90° Phase Control of SCR.
In ac circuits, the SCR can be turned on by the gate at any angle a with respect to the applied voltage. This angle α is called the firing angle. Power control is obtained by varying the firing angle and this is known as phase control. In the phase-control circuit given in fig. 1, the gate triggering voltage is derived from the ac supply through resistors R1, R2 and R3. The variable resistance R2 limits the gate current during positive half cycles of the supply. If the moving contact is set to the top of resistor R2, resistance in the circuit is the lowest and the SCR may trigger almost immediately at the commencement of the positive half cycle of the input. If, on the other hand, the moving contact is set to the bottom of resistor R2, resistance in the circuit is maximum, the SCR may not switch on until the peak of the positive half-cycle. By adjusting R2 between these two extremes, SCR can be switched on somewhere between the commencement and peak of the positive half-cycle, that is between 0° and 90°. If the triggering voltage VT is not large enough to trigger SCR at 90°, the device will not trigger on at all, because VT has the maximum value at the peak of the input and decreases with the fall in voltage. This operation is sometimes referred to as half-wave variable-resistance phase control. It is an effective method of controlling the load power.
Diode D is provided to protect the SCR gate from the negative voltage that would otherwise be applied during the negative half cycle of the input. It can be seen from the circuit diagram shown in fig.a, that at the instant of turning on of the SCR gate current flows through RL and diode. So
VT=VD + VG + IGRL
180 degree Phase Control.
The circuit shown in figure, can trigger the SCR from 0° to 180° of the input waveform. In the circuit shown here, the resistor R and capacitor C determine the point in the input cycle at which the SCR triggers. During the negative half cycle of the input, capacitor C is charged negatively (with the polarity shown in the figure) through diode D2 to the peak of the input voltage because diode D2 is forward-biased. When the peak of the input negative half cycle is passed, diode D2 gets reverse-biased and capacitor C commences to discharge through resistor R. Depending upon the time constant, that is CR, the capacitor C may be almost completely discharged at the commencement of the positive half cycle of the input, or it may retain a partially negative charge until almost 180° of positive half cycle has passed. So long as the capacitor C remains negatively charged, diode D1, is reverse-biased and the gate cannot go positive to trigger the SCR into conduction. Thus R and /or C can be adjusted to affect SCR triggering anywhere from 0° to 180° of the input ac cycle.
Pulse Control of an SCR.
The simplest of SCR control circuits is shown in figure. If SCR were an ordinary rectifier, it would develop half-wave rectified ac voltage across the load RL. The same would be true if the gate of the SCR had a continuous bias voltage to keep it on when the anode-cathode voltage VAK goes positive. A trigger pulse applied to the gate can switch the device at any time during the positive half-cycle of the input. The resultant load waveform is a portion of positive half cycle commencing at the instant at which the SCR is triggered. Resistor RG holds the gate-cathode voltage, VG at zero when no trigger input is present. The instantaneous level of load current can/be determined from the following relation