SCR Triggering Circuit Requirements

Thyristor Firing Circuit Requirements;

In this post, we are going to see about the various requirements to be met for the trigger circuits of SCR.

Various methods are available to turn on the thyristor. Please click here to know more about Various SCR firing methods.
Among them, the most general ( practical) method to turn on the SCR is gate triggering. In gate triggering method, instead of applying continuous signal, pulse triggering is most preferable.

What are the conditions to be satisfied to turn ON the thyristor?

To turn on the SCR, following three conditions are required to be satisfied

  1. SCR should be forward biased
  2. The load impedance should not be too high, so that, if SCR turns on, the current in the SCR would reach more than the latching current value.
  3. The gate terminal must be made positive with respect to the cathode.

[Note: As the TRIACs can be triggered in both directions, the first and third conditions are not applicable]

What are the requirements to be satisfied to turn ON the SCR?

To fulfill three conditions mentioned above, a trigger circuit ( pulse triggering circuit) should satisfy the following requirements

  • Trigger pulse train is applied to a SCR staring at the instant it is turned ON. The rise time and the duration of the individual pulse is consistent with the Turn ON requirements of the device and the load.
  • A pulse train is applied to the thyristor over the entire interval over which it conducts.
  • A provision is made by accommodating the "end-stop pulse" with the pulse train.
  • For feedback control, the delay angle is controllable by a low-level DC voltage Vc
  • The driver circuits are provided with an INHIBIT input terminal which is used for the logical control of the converter.

Read More:
How Snubber Circuit protects SCR?
How to test SCR with ohmmeter?
Sensitivity Analysis in Electrical Circuits

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