Thyristor Switch

Normally, two pushbuttons are needed to control a thyristor, namely — you guessed it — one to switch it on and one to switch it off. With the circuit shown here, it is possible to control the thyristor using only one pushbutton. If S1 is pressed, a positive voltage is applied to the gate and the thyristor is triggered on. The LED lights up at the same time, the relay pulls in and the transistor conducts. This causes C1 to be charged, so that the next time the button is pressed, the ground potential is applied to the gate instead of a positive voltage. The thyristor then returns to the nonconductive state.