When cathode is positive with respect to anode in an SCR, the number of blocked p-n junctions is

When cathode is positive with respect to anode in an SCR, the number of blocked p-n junctions is Correct Answer 2

The correct answer is 'option 2'

Concept:

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There are three modes of operation for an SCR depending upon the biasing given to it.

Forward blocking mode:

• In this mode of operation, the Silicon Controlled Rectifier is connected such that the anode terminal is made positive with respect to the cathode while the gate terminal is kept open.
• In these state junctions, J1 and J3 are forward biased and the junction J2 reverse biased.

Forward Conduction mode:

• In this mode, SCR or thyristor comes into the conduction mode from blocking mode.
• It can be done in two ways either by applying a positive pulse to the gate terminal or by increasing the forward voltage (or voltage across the anode and cathode) beyond the break-over voltage of the SCR.

Reverse blocking mode: 

• In this mode of operation, the cathode is made positive with respect to the anode.
• Then the junctions J1 and J3 are reverse biased and J2 is forward biased.
• This reverse voltage drives the SCR into the reverse blocking region results to flow a small leakage current through it and acting as an open switch.

Solution: 

The question has asked about the condition where the cathode is given positive voltage w.r.t anode, this is the case of reverse blocking mode. The number of junctions that are blocked is 2(J1 and J2).