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Forward and Reverse Blocking Mode in Thyristor

 

Forward and Reverse Blocking Mode in Thyristor

A thyristor (such as an SCR — Silicon Controlled Rectifier) has three terminals: Anode (A), Cathode (K), and Gate (G).

Its operation depends on the polarity of the applied voltage and gate signal.

1. Forward Blocking Mode

In this mode:

  • Anode is positive with respect to cathode.
  • Gate signal is not applied.

So the SCR is forward biased, but it still does not conduct.

What happens internally?

  • Junctions J1 and J3 are forward biased.
  • Junction J2 is reverse biased.
  • Only a very small leakage current flows.
VA > VK

If the forward voltage exceeds the forward breakover voltage, the SCR turns ON automatically even without gate current.

This mode acts like an open switch waiting for a gate pulse.

2. Reverse Blocking Mode

In this mode:

  • Cathode is positive with respect to anode.
  • The SCR is reverse biased.

What happens internally?

  • Junctions J1 and J3 become reverse biased.
  • Junction J2 becomes forward biased.
  • Only a very small reverse leakage current flows.
VK > VA

If reverse voltage exceeds the reverse breakdown voltage, the SCR may get damaged due to heavy reverse current.

This mode prevents current flow in the reverse direction.

Comparison Table

Feature Forward Blocking Reverse Blocking
Anode Voltage Positive Negative
Cathode Voltage Negative Positive
Gate Signal Not Applied Usually Irrelevant
Current Flow Very Small Leakage Current Very Small Leakage Current
Main Blocking Junction J2 J1 and J3
SCR State OFF OFF
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