Turning Off SCR (Commutation)

Methods of Turning OFF SCR:

We have already discussed about various turn on methods of 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.

Read Further:

AC Voltage Converters (Phase Controlled Converters)
Four Quadrant Operation
Diode Power Loss Calculation
Power Electronics Interview Questions: Set-1 

Thanks for reading about scr commutation.... Please leave your comments below...

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33 Responses

  1. tabrez alam says:

    just good.superb to study.plz add more topics about thyristers

  2. tabrez alam says:


  3. Leader says:

    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:

    Dear Admin Sir,
    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

  8. ammad says:

    It is very nice to read...withing short time....please give such electronics related information..it is usefull while readin

  9. abhay kumar says:

    superb solution and description go ahead

  10. Prateek says:

    Nice.. Thankss

  11. santhosh says:

    Give me more importation i will expert this site it's better to study compare to other site ..

  12. Ragavendiran says:

    good, need more information

  13. Shubham ji says:

    e-study are vry useful for us from there.....thanx

  14. Mahid amitsinh says:

    Thank u veryyyy much.....for giving a detail descreption abt a to z power electronics....carry on..and add more detail.

  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:

    Nice work admin.

  19. CHIBUEZE says:


  20. sahil says:

    cool explanation and presentation. 🙂

  21. mitisha goswami says:

    awsme nd esy to understnd nd thnxx for such an xplanation..

  22. rupesh bansod says:

    wonderful explanation

  23. nikita chavhan says:

    Really easy to understand
    Thankx a lot
    Plz give more information
    About class A ,B,C

  24. Nandhini says:

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

  25. Niyomugabo Eric says:

    Ok am agree with you what you did ok post more in order to inrease our studys as a students

  26. Amr Nahhas says:

    Many Thanks ! very Helpful !!

  27. Devraj says:

    Honestly hat's off to the writer. There's a difference between understanding and knowing. As Sir Einstien said, if u do not explain it simple, means u haven't understood it yet... And I feel the writer has both the talents... Great job and thanks a ton...

  28. Sahithi says:

    Good easy to learn

  29. suman paudel says:

    easy to understand
    thank you

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