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MEASUREMENTS AND INSTRUMENTATION
Prepared ByCHARLES S.
Course Synopsis
• This course introduces the principles of instrumentation and measurements. It explores the working principles of DC & AC meters, oscilloscope and signal generators as well as the operation and application of various sensors and transducers
SYLLABUS SYLLABUS SYLLABUS SYLLABUS
UNIT – I STANDARDS AND INDICATING INSTRUMENTS
• SI units – Units for charge, voltage, current, power, energy, flux. Standards – Brief Introduction.
• Deflecting, controlling and damping forces in indicating instruments –
• Moving iron : attraction and repulsion type instruments, errors.• Moving coil instruments – Permanent magnet moving coil
instruments, Dynamometer type moving coil Instruments, Torque equations, errors.
• Ballistic Galvanometer – extension of ranges, use of shunts and Instrument Transformers.
SYLLABUSSYLLABUSSYLLABUSSYLLABUS
UNIT - II MEASUREMENT OF POWER AND ENERGY
• Dynamometer type wattmeter – Torque expression, Errors. Energy meters, Calibration of energy meters.
• Measurement of power using Instrument Transformers.
• Maximum demand indicator, Trivector meter, Power factor meter
• Power Analyzer.
SYLLABUSSYLLABUSSYLLABUSSYLLABUS
Unit III - BRIDGES
• Resistance measurement - Wheatstone bridge, Kelvin double bridge, substitution method. –Transformer ratio bridges – Measurement of Earth resistance.
• Measurement of inductance and capacitance –Maxwell’s Bridge, Anderson Bridge, DeSauty’sBridge and Schering bridge.
SYLLABUSSYLLABUSSYLLABUSSYLLABUS
UNIT - IV TRANSDUCERS AND MEASUREMENTOF NON-ELECTRICAL QUANTITIES
• Transducers – Classifications, Principle of operation of Resistance potentiometer, LVDT, Strain Gauge and Piezo-electric transducers.
• Encoders. • Measurement of Pressure and Flow – Measurement of
Temperature: Resistance thermometers, thermistors, thermocouples, optical and Radiation pyrometers.
SYLLABUSSYLLABUSSYLLABUSSYLLABUS
UNIT – V DIGITAL INSTRUMENTS
• Electronic voltmeter – Digital voltmeter of ramp and integrating types.
• Digital Multi meter – block diagram. • Block diagram of dual channel oscilloscope. • Spectrum Analyzer.• Signal and function generators. • Harmonic distortion analyzer. • Strip chart and X-Y recorders, Field Bus
Instrumentation.
Introduction to Instrumentation and
Measurements
Measurement?
• The Measurement of a given quantity is essentially an act (or) the result of comparison between the quantity and predefined standard. It expressed in numerical values
• Measurement is the process by which one can convert physical parameters to meaningful numbers
Measurement?
Simple instrument method
Measurement?
In order that the results of measurement are meaningful there are two basic requirements
• The standard used for comparison purposes must be accurately defined and should be commonly used
• The apparatus used and the method adopted must be provable
Methods of measurement
1. Direct method : The unknown quantity is directly compared against a standard . The result is expressed as a numerical value e.g.. Length , Mass , Time
2. Indirect method: Measurement of indirect methods are not always possible, feasible, and practicable. These methods in most of the cases are inaccurate because they involve human factors
Instrument and Measurement Systems
• Measurements involve the use of instrumentsas a physical means of determining quantities (or) Variables
• In simple cases, an instrument consists of a single unit which gives an output reading (or) signal according to the unknown variable applied to it.
• In more complex measurementsituations a measuring instruments may consists of several separate instruments. These elements may consists of transuducing elements which convert the measured to an analogues form.
Instrument and Measurement Systems
Process Instrumentation System
Classification of the instruments
They are many ways in which instruments can be Classified broadly
1. Absolute Instruments These instrument give the magnitude of the quantity under measurement in terms of physical constants of the instrument
2. Secondary InstrumentsThese instruments are so constructed that the quantity being measured can only be measured by observing the output indicated by the instrument
Measurement Standards
• Based on definition of the seven
fundamental SI units of measurement
• Categorized into four:– International standard (SI)– Primary standards– Secondary (transfer) standards– Working standards
Base Units of Measurement
Base Units of Measurement
Deflection type instrument
• The deflection of the instrument provides a basis for determining the quantity under measurement
• The measured quantity produces some physical effect with deflects (or) produces a mechanical displacement of the moving system of the instrument
• An opposing effect is built in the Instrument which tries to oppose the deflection (or) the mechanical displacement of the moving system
Deflection type instrument
• The opposing effect is closely related to the deflection
• The opposing effect is so designed that is magnitude increases with the increase of deflection
• The balanced is achieved when opposing effect equals to cause producing the deflection.
