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Thyristors

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.

Cathode
(negative)
Anode
(positive)
Gate
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 lights up.

 

• 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.

Anode

Gate

Cathode

Circuit Symbol

Cathode
(negative)
Anode
(positive)
Gate

Pin Layout