Chapter no.5

Thyristor application & photosensitive control circuits

Contents: Ac power control using TRIAC. Light dimmer. Automatic battery charger. Emergency light system. Temperature controller. Fan speed regulator. Opto coupler. Burglar alarm . Batch counter. Smoke detector.

Ac phase control using triac:

Phase control is also called as firing angle control. so phase control is basically the control of firing angle of the triac. The phase control is used to precisely control the amount of power delivered to the load such as fans or lamp load etc..

Operation:

Operation in positive half cycle: In the positive half cycle , live point (L)

is positive with respect to the neutral point (N).

The charging current for the capacitor C₁ flows through R₁ as shown in figure.

As soon as the voltage across C₁ reaches the break-over voltage of the Diac, it is turn on to supply gate current for the Triad and the Triad will be turned on.

The conducting Triac is equivalent to a closed switch. So the R₁C₁ and Diac short circuited. The load voltage is equal to the instantaneous supply voltage.

Operation in negative half cycle: In the negative half cycle, the live point (L) is negative

with respect to the neutral (N). The Triac and Diac both are in the off state. The

charging current for the capacitor C₁ flows through R₁ as shown in fig.

Voltage on C₁ is now negative. As soon as this voltage reaches the break-over voltage of the Diac, it is turned on and supplies gate current to the Triac.

The Triac is then turned on. The Diac and R₁C₁ circuit is short circuited.

The load current reverses its direction and the voltage across the load will be negative equal to the instantaneous ac supply voltage.

Thus Diac being a bi-directional device can turn on the Triac in both the half cycles of the input ac supply. The capacitor C₁ must be a non-polarized capacitor, being capable of charging to positive as well as negative voltages.

The firing angle (α) i.e. the instant at which the Triac is turned on in the positive as well as negative half cycles of the ac supply voltage can be controlled by making the resistance R₁ variable.

This variable resistance would then decide the charging rate of capacitor C₁ and hence firing angle (α) in each half cycle. As the control circuit operates directly on AC supply, it is automatically synchronized with the supply. The load voltage waveforms for a resistive load are shown above.

Light dimmer using triac:

 

Light dimmer using triac:

Battery charger:

Waveforms of battery charger:

Operation: The input transformer T is a step down transformer reduces the

230V AC mains 15V. The secondary voltage rectified by the full wave rectifier circuit.

The zener diode Z1 maintains a constant voltage 15V, at point “x”

The rectifier voltage waveform at “A” .The dotted line in this figure indicates the battery voltage.

When voltage at point A is greater than the battery voltage the SCR1 is forward biased and can conduct if the gate junction is a forward biased.

Thus SCR1 conducts from P to R as shown in figure and charges 12 volt battery connected in the circuit.

As the battery accumulates more and more charge, the dotted line goes up and the point P and R come closer to Q in figure, thus a reducing the conduction time for SCR1, and hence increasing the charging time , of battery.

When the battery is fully charged say about 14 volts the cathode of SCR1 is at 14V and the gate is at 14.3V. This difference of 0.3V between the gate and cathode can not forward biased as gate junction and will not be triggered. Thus the battery is cut off from the supply and charging will stop automatically.

Emergency light system:

The basic emergency lightning system is as shown in figure. This system includes the facility to charge the 6 volt battery and switches automatically from the AC supply failure takes place.

Operation:

Mode 1 (when AC supply is on) : The diode D1and D2 along with the center tap transformer T1 from full wave rectifier. They provide DC voltage for the 6V lamp load when the AC supply is ON. Diode D3 and R1 supply the battery charging current which can be varied by R1. The anode gate of SCR1 is kept at the battery voltage. While the cathode of SCR1 is kept at a higher potential by C1. Therefore as long as the AC supply is ON, the SCR1 remains reverse biased.

Mode 2 (when AC supply is OFF): As soon as the AC supply is interrupted, the output of the rectifier formed by D1 and D2 goes to zero. The cathode potential of SCR1 falls below battery voltage. The gate current is supplied to SCR1 through R3 and the SCR is triggered. This connects the 6v battery across the lamp. When the AC input reappears the SCR1 is turned OFF automatically and the charging of the battery will begin.

Temperature controller:

Phase control circuits may also be used for regulating temperature. Fig. shows the connection diagram of temperature controller.

Operation: It is a simple full-wave phase control circuit. By adjusting resistance R₁ and pot R₂ we can fix the resistance

temperature for the load. Z₁ is a zener diode which gives a fixed voltage across it. this

voltage appears across the thermistor R₄. When the voltage across the thermistor R₄ is sufficient to

charge the capacitor C₁ to a voltage equal to or more than the break-over voltage of the Dias, the Dias triggered and sends a trigger pulse to the gate of the Triac.

The Triac starts conducting, thus connecting the heater element in the circuit.

As the temperature increases, the thermistor resistance decreases and as such, the voltage across capacitor C₁ is reduced. This increases the firing angle of the Triac thus reducing the voltage across the heater element accordingly and consequently reduction of heat takes place.

Gradually, a stage comes when the voltage across the capacitor C₁ becomes insufficient to trigger the Diac and the Triac is automatically switched off.

This results in the disconnection of the heater element from the circuit.

R1 is limiting resistor. The function of Z₁ is to avoid the effect of the supply

fluctuations on the performance of the circuit. For a 220V, 50Hz A.C. supply, R₁ may be chosen 47k,

2W and the capacitor C₁ of 0.1μF value. The rating of zener diode Z₁ may be decided by the

break-over voltage of the Dias. The Triac specification depends on the load to be controlled.

