Question

Explain the operation of RC triggering circuit with neat diagram.

Answer 1

RC triggering circuit:

The RC half-wave triggering circuit is as shown in the figure. During negative halfcycle of supply voltage v_s, the capacitor C charges through diode D₂ and load, with lower plate positive. The charging time constant depends upon C and load impedance. If this charging time constant is low, the capacitor gets quickly charged to supply voltage and may attain peak value V_m at -90° as shown in the waveform diagram. At this instant, as both capacitor voltage and supply voltage are equal and opposite, capacitor current is zero. During period after ωt = -90° to ωt = 0°, the supply voltage vs drops from Vm as capacitor discharges. The discharging current passes through supply source, load and R. At ωt = 0°, the capacitor voltage reduces to a value less than Vm, represented by OA in the waveform diagram. After ωt = 0°, the supply voltage become positive, it now helps the discharging current and therefore, the capacitor gets discharged at faster rate. Its negative voltage (lower plate positive) get reduced to zero at a particular instant. The discharging current further continues in the same direction to charge capacitor with upper plate positive. When the capacitor voltage reaches to gate trigger voltage, the SCR is fired. After firing, voltage across SCR drops to low value and hence capacitor voltage also get reduced to low value as shown. The discharging of negatively charged (lower plate positive) capacitor and further charging with upper plate positive is through load and R. Therefore, R determines the discharging and positive charging time-constants. If R is increased, the time-constant get increased and firing is delayed. The SCR can never be triggered at 0° & 180° because the supply voltage is zero and less than finger voltage.