# Power Electronics Questions and Answers – Thyristors-1

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Thyristors-1”.

1. A thyristor (SCR) is a
a) P-N-P device
b) N-P-N device
c) P-N-P-N device
d) P-N device

Explanation: An SCR (silicon controlled rectifier) is a four layer p-n-p-n type device.

2. Which terminal does not belong to the SCR?
a) Anode
b) Gate
c) Base
d) Cathode

Explanation: The SCR is having three terminals viz. anode, cathode and the gate.

3. An SCR is a
a) four layer, four junction device
b) four layer, three junction device
c) four layer, two junction device
d) three layer, single junction device

Explanation: SCR is a four layer p-n-p-n device which forms three p-n junctions.

4. Choose the false statement.
a) SCR is a bidirectional device
b) SCR is a controlled device
c) In SCR the gate is the controlling terminal
d) SCR are used for high-power applications

Explanation: It is a unidirectional device, current only flows from anode to cathode.

5. In the SCR structure the gate terminal is located
a) near the anode terminal
b) near the cathode terminal
c) in between the anode & cathode terminal
d) none of the mentioned

Explanation: The gate is located near the cathode, because it allows fast turning on of the device when the gate signal is applied by forward basing the second junction.
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6. The static V-I curve for the SCR is plotted for
a) Ia (anode current) vs Ig (gate current), Va (anode – cathode voltage) as a parameter
b) Ia vs Va with Ig as a parameter
c) Va vs Ig with Ia as a parameter
d) Ig vs Vg with Ia as a parameter

Explanation: The curve is plotted for Ia vs Va for different values of gate current Ig.

7. If the cathode of an SCR is made positive with respect to the anode & no gate current is applied then
a) all the junctions are reversed biased
b) all the junctions are forward biased
c) only the middle junction is forward biased
d) only the middle junction is reversed biased

Explanation: The device is in the reverse blocking state (3rd quadrant) & only the middle junction is forward biased whereas other two are reversed biased.

8. For an SCR in the reverse blocking mode, (practically)
a) leakage current does not flow
b) leakage current flows from anode to cathode
c) leakage current flows from cathode to anode
d) leakage current flows from gate to anode

Explanation: In the reverse blocking mode, the gate current is zero & a reverse voltage is applied at the cathode-anode.

9. With the anode positive with respect to the cathode & the gate circuit open, the SCR is said to be in the
a) reverse blocking mode
b) reverse conduction mode
c) forward blocking mode
d) forward conduction mode

Explanation: The SCR is in the forward blocking mode with its top and bottom junctions forward biased and the middle junction reversed biased.

10. For an SCR in the forward blocking mode (practically)
a) leakage current does not flow
b) leakage current flows from anode to cathode
c) leakage current flows from cathode to anode
d) leakage current flows from gate to anode

Explanation: In the forward blocking mode, the gate current is zero & only the middle J2 junction is reversed biased.

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