A thyristor is a component that can be used as an electronic latch. Its circuit diagram
looks like a diode but with an extra input added: the gate.
A bistable circuit is one which can be made to latch in either the on or the off
state. Bi means two.
A practical thyristor circuit
How the circuit above works
• Normally the lamp is switched off.
• When switch 1 is pressed current flows from the battery, through resistor R1 and
into the gate of the thyristor and this makes the thyristor turn on and the lamp
• When the switch 1 is released however the thyristor stays switched on and the lamp
stays on, the thyristor is said to be latched.
• To switch off the lamp you need to interrupt the current flowing through the thyristor,
this will make it unlatch. In the circuit above this is done by pressing switch 2
which shorts out the anode and cathode therefore stopping any current flowing through
it. Once switch 2 is released the thyristor unlatches and the lamp will go out.
Another thyristor circuit
The circuit shown here is a modified version of the above circuit. In this case the
reset switch is in the form of a push to break switch (PTB) instead.
Switch 1 can be used to reset the thyristor because it interrupts the current flowing
between the anode and the cathode when it is pressed.