of 3 kHz at -6dB and a figure of 6 kHz at -60 dB for the stop band would have a shape factor of 2:1. For this figure to have real meaning the two attenuation figures should also be quoted. As a result the full shape factor specification should be 2:1 at 6/60 dB.

Quartz crystal filter design parameters

When a quartz crystal filter is designed factors such as the input and output impedance as well as bandwidth, crystal Q and many other factors need to be taken into account.

Some of the chief factors are obviously the bandwidth, shape fact, and ultimate cutoff. Although it is very much a simplification, these factors are dependent upon the number of poles (equivalent to the number of crystals), their Q value, and their individual frequencies.

Further factors such as the maximum bandwidth that can be achieved is controlled by the filter impedance and also the spurious responses that are present in the individual quartz crystal elements. The location of the important responses for quartz crystal filters can be controlled by

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the size of the plates deposited onto the crystals. By making them smaller the responses also become less critical. The down side of this is that the impedance of the overall quartz crystal filter rises. This means that the quartz crystal filter will need impedance transformers at the input and the output. This obviously needs to be avoided if at all possible, but for wide band filters it is often the only option.

Summary

Quartz crystal filters are widely used in many applications, and particularly for radio applications. Here these quartz crystal filter provide an exceptional level of performance, and bearing this in mind their cost is very reasonable.

Monolithic crystal filter- summary, tutorial or overview of the basics of the monolithic crystal filter describing its operation and giving its equivalent circuit.

Quartz crystal filters are widely used in many areas, and in particular in high performance radio receivers. They are able to offer unparalleled levels of performance at a cost that represents excellent value for the performance.

A development of the basic idea of a crystal filter is the monolithic crystal filter. These monolithic crystal filters are able to offer even higher levels of performance in some respects while costs are reduced slightly.

What is a monolithic crystal filter?

Traditionally a crystal filter is made from a number of discrete crystals with the circuit often based around the half lattice network. However these designs require the use of a number of individual crystals - often six or eight are used to give the required performance.

Rather than having a number of discrete crystals, a monolithic filter uses a single crystal element and two sets of electrodes plated onto the surface. There are two ways in which this can be done. The first is to have two sets of identical electrodes, top and bottom. Alternatively a single electrode can be plated onto one surface acting effectively as a common ground with two electrodes, one for the input and the other for the output at the top.

Circuit diagram of a half lattice crystal filter

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Like most crystals used for radio frequency applications an AT cut is used - the cut of the crystal is defined by the angle at which the blank is cut from the original quartz crystal and the angle of cut defines many of the properties of the crystal blank and hence the overall filter.

The two sets of electrodes are placed onto the crystal. The electrodes are coupled to each other by the mechanical resonances in the crystal to give a highly selective filter.

Diagram of a monolithic crystal filter

The filter crystal is grown and cut in the same way as that used for normal crystals. Although quartz occurs naturally, and was used at one time for crystal manufacture, most of the material used today is manufactured synthetically. This has the advantage that the crystals are formed under much tighter conditions quality is more uniform. Additionally some of the flaws existing in natural quartz are not present making the overall quality much higher.

Once the raw crystal has been formed it is cut into blanks which are lapped and polished to a very high degree. The final stages of manufacture usually involve chemical etching as this gives a much finer finish. As a result the effects of ageing are greatly reduced. The size is very important because this determines the final resonant frequency.

Once the blank has been completed the electrodes are deposited onto the quartz. These are normally aluminium, silver or gold, and they have to be deposited under very controlled conditions so that they cover the required area and have the correct thickness. The thickness of the electrodes can be used to trim the filter to its exact resonant frequency. Making the electrodes slightly thicker reduces the frequency allowing the final performance to meet very stringent requirements.

Monolithic crystal filter operation

Like the standard quartz crystal, the monolithic crystal filter relies upon the piezo-electric effect that is exhibited in quartz for its operation. When signals appear across one pair of electrodes they set up mechanical vibrations on the crystal. These are affected by the mechanical resonances of the crystal element, and only those signals within the pass-band of the filter are allowed across the crystal to be picked up by the second pair of electrodes.

The primary resonant regions of the filter are between the electrodes on the upper and lower surfaces. The thickness of the wafer between the electrodes will be equal to nλ/2 where λ is the wavelength of the vibration in the crystal.

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Monolithic crystal filter resonance

The operation of the filter can be explained in terms of an equivalent circuit. There are several elements to this, each adding to the overall performance of the filter.

Equivalent circuit of a monolithic crystal filter

There are several element to the operation of the monolithic crystal filter circuit.

L1 / C1 & L2 / C2 :   These are the two series resonant circuits. The actual values of these equivalent components are determined by the mechanical dimensions and properties of the quartz element.

L3:   This represents the internal coupling within the internal coupling between the two resonant circuits. It is typically equal to k x L1, assuming L1 = L2. It can be calculated from the bandwidth divided by the centre frequency (B / F0). Typical values for the coupling constant are around 0.0005.

Co:   This is the parasitic capacitance between the top and bottom electrodes of the input and output - they are assumed to be the same. This capacitance can be accommodated within the input and output matching networks that are outside the filter and within the external circuitry.

Cp:   Cp is the parasitic leakage capacitance across the resonant element of the monolith crystal filter. This capacitance needs to be kept as small as possible to prevent signal leakage across the filter which impairs its stop band attenuation performance

Monolithic crystal filters can be designed for use over a wide range of frequencies. Costs rise, though, as the frequencies increase as manufacturing becomes more exacting and reject rates rise as the crystals become much smaller and more fragile.

Where operation at high frequencies is required, these monolithic filters can be run in an overtone mode, and this results in the crystal elements being larger, and hence more robust, than if they were designed for fundamental frequency operation.

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Monolithic filter impedance

When using any form of filter it is necessary to ensure that it is terminated with the correct impedance. The same its true for monolithic crystal filters. When designing the filter it is necessary to be able to calculate its impedance. Typically it is between 500Ω and 10kΩ. It is relatively easy to make a good estimate of the impedance of the input and output.

Where:Z = impedance of impedance of filterB = bandwidth in kHzfo = centre frequency in MHzn = overtone used for the filterR is a constant - typically between 1 kΩ and 2 kΩ

Multi-stage monolithic filters

In order to improve the performance of a monolithic crystal filter, it is possible to add further poles. This will increase the rate of cutoff and stopband attenuation. However as the number of poles increases within a single crystal unit, unwanted modes become more difficult to control.

To overcome this, while still increasing he performance higher order filters are generally made by connecting several monolithic crystal filters in series. This will enable the performance of multi-section crystal filters to be replicated in a monolithic format.

Filter made using multiple sections are often called tandem monolithic crystal filter.

Although often cheaper than a crystal filter made from discrete components, a monolithic crystal filter can still be expensive. Nevertheless monolithic crystal filters are able to provide a high level of performance Also with quartz crystal technology improving, their cost is likely to fall somewhat over the years and the performance improve steadily.

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