• The value of measured quantity can be measured from the deflection, at the point of balance
PMMC type Instrument
Spring
θ
Td
Tc
Td = G.I [ where G is depends Flux density, number of turns are the moving coil
Tc = K.θ [ where K is the spring constant which depends upon the material and dimensions of the spring
I
PMMC type Instrument
Advantages: 1. Uniform Scale 2. Since Driving power is small, Power Consumption is
Low 3. Due to aluminum or Copper former hysteresis loss is
absent 4. Using shunt or multipliers the ranges can be extended
Disadvantages: 1.Ac quantity cannot be measured 2. Compare than the PMMI, its costlier 3. Friction, Temperature, ageing of control springs might
introduce errors
• In this instrument, a zero (or) null indicationleads to determination of the magnitude of the measured quantity
• A null type instrument attempts to maintain the deflection at zero by suitable application of an effect opposing that generated by the measured quantity
Null type instrument
Null type instrument
Slide WireSliding Contact
DC Potentiometer
a b
Summarize the above two instruments
• Null Type instrument are more accurate than the deflection type instrument
• Null Type instrument is highly sensitive as compared with the deflection type instrument
• Deflection type instrument are more suited for measurements under dynamic conditions
Damping Systems
• Definition:The action of reducing the vibration of an object. This tends to return the vibrating object to its original position.
• The damping torque should be such of a magnitude that pointer quickly comes to its final steady position, without overshooting
1. Under damped 2. critical damped 3. Over damped
Damping Systems
• The Damping device should be such that it produces a damping only While the moving system is in the motion
• The effective damping torque should be proportional to the velocity of the moving system
Damping Systems
1. Under damped:The moving system will oscillate about the final steady state position with a decreasing amplitude and will take some time before it comes to rest.
2. Critical damped:When the moving system moves rapidly but smoothly to its final steady position, the instrument is said to critical damped
3. Over damped:If the damping torque is more than what is required for critical damping, the instrument is said to be over damped
Damping Systems
The methods of producing damping torque
1. Air friction damping
2. Eddy current damping
Air friction damping
When the piston moves in to chamber, the air inside is compressed and pressure of air thus build up, opposes the motion of piston
When the piston moves out of chamber , the pressure in closed space Falls, and the pressure on the open side is greater than the other side
Eddy current damping
In Fluid Friction damping oil is used in place of Air
� Viscosity of oil is greater
�The damping force is also correspondingly greater
�When the moving system moves, the disc moves in oil and a frictional drag is produced
�The Frictional drug always opposes the motion
Eddy Current damping
• When a conductor moves in a magnetic field an emf is inducedin it if a closed path is provided, then the current flows, this current known eddy current
• This current interact with the magnetic field to produce electromagnetic torque which opposes the motion
• This torque is proportional to the strength of the magnetic field and current product
T α ϕ α I• The current is proportional to emf which in turn is proportional
to velocity of the conductor
I α emf α velocity of the conductor
Electromagnetic Damping
• The movement of a coil in a magnetic field produces a current in the coil which interacts with the magnetic field to produce a torque
• This torque opposes the movement of the coil and slows the response
Moving Iron Instrument
• In moving iron are movable system consists of one (or) pieces of specially shaped soft iron
• Which are so pivoted (central point) as to be acted upon by the magnetic field produced by the current in the coil
1. Attraction type2. Repulsion type
Moving Iron Instrument
Moving Iron Instrument
• A plate or vane of high permeability steel forms the moving element of the system
• This iron van is so situated that it can be move in a magnetic field produced by the stationary coil
• The coil is excited by the current (or) voltage under measurement
Moving Iron Instrument
• When the coil is excited, it becomes an electromagnet and the iron van moves in a such a way so as to increase the flux of the electromagnet
• This is because iron vane tries to occupy a position of minimum reluctance
• Thus the force produced is always in such a direction
1. Attraction type
2. Repulsion Type
Repulsion Type
Moving Iron Instrument
Attraction Type Repulsion type
Permanent Magnet Moving Coil Instrument [PMMC]
Permanent Magnet Moving Coil Instrument [PMMC]
The general theory of moving-coil instruments may be dealt with considering a rectangular coil of N turns, free to rotate about a vertical axis.
