How Thyristor works?

Thyristors is one of the markable innovations in semiconductor sector?because of their unique properties and ability to control electric power and current by acting as a switch. They are capable of handling large voltages and currents, making them ideal for power control applications such as motor speed controllers, liquid level regulators, and pressure control systems. They are also commonly used in power electronics such as inverters and converters to convert AC to DC and vice versa. Additionally, thyristors find their use in light-dimming circuits in the domestic arena.

A thyristor is a four-layer solid-state semiconductor device that contains 3 PN junctions in series having 3 terminals called anode, cathode, and gate. It is a unidirectional device and can be used as an open-circuit switch. The silicon wafer in a thyristor is doped with four alternate P and N types, which look like two transistors connected back-to-back. Here, the P (cathode) and the N (anode) are joined in series, thus we get three terminal pins: anode, gate, and cathode.

When we forward bias the anode and cathode i.e. anode and cathode connected to the positive and negative terminals of a battery, the first PN junction and last PN junction become forward biased due to the break of the depletion layer. The middle junction remains as reversed-biased as no current is provided to the gate. When we supply current to the gate, then the middle junction layer starts breaking and the current starts flowing in the circuit.

I have created an equivalent circuit of thyristor using npn & pnp transistor as shown in the above video. A once positive voltage greater than the threshold is applied on the gate terminal thyristor will be in On-state and start conducting. But the only difference is unlike other semiconductor devices thyristor will not turn off when we make gate voltage zero.

We have to apply a negative voltage pulse on the gate to turn off the thyristor. In the above video, I am giving 3V to turn on and -3v to turn off the thyristor. This video shows a basic example of a thyristor with DC applications.