What is a Thyristor?

#Technical studies- study 11

Introduction

Thyristor is a unidirectional semiconductor device. It allows current to flow only in one direction like diodes. SCR and TRIAC are major thyristors. It operates only in two states either open or close, like a latch. So it can be used for switching applications.

Below drawing shows an equivalent circuit of thyristor.

Equivalent circuit of Thyristor

In circuit T1 is a PNP transistor and T2 is an NPN transistor. The base of T1 is connected to the collector of T2 and the base of T2 is connected to collector of T1.

In order to make this circuit on we need to apply a large Vcc to T1. This will make current flow in collector of T1. Because of this, base current of T2 will increase. As base of T2 triggered it will provide feedback to T1. Therefore both transistors are driven to saturation mode. This process of switching on the thyristor is called breakover method.

To switch off this circuit we need to decrease the supply voltage to zero. This will drive the transistor to cutoff mode. This process of switching off the thyristor is called low-current drop out method.

Thyrister is also called four layer diode, the below drawing shows the structure of the thyristor.

Structure of a thyristor
Thyristor Symbol

Silicon Controlled Rectifier (SCR):

Silicon controlled rectifier is most commonly used thyristor. It is used in very large current switching applications. We studied that a thyristor can be switched on using breakover method and switched off using low-current drop out method.

We can have a second way to switch on the thyristor by adding a terminal to the base of the second transistor of the thyristor for triggering. That is what called what SCR is, means an SCR is a four layer diode with a triggering terminal. This terminal is called gate. So SCR has three terminals, the trigger terminal gate, anode and cathode.

See the below drawing for better understanding.

Equivalent circuit of SCR
Structure of SCR
Symbol of SCR

Triggering of SCR:

SCR Triggering

See the above drawing; a trigger signal is applied to gate. This increases the base current of T2 and the positive feedback started. Therefore  the device is driven to saturation. This makes the SCR on.

As the gate terminal is connected to the base of a transistor minimum 0.7V is required to trigger the SCR.

Switching off the SCR:

When a trigger signal is applied to an SCR it turns on, but when the trigger signal is turned off the SCR not goes to off state. So it requires extra effort to switch of the SCR.

By reducing the Vcc the anode-cathode current can be reduced to below holding current (the low value of current at which a transistor drives saturation to cutoff). This will make the SCR off.

There are two more methods to switch off an SCR.

  1. Current interruption
  2. Forced communication

Current interruption:

In this method we can either open the series switch above the anode or close the parallel switch to anode and cathode.

Switching off the SCR by opening the series switch
Switching off the SCR by closing the parallel switch

Forced communication:

In this method the SCR is connected in reverse bias. This decreases the holding current and the SCR turns off.

Switching off the SCR using forced communication

VI characteristics of SCR:

Current voltage graph of SCR

The above drawing shows the relation between current and voltage in an SCR.

Difference between FET and SCR:

FET SCR
  • The gate voltage can switch on and switch off the device.
  • The gate voltage can only switch on the device.
  • When the input is high output is low and when the input is low the output is high. 
  • When the input is high output is low and when the input is low output remains low. 
  • It is similar to a push button switch. 
  • It is similar to a single pole single through switch. 

Bidirectional thyristor:

SCR is a unidirectional device. DIAC and TRIAC are bidirectional thyristors. These devices can control in both directions.

DIAC:

The above drawing shows the equivalent circuit of a DIAC. This device is a combination of two four layer diodes in reverse parallel connection. Like four layer diodes DIAC also switches on by breakover method.

DIAC Symbol
DIAC equivalent circuit

TRIAC:

TRIAC is a combination of two SCRs in reverse parallel connection. It can control current in both directions.

TRIAC Equivalent circuit
TRIAC Symbol

Please write in the comment box below if you have any questions.

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