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In Tutorials SCR Applications The ability of an SCR to control large currents to a load by means of small gate current makes the device very useful in switching and control applications. Here we will consider six applications of SCR like power control, switching, zero-voltage switching, over-voltage protection , pulse circuits and battery charging regulator. Power Control. SCR Power Control Circuit Because of the bistable characteristics of semiconductor devices, whereby they can be switched on and off, and the efficiency of gate control to trigger such devices, the SCRs are ideally suited for many industrial applications.

SCRs have got specific advantages over saturable core reactors and gas tubes owing to their compactness, reliability, low losses, and speedy turn-on and turn-off. The bistable states conducting and non-conducting of the SCR and the property that enables fast transition from one state to the other are made use of in the control of power in both ac and dc circuits.

This is known as phase control. A simple half-wave circuit is shown in figure a. The load current, load voltage and supply voltage waveforms are shown in figure b. The SCR will turn-off by natural commutation when the current becomes zero. The power consumed by the load decreases with the increase in firing angle a. The reactive power input from the supply increases with the increase in firing angle. The load current wave-form can be improved by connecting a free-wheeling diode D1, as shown by the dotted line in fig-a.

With this diode, SCR will be turned-off as soon as the input voltage polarity reverses. After that, the load current will free wheel through the diode and a reverse voltage will appear across the SCR.

The main advantage of phase control is that the load current passes through a natural zero point during every half cycle. So, the device turns-off by itself at the end of every conducting period and no other commutating circuit is required. Another important application of SCRs is in inverters , used for converting dc into ac. The input frequency is related to the triggering frequency of SCRs in the inverters.

Thus, variable frequency supply can be easily obtained and used for speed control of ac motors, induction heating, electrolytic cleaning, fluorescent lighting and several other applications. Potentiometer R controls the angle of conduction of the two SCRs. The greater the resistance of the pot, lesser will be the voltage across capacitors C1 and C2 and hence smaller will be the time duration of conduction of SCR1 and SCR2 during a cycle.

When the capacitor gets fully charged, charge on the capacitor depending upon the value of R it discharges through Zener diode Z. This gives a pulse to the primary and thereby secondary of the transformer T2. During negative half cycle similar action takes place due to charging of capacitor C1 and SCR1 is triggered. Thus power to a load is controlled by using SCRs.

The input voltage is alternating and the trigger pulses are applied to the gates of SCRs through the control switch S. Resistance R is provided in the gate circuit to limit the gate current while resistors R1 and R2 are to protect the diodes D1 and D2 respectively.

For starting the circuit, when switch S is closed, SCR1 will fire at the beginning of the positive half-cycle the gate trigger current is assumed to be very small because during positive half cycle SCR1 is forward biased. It will turn-off when the current goes through the zero value.

As soon as SCR1 is turned-off, SCR2 will fire since the voltage polarity is already reversed and it gets the proper gate current. The circuit can be broken by opening the switch S. Opening of gate circuit poses no problem, as current through this switch is small.

As no further gate signal will be applied to the SCRs when switch S is open, the SCRs will not be triggered and the load current will be zero. The maximum time delay for breaking the circuit is one half-cycle. The above circuit is also called the static contactor because it does not have any moving part. The circuit is broken by turning-off SCR1. This discharge current is in opposite direction to that flowing through SCR1 and when the two become equal SCR2 turns-off.

Now capacitor C gets charged through the load and when the capacitor C gets fully charged, the SCR2 tums-off. Thus the circuit acts as a dc circuit breaker.

The resistor R is taken of such a value that current through R is lower than that of holding current. Zero Voltage Switching. Only half-wave control is used here.

Similarly, when switch S is closed, SCR1 will stop conducting at the end of the present or previous positive half-cycle and will not get triggered again. Over-Voltage Protection. Over Voltage Circuit Protection SCRs can be employed for protecting other equipment from over-voltages owing to their fast switching action. The SCR employed for protection is connected in parallel with the load. Two SCRs are used—one for the positive half-cycle and the other for negative half-cycle, as shown in figure.

Resistor R1 limits the short-circuit current when the SCRs are fired. Pulse Circuits. The output circuit is designed to have discharge current of less than a milli-second duration.

The capacitor will again get charged in the following positive half-cycle and the SCR will be triggered again in the negative half-cycle. Thus the frequency of the output pulse will be equal to the frequency of the input supply. For limiting the charging current resistor R is used. Battery Charging Regulator. Battery Charging Regulator The basic components of the circuits are shown in figure.

When the full-wave rectified input is large enough to give the required turn-on gate current controlled by resistor R1 , SCR1 will turn on and the charging of the battery will commence. The capacitor C is included in the circuit to prevent any voltage transients in the circuit from accidentally turning on of the SCR2.

As charging continues, the battery voltage increases to a point when VR is large enough to both turn on the When this occurs, the battery is fully charged and the open-circuit state of SCR1 will cut off the charging current. Thus the regulator charges the battery whenever the voltage drops and prevents overcharging when fully charged. There are many more applications of SCRs such as in soft start circuits, logic and digital circuits, but it is not possible to discuss all these here.


SCR Applications





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