Permanent Magnet Moving Coil Instrument [PMMC]
• A moving coil instrument consists basically of a permanent magnet to provide a magnetic field and a small lightweight coil is wound on a rectangular soft iron core that is free to rotate around its vertical axis
• When a current is passed through the coil windings, a torque is developed on the coil by the interaction of the magnetic field
• The aluminium pointer attached to rotating coil and the pointer moves around the calibrated scale indicates the deflection of the coil
Permanent Magnet Moving Coil Instrument [PMMC]
• Principle of operation It has been mentioned that the interaction between the induced field and the field produced by the permanent magnet causes a deflecting torque, which results in rotation of the coil.
1.Deflecting Torque
2. Controlling Torque
3.Damping Torque
Permanent Magnet Moving Coil Instrument [PMMC]
• Deflecting Torque
If the coil is carrying a current of .iamp ,
The force on a coil side = BilN (newton, N).
∴Torque due to both coil sides = (2r)BilN (Nm)= Gi (Nm)
Where, GI- is the Galvanometer constant and it is expressed as
G = 2rBlN (nm/amp)= NBA
(note A= 2rl -area of the coil.)
Permanent Magnet Moving Coil Instrument [PMMC]
• Controlling Torque
The controlling torque is directly proportional tothe angle of deflection of the coil.
Control Torque = Kθ
Whereθ – Deflection angleK – Spring Constant Nm/rad
Permanent Magnet Moving Coil Instrument [PMMC]
• Damping Torque: It is provided by the induced currents in a
metal former or core on which the coil is wound or in the circuit of the coil itself
As the coil moves in the field of the permanent magnet, eddy currents are set up in the metal former or core.
The pointer will therefore swing more slowly to its proper position and come to rest quickly with very little oscillation.
Types of strain gauges
• Unbonded metal strain gauge• Bonded metal wire strain gauge• Bonded metal foil strain gauge• Vacuum deposited thin metal film strain
gauges.• Sputter deposited thin metal film strain gauge.• Bonded semiconductor strain gauges.• Diffused metal strain gauge.
Unbonded metal strain gauge
• Used almost exclusively in transducer applications.
• At initial preload , the strains and resistances of the four arms are normally equal, with the result the output voltage of the bridge, e0=0.
• Application of pressure produces a small displacement , the displacement increases tension in 2 wires and decreasing the resistance of the remaining 2 wires.
• This causes an unbalance of the bridge producing an output voltage which is proportional to the input displacement and hence to the applied pressure.
Bonded metal wire strain gauge
• It consist of a grid of fine resistance wire of diameter of about 0.025mm.
• The wire is cemented to a base.
• The base – thin sheet of paper or bakelite.
• Wire is covered with a thin sheet of material so that it is not damaged mechanically.
• The spreading of wire permits a uniform distribution of stress over a grid.
Bonded metal foil strain gauge
�Extension of the bonded metal wire strain gauge.
�The bonded metal wire strain gauge have been completely superseded by bonded foil strain gauge.
�The local increase in area reduces the transverse sensitivity which is a spurious input since the gauge is designed to measure the strain component along the length of grid elements
Metal foil strain gauge
Semiconductor strain gauge.
The resistance of the semi conductors changes with change in applied strain
The semiconductor strain gauge depends for their action upon piezo resistive effect. (i.e. the change in the value of the resistance due to change in resistivity.)
Measurement of pressure and flow
• Measurement of flow – It mainly operate on the principle of placing
on obstruction in the path of fluid causing a change in fluid pressure which is dependent upon the rate of flow
– Thus by measuring the difference in pressure before and after the obstruction by means of a differential pressure sensors, the rate of flow may be determined
– The examples of these flow meters Orifice plate, Venturi tube, Rotameter
•The turbine flow meter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow The turbine tends to have all the flow traveling around it.
