Fig.3. Symbol of thyristor.
A thyristor is generallya four layer three-terminal device. Four layers are formed by
alternating p-type and n-type semiconductor materials. Subsequently, there are three pn
junctions formed in the device. It is a bistable device. The three terminals of this device are
called anode (A), cathode (K) and gate (G) respectively. The gate terminal is control terminal
of the device. That means, the current flowing through the device is controlled by electrical
signal applied to the gate terminal. The anode and cathode are the power terminals of the
device handle the large applied voltage and conduct the main current through the thyristor.
Fig.4. P-N junctions of thyristor.
A thyristors acts as an on-off switch which is used to control output power of an electrical
circuit by switching on and off the load circuit periodically in a pre-set interval. The main
difference of thyristors with other electronics switches is that, a thyristor can handle large
current and can withstand large voltage, whereas electronic switches handle only small
current and voltage. When positive potential applied to the anode with respect to the cathode,
ideally no current will flow through the device and this condition is called forward – blocking
state but when appropriate gate signal is applied, a large forward anode current starts flowing,
with a small anode-cathode potential drop and the device becomes in forward-conduction
state. Although after removing the gate signal, the device will remain in its forwardconduction mode until the polarity of the load reverses.Therefore, thyristor is also called a
latching device. Some thyristors are also controllable in switching from forward-conduction
back to a forward-blocking state.