The process of turning ON SCR (silicon controlled rectifier) is called as triggering. The various methods of SCR triggering by gate control are given below.
- Triggering by dc gate signal.
- Triggering by pulse gate signal.
- Triggering by ac gate signal.
Triggering by DC gate signal:
With SCR in forward blocking state, a DC voltage of proper polarity and magnitude is applied between the gate and the cathode. The gate current starts flowing and turns ON the device. The disadvantages of this triggering are continuous gate current causes power losses at the gate junction and there is no isolation of the gate control circuit from the main power circuit.
Triggering by pulse gate signal:
In this the gate drive consists of a single pulse appearing periodically or a sequence of high frequency pulses which is also called as carrier frequency gating. The advantages of triggering by pulse gate signal is that the gate losses are very much reduced since the drive is discontinuous. The gate pulses are applied through pulse transformer which gives isolation. The firing angle varies from 0 to 180 degree.
Triggering by AC gate signals:
In many power control circuit that uses AC input, the gate to cathode voltage is obtained from a phase shifted AC voltage derived from the main supply. The advantage is that it has proper isolation of power and control circuits are provided.