•The output is usually in the form of a digital electrical signal whose frequency is directly proportional to flow rate and whose total count is proportional to the total quantity , as each pulse represents a discrete volume
Temperature measurement
• One of the most common devices for measuring temperature is theglass thermometer.
• This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid.
• Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid.
• Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer.
• Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is thegas thermometer.
Classifications
• Other important devices for measuring temperature include:
• Thermocouples• Thermistors• Resistance Temperature Detector (RTD)• Pyrometers• Langmuir probes (for electron temperature of
a plasma)• Infrared
Resistance Thermometers• The resistance of the conductor changes when
its temperature is changed. This property is utilized for measurement of temperature.
• The variation of resistance R with temperature T can be represented by the following relationship for most of the metals as
R=R0(1+α1T+ α2T2+…+ αnTn+………)• Where R0=resistance at temperature T=0
and • α1 ,α2,αn are constants.• The resistance thermometer uses the change in
electrical resistance of conductor to determine the temperature
• It’s a positive temperature coefficient.
R T
Requirements of a conductor material to be used in (Resistance
Temperature Detector) RTDs are
• The change in resistance of material per unit change in temperature should be as large as possible.
• The material should have a high value of resistivity so that minimum volume of material is used for the construction of RTD.
• The resistance of material should have a continuous and stable relationship with temperature.
• Contraction of a term “thermal resistors”
• Its composed of semiconductor materials.
• Used in applications which involve measurements in the range of -60oC to 15oC
• The resistance of thermistors ranges from 0.5ς to 0.75 Mς
Thermistors
Thermistors
Composed of sintered mixture of metallic oxides such as manganese, nickel , cobalt, copper, iron and uranium.
Typical resistance–temperature characteristics of thermistor materials.
Applications thermistors
1. Measurement of Temperature
2. Control of temperature
3. Temperature compensation
4. Measurement of power at high frequencies
5. Measurement of thermal conductivity
6. Measurement level, flow and pressure of liquids
Thermocouple
• When two metals having different work functions are placed together, a voltage is generated at the junction which is nearly proportional to the temperature. This junction is called a thermocouple.
• A thermocouple or thermocouple thermometer is a junction between two different metals that produces avoltagerelated to a temperature difference.
Construction of thermocouples
Measurement of Thermocouple output
• The output emf of a thermocouple as a result of difference between temperatures of measuring junction and reference junction can be measured by the following methods
1. Measuring the output voltage directly with PMMC
2. Measuring the output voltage with the help of DC potentiometer
3. Measuring the output voltage after amplifying it
Pyrometer
• It is a temperature-measuring device, originally an instrument that measures temperatures beyond the range of thermometers, but now in addition a device that measures thermal radiation in any temperature range
• A pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry. This device can be used to determine the temperature of an object's surface.
• These pyrometers find applications for temperatures which are above the range of thermocouples and also for rapidly moving objects
Optical Pyrometers
The Radiations from a heated body at high temperatures fall with in the visible region of the electromagnetic spectrum
For a given wavelength in the visible region the energy radiated is greater at high temperatures
Within the visible region a given wavelength has a fixed colour and the energy of radiation is interpreted as intensity or brightness
Therefore if we measure brightness of the light of a given colour emitted by a hot source , we can have an indication of temperature
Optical Pyrometers
• In an optical pyrometer the wavelength of radiation accepted is restricted by means of a colour filter and the brightness is measured by comparison with standard lamp.
• An image of the radiating source is produced by a lens and made to coincide with the filament of electric lamp.
• The current through filament is made variable so that lamp intensity can be adjusted.
• The current through filament is adjusted until the filament and the image are of equal brightness
Optical Pyrometers
• When the brightness of image produced by the source and brightness produced by the filament are equal, the outline of the filament disappears
• The range of temperature which can be measured in the order of 1400 0 C
• Applications1. Temperature of furnace2. Temperature of molten metals and other heated materials
Radiation Pyrometers
Radian pyrometry measures the radiant heat emitted or reflected by a Hot object
Thermal radiation is electromagnetic radiation emitted as a result of temperature
The radiation detector is either a thermal detector, which measures the temperature rise in a black body at the center point of the optical system
Radiation Pyrometers
• Thermal detectors respond equally to all wavelengths in the frequency spectrum, and consist of either thermopiles, resistance thermometers or thermistors.
• All of these typically have time constants of several milliseconds, because of the time taken for the black body to heat up and the temperature sensor to respond to the temperature change.
