A Thyristor (silicon controlled rectifier or SCR) is a little like a transistor. When a small current flows into the GATE (G), this allows a larger current to flow from the ANODE (A) to the CATHODE (C). Even when the current into the gate stops the thyristor continues to allow current to flow from anode to cathode. It latches on.

The circuit opposite represents a steady hand game which consists of a wire loop that has to be moved around a wire course without touching it. If the wire course is touched by the loop, current flows into the 'gate' of the thyristor and the buzzer sounds.
The buzzer will continue to sound after the loop has touched the wire course. This is due to the thyristor which once activated cannot be deactivated until all power is turned off.
This type of circuit is also known as a ‘latching circuit’

ALARM CIRCUIT

The circuit below is an alarm circuit and it incorporates a thyristor. When the house holder leaves he/she turns on the master power switch and the exit switch. If an intruder steps on the pressure pad the alarm sounds and ‘latches’ on (stays on) because of the thyristor.

2. Explain how a thyristor works.

3. Draw a circuit which includes a thyristor and explain how the circuit works.

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