# Turning Off SCR (Commutation)

### Methods of Turning OFF SCR:

It is recommended to refresh about the SCR basics and VI characteristics before proceeding further...
In this post let us discuss about the various turn OFF process also known as commutation process.

• To tum on a thyristor, a low voltage, short duration pulse is applied to the gate (typically 4V, 100µs).
• Once the thyristor is turned-on, the gate loses control and the thyristor will only turn off when the load current falls virtually to zero, or the thyristor is reverse biased.
• The thyristor will turn off naturally with a.c. supplies as the voltage reverses (which is called as Natural Commutation), but no such reversal occurs with d.c. supplies and it is necessary to force a voltage reversal if tum-off is to occur. This process is called Forced Commutation.

This post will give you a brief introduction about SCR commutation.

#### What is meant by Commutation?

The process of turning OFF SCR is defined as "Commutation".

• In all commutation techniques, a reverse voltage is applied across the thyristor during the turn OFF process.
• By turning OFF a thyristor we bring it from forward conducting to the forward blocking mode.
• The condition to be satisfied in order to turn OFF an SCR are:
1. IA < IH ( Anode current must be less than holding current)
2. A reverse voltage is applied to SCR for sufficient time enabling it to recover its blocking state.
• There are two methods by which a thyristor can be turned OFF.
1. Natural Commutation
2. Forced Commutation

#### Natural Commutation:-

• In AC circuit, the current always passes through zero for every half cycle.
• As the current passes through natural zero, a reverse Voltage will simultaneously appear across the device.
• This will turn OFF the device immediately.
• This process is called as natural commutation, since no external circuit is required for this purpose. It is also known as line commutation.
• AC voltage controllers or phase voltage controllers are the example for the natural commutation.

#### Forced Commutation:

• To turn OFF a thyristor, the forward anode current should be brought to zero for sufficient time to allow the removal of charged carriers.
• In case of DC circuits the forward current should be forced to zero by means of some external circuits.
• This process is called as forced commutation.
• The forced commutation is further classified as
(1) Voltage Commutation (charge stored in the capacitor will be applied to the SCR to turn it off)
(2) Current Commutation (field stored in the inductor will be applied to the SCR to turn it off)
• Jones chopper is an example for the voltage commutated chopper
• Morgan chopper is an example for the current commutated chopper.

### 33 Responses

1. tabrez alam says:

2. tabrez alam says:

good

What is the forced commutation circuit for the operation of chopper in which the main thyristor is turned off by sudden application of reverse voltage across the main thyristor???

4. Sonu kumar says:

Very easy to understand. Thanx a lot. Keep updating.

5. Chinmoy Das says:

what is class B or resonant pulse commutation?

6. ravi says:

please post some real time daily application videos if possible.

7. pundalik says:

hi can u plez update force commutation in detail...
class A
class B
AN ALL

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g

9. abhay kumar says:

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10. Prateek says:

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12. Ragavendiran says:

13. Shubham ji says:

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14. Mahid amitsinh says:

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15. vinay says:

good importion.we can easily understand

16. obey muchuweni says:

how do you test the integrity of an semikron skkt 132/14e thyristor with a digital multimeter?

17. dheeraj says:

How to test an igbt circuit

18. Tonney Ogolla says:

19. CHIBUEZE says:

I NEED A COMPLETE CIRCUIT DIAGRAM ON VARIABLE SPEED BI-DIRECTIONAL DC MOTOR CONTROL AND THE DESIGN ANALYSIS

20. sahil says:

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21. mitisha goswami says:

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22. rupesh bansod says:

wonderful explanation

23. nikita chavhan says:

Really easy to understand
Thankx a lot

24. Nandhini says:

What is the external circuit for forced commutation? Do tell me the circuit names for forced commutation?

25. Niyomugabo Eric says:

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26. Amr Nahhas says:

Many Thanks ! very Helpful !!

27. Devraj says:

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28. Sahithi says:

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29. suman paudel says:

easy to understand
thank you