Radiation Pyrometers
• The operation of thermal radiation pyrometers is based upon blackbody concepts
• The total thermal radiation emitted by blackbody is:
•4 2
9 2
0
/
tan 57.2 10 /
bq T W m
where Boltzmann cons t W m
T absolutetemp
σσ −
=
= = ×=
UNIT V
Digital Instruments
Digital Voltmeter [DVM]
• A digital voltmeter displays the value of a.c or d.c voltage being measured directly as discrete numerals in the decimal number system
• Advantages:
• It eliminates the observational errors committed by operators
• Parallax and approximation are entirely eliminated • Increase the speed which readings can be taken • The output values we can store for future computations
by using memory devices
Digital Voltmeter
Types :
1. Ramp Type DVM
2. Integrating type DVM
3. Potentiometric DVM
4. Successive approximation type DVM
5. Continuous balance type DVM
1. Ramp type voltmeter
• The digital voltmeter is to measure the time that a linear ramp voltage takes to change from level of input voltage to zero voltage (vice versa)
• This time interval is measured with an electronic time interval counter and the count is displayed as a number of digits on electronic indicating tubes of the output readout of the voltmeter
Linear ramp type DVM
Ranging &
Attenuator
Ramp
Generator
Oscillator Gate CounterRead out
Sampled RateMV
Input Voltage
Input
ComparatorStart pulse
Ground
Comparator
Stop pulse
Integrating Type DVM ( Voltage to Frequency Conversion )
• A constant input voltage is integrated and the slope of the output ramp is proportional to the input voltage
• When the output voltage reaches a certain value, it is discharged to 0 and another cycle begins
• The frequency of the output waveform is proportional to the input voltage
Integrating Type DVM
Digital Frequency
Meter
Pulse Generator
e0
er
eiR1
R2
t1 t2Pulse
er Waveformsat output of the integrator
Output of the Pulse generator
Multimeter
• A multimeter or a multitester , also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit.
• A typical multimeter may include features such as the ability to measure voltage, current, resistance, etc……….
• There are two categories of multimeter,1. Analog multimeter and2. Digital multimeter
Digital Multimeter
• The current is converted to voltage by passing it through low shunt resistance.
• The a.c quantities are converted to d.c by employing various rectifier and filtering circuits
• While for the resistance measurements the meter consists of a precision low current source.
Digital Multimeter
signal generator
• signal generator is an electronic device that generates repeating or non-repeating electronic signals (in either the analog or digital domains).
• These instruments usually produce a fixed frequency sine wave, whose output can be frequency or amplitude modulated by another signal.
• The instruments cover a frequency range of 0.001Hz to 50GHZ
signal generator
Oscillator Amplitude
Modulation Circuit
Attenuator Output amplifier
Detector
Comparator FeedbackAmplifier
Input for Amplitude Modulation
Input for Frequency Modulation
Output
signal generator
• Frequency ModulationIt is achieved by varying the voltage across
a variable capacitance diode in the tuning circuit of the oscillator
This gives a system with low output distortion, for modulation depths below 1% of the carrier frequency
During the frequency modulation manual or automatic methods may be used to keep the amplitude of the output constant
signal generator
• Amplitude Modulation It is most conveniently done by varying the
supply voltage to the oscillator
• Feedback – can be used to reduce to reduced output distortion
• Output To obtain the modulation envelop,
comparing this with the amplitude modulation input, then amplifying and feed back the difference as the modulation signal
Function Generator
• It’s a Versatile instrument that delivers a choice of different waveforms whose frequencies are adjustable over a wide range
• The outputs are – Sine wave – Triangular Wave– Square Wave– Saw tooth Wave
Simple Oscillator Circuit
Threshold Decision Oscillator
Function generator
Function Generator
• Features – These various outputs of the generator are available
simultaneously
– Useful feature of the function generator is that it can be phase locked to an external source.
– One function generator can be phase locked with second function generator, the two output signals can be displaced in phase by an adjustable amount.
– The function generator can be phase locked to a standard frequency of the source.
– Output waveforms of the generator will have same accuracy and stability as that of standard source
STORAGE AND DISPLAY DEVICES
� Magnetic disk and tape� Recorders� digital plotters� Printers� CRT display� digital CRO� LED� LCD � Dot Matrix Display.
