How Does A Thyristor Work?

By Ashwini Kumar Sinha

2811
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Before going deep into the thyristor let’s explain why we need a thyristor while we already have a tiny component named transistor that can help us in switching, amplification.

Well, transistors can do the switching thing with a transistor as well but they are not good to handle a large amount of current and another problem with the transistor is that when we remove the switching current they turned off.

When we want do triggering the switching current is removed then we need another device because here the transistor fails. So we have another device called as Thyristor that can solve both of the above problems.

The Thyristor can handle a large amount of current and also run continuously even if we remove the switching current.

A thyristor is a four-layer semiconductor device and contains 3 PN junction in series having 3 terminals named Anode, Cathode, and Gate. Like Diode Thyristor is also a unidirectional device but unlike diode, it can also be used as an open circuit switch.

Working Principle of a Thyristor.

In thyristor, the silicon wafer is doped with four alternate P and N types and this makes you look like a two-transistor connected back to back. As you can see in the pic below.

Here the P(Cathode) and N junction are joined in series and we got anode, gate and cathode three terminals pins.

When we provide the anode and cathode with forward-biased i.e Anode connected to + and cathode connected to -Ve then first PN junction and last PN junction (j1 and j3 ) become forward biased due to breaking of the depletion layer. While the junction j2 remains as reversed biased we haven’t provided any current to the gate. So, now when we provide the supply to the gate then the j2 junction layer starts breaking and the current starts flowing and the circuit.

Here we got its three working states:

Forward Blocking Mode

The Junction J1 and J3 is in forward working state and j3 in reverse-biased state and not allowing the current to flow.

Forward Conduction Mode

Here we apply a positive voltage to the gate terminal and make the j2 depletion reason breakdown. Due to the break down of depletion reason j2 current starts flowing the circuit and hence result in switch on mode

Reverse Blocking Mode

Here we give the negative voltage to anode and positive to the cathode and the gate is in open circuit. So this causes the J1 and J3 in reverse biased and j2 in forward biased. Due to the reverse bias of J1 and j2, the flow of current gets stopped.

Uses Of Thyristor

There are generally 3 different kinds of thyristor:

SCR (Silicon Controlled rectifier)

SCR has the ability to control huge current generally used as hight frequency switch in an electrical circuit.

GTO (Gate Turn OFF)

They are used in inverters, AC drives, induction heater etc.

IGBT (Insulated Gate Controlled Bipolar Transistor)

They are used in SMPS, Motor control, induction heating, etc.

We use thyristor as switching circuit where we control motor, lamps to turn on and off.

Video Tutorial explaining the Thyristor

Video Courtesy: Learn Engineering

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