Fan speed regulator using triac:

The fan motor is a single phase induction motor, the speed of which depends on the rms value of voltage applied to it.

Fig shows how a triac can be used to control the speed of fan motor.

Operation: In both the half cycle of the input supply voltage, the

capacitor C will charge through the resistor R. As soon as the voltage across the capacitor reaches the

breakdown voltage of the diac, a triggering pulse is generated and the triac is turned on.

As soon as the triac is turned on, the voltage between points A and N will drop down to on state voltage drop of the triac.

The voltage on the C is negligibly small .The capacitor can charge again after the triac is turned off.

The charging rate of the capacitor C is decided by the variable resistor R.

If the value R is low, the capacitor will charge to the diac breakdown voltage in less time, that is the firing angle will be small, the rms voltage applied to the motor will be higher and fan speed increases.

With increase in the value of R, the capacitor will charge slowly increasing the firing angle ()of the triac thus reducing the speed.

In this way, the fan speed can be regulated by controlling the firing angel of the triac in both half cycles.

Opto-coupler:

In electronics, an opto-isolator (or optical isolator, optocoupler, photocoupler, or photoMOS) is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated — since the signal goes from an electrical signal to an optical signal back to an electrical signal, electrical contact along the path is broken.

Operation: A common implementation involves a LED and a

phototransistor, separated so that light may travel across a barrier but electrical current may not. When an electrical signal is applied to the input of the opto-isolator, its LED lights, its light sensor activates, and a corresponding electrical signal generated at the output. Unlike a transformer, the opto-isolator allows for DC coupling and provides significant protection from serious overvoltage conditions in one circuit affecting the other.

With a photodiode as the detector, the output current is proportional to the amount of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode.

In photovoltaic mode, the diode acts like a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity.

In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light.

An opto-isolator can also be constructed using a small incandescent lamp in place of the LED; because the lamp has a much slower response time than an LED, will filter out noise or half-wave power in the input signal. In so doing, it will also filter out any audio- or higher-frequency signals in the input. It has the further disadvantage, of course, (an overwhelming disadvantage in most applications) that incandescent lamps have finite life spans.

The optical path may be air or a dielectric waveguide. The transmitting and receiving elements of an optical isolator may be contained within a single compact module, for mounting, for example, on a circuit board; in this case, the module is often called an opto-isolator or opto-isolator.

The photosensor may be a photocell, phototransistor, or an optically triggered SCR or Triac.

An opto-coupler, also called opto-isolator, is an electronic component that transfers an electrical signal or voltage from one part of a circuit to another, or from one circuit to another, while electrically isolating the two circuits from each other.

It consists of an infrared emitting LED chip that is optically in-line with a light-sensitive silicon semiconductor chip, all enclosed in the same package.

The silicon chip could be in the form of a photo diode, photo transistor, photo Darlington, or photo SCR.

Burglar alarm:Fig. shows simple burglar alarm circuit which makes enclosure or on the door for the purpose of protection against burglary , scr is connected in the circuit as shown in fig. limiting resistor R and the micro switch are connected at the gate of the scr. The micro (door) switch remains in the OFF position when the door is closed, whereas the switch is put on automatically when the door is opened. With the reset switch S closed, working of the circuit as follows:

Operation: Condition (1): when the door is closed. The d.c. supply is available at input terminals of the alarm. The micro switch being in the off position, does not allow any signal at the gate of the scr. The scr does not conduct in this condition and therefore return path is not available for the current to flow through the alarm and the alarm is not energized.

Condition (2): when the door is opened. micro switch in this condition, being in the ON position allows the required signal at the scr gate. Resistor R is for limiting the current. As soon as ,the gate signal is received, the scr starts conducting and the alarm is energized. For putting the alarm off, the scr is de-energized by opening the reset switch s. the alarm may be replaced by lamp, as shown in the dia.

Batch counter: Batch counter is special purpose counter, which is used to count the number of opaque object moving on a conveyer belt. The block diagram of a batch counter system is shown in fig.

Operation:

This system is used for counting the number of objects that are passing on a conveyer belt.

A light beam is focused continuously on LDR and the reverse photocurrent is flowing through it.

As soon as an object passes and interrupts the beam of light, the photocurrent flowing through the LDR will reduce to zero.

Hence corresponding to every object passing through we get low going pulse as shown in fig.

An electronic counter counts these low going pulse and displays them on seven segment display.

As each counted pulse correspond to an object, the displayed number corresponds to the number of objects.

Smoke detector:

Operation: Operation in presence of smoke: When the smoke is present between the light source and LDR, it

interrupt the light falling on LDR. The resistance of LDR increases. This increase the base voltage of Q1 and turn it on. The collector

voltage of Q1 now reduce to a low value. Due to this the base voltage of Q2 will reduce to a very low value

and Q2 is turn off. The relay is de-energized and the N.C. contact is closed to connect

supply to the alarm and the alarm will be operated to indicate the presence of smoke.

Operation in absence of smoke: When the smoke is not detected, the light from the light source falls

on LDR without any interruption. The resistance of LDR is low and the base voltage of transistor Q1 is low.

i.e. Q1 is in the off state. This raises the collector voltage of Q1 to Vcc

Due to high collector voltage of Q1, the base of Q2 receives a sufficiently high voltage and Q2 is turned on.

The collector current of Q2 flows through the relay coil to energies the relay. The N.C. contact of the relay is open circuited and the alarm does not sound.

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