Recorders
� A recorders records electrical and non electrical quantities as a function of time.
� They are used for having permanent records. It monitors continuously the condition state or value of the process variable such as flow, force, pressure, temperature, current , voltage, electrical power etc.
� Current and voltage can be recorded directly while the non electrical quantities are recorded indirectly by first converting them to equivalent currents or voltages with the help of sensors or transducers.
Recording device
Analog recorders Digital recorders
Incremental digital Synchronous digitalOscillographic recorders Magnetic tape recorders
Graphic recorders
Strip Chart recorders
X-Y recorders
Circular Chart recorders
Galvanometer type
Null type
LVDT recorders
Potentiometric recorders Bridge recorders
Strip Chart
• Strip Chart Recorder are those in which data is recorded on a continuous roll of chart paper moving at a constant speed
• The recorder records the variation of one of more variables with respect to time
Strip Chart
• It consists of – A long roll of Graph paper moving vertically
– A system for driving the paper at some selected speed (1 – 100 mm/s)
– A stylus for making marks on the moving graph paper. The stylus moving horizontally in proportional to the quantity being measured
– A stylus driving system which moves the stylus in a nearly analog of the quantity being recorded
Strip Chart
Stylus
• Most recorders use a pointer attached to the stylus
• So that the instantaneous value of the quantity being recorded can be measured directly on the calibrated scale
Strip Chart
• Most strip chart recorders use a servo feedback system, to ensure that the displacement of the pen across the paper tracks the input voltage in the required frequency range
• A potentiometer system in generally used to measure the position of the writing head
• The Chart paper drive system generally consists of a stepping motor which controls the movement of the chart paper at a uniform rate.
Strip Chart
• Basically it having three types of mechanisms
– Paper Drive Systems– Marking mechanism – Tracing systems
Paper Drive Systems
The paper drive system should move the paper at a uniform speed
A spring wound mechanism may be used in most of the recorders a synchronous motor is used for making marks on the paper
Strip Chart
• Marking mechanism– There are many types of mechanisms used for
making marks on the paper
1. Marking with ink filled stylus
2. Marking with heated stylus3. Chopper Bar
4. Electric Stylus Marking
5. Electrostatic Stylus 6. Optical Marking method
Strip Chart
• Types of Strip Chart Recorders
– Galvanometer Type
– Null Type recorder
Galvanometer Type
Null Type recorder
Strip Chart
X-Y Recorders
Oscilloscope
• An oscilloscope(abbreviated sometimes asscopeor O-scope) is a type ofelectronic test instrumentthat allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electricalpotential differences(vertical axis) plotted as a function of time or of some other voltage
• The CRO depends on the movement of an electron beam, which is bombarded on a screen coated with a florescent material, to produce a visible spot.
• If the electron beam is deflected on both the conventional axes (i.e X-axis and Y-axis)
Oscilloscope
• The Oscilloscope is basically an electron beam voltmeter.
• The heart of the oscilloscope is the Cathode Ray Tube (CRT) – Which makes the applied signal visible by the
deflection of a thin beam of electrons.
Cathode Ray tube
H.T. supply
heater supply
Vertical Deflection plates
+-phosphor screen
anode
HorizontalDeflection plates
Electron Gun
Cathode Ray tube
• The Electron gun assembly produced a sharply focused beam of electrons which are accelerated to high velocity
• The focused beam of electrons strikes the Fluorescent screen with sufficient energy to cause a luminous spot on the screen
• After leaving the electron gun, the electron beam passes through two Paris of “Electrostatic Deflection” plates
Cathode Ray tube
• Voltage applied to these plates deflect the beam
• Voltages applied to one pair of plates move the beam vertically up and down – and the voltage applied to the other pair of plates
move the beam horizontally from one side to another.–
• These two movements ( horizontal and vertical are independent of each other and thus the beam may positioned anywhere on the screen)
summary
H.T. supply
heater supply
y plates x plates
+-phosphor screen
anode
electron gun produces a beam
of electrons light produced on the screen by electron beam
a p.d. across the y plates deflects the
trace vertically
a p.d. across the x plates deflects the trace horizontally
Cathode Ray tube
• The main parts of a CRT
– Electron gun assembly
– Deflection plate assembly
– Fluorescent Screen
– Glass Envelop
– Base, through which connections are made to various parts