The two transistor equivalent circuit of SCR is:

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Question

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KVS TGT WET (Work Experience Teacher) Official Paper

Answer 1

Two transistor model of the SCR bisecting the SCRs middle two layers.

The two-transistor equivalent circuit shows that the collector current of the NPN transistor TR2 feeds directly into the base of the PNP transistor TR1, while the collector current of TR1 feeds into the base of TR2.
These two inter-connected transistors rely upon each other for conduction as each transistor gets its base-emitter current from the other’s collector-emitter current.
So, until one of the transistors is given some base current nothing can happen even if an anode-to-cathode voltage is present.