MCQs on Power Electronics

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01․ For an SCR gate cathode characteristic is a straight line of 130. For triggered source volume of 15 V and allowable gate power dissipation of 0.5 W compute the gate source resistance?
111.9 ohm
11.19 ohm
108 ohm
115 ohm


02․ Latching current for an SCR inserted between a dc voltage source of 200 V and load is 100 mA. Compute the minimum rate of width pulse required to turn ON the SCR in case load consists of R = 20 Ω in series with L = 0.2 H. width=263
200 µs.
300 µs.
150 µs.
100 µs.

\ So, minimum width of pulse is 100 µs.

03․ The latching current of SCR is 20 mA. Its holding current will be
23 mA.
40 mA.
10 mA.
60 mA.

As holding current is always less than leaking current.

04․ Thyristor can be protected from over voltages by using
voltage clamping device.
heat sink.
snubber circuit.

Non linear or voltage clamping device is used to protect thyristors under surge conditions by providing low impedance path.

05․ Example of a voltage clamping device
fast acting fuse.
snubber circuit.
metal oxide varistor.
aluminium block.

Varistor is a non - linear resistor which is used as voltage clamping device.

06․ Under over voltage condition impedance offered by the voltage clamping device is

Voltage clamping device under surge condition provides low impedance path to protect SCR.

07․ Under normal operating condition voltage clamping device offers impedance of
high value.
low value.
zero value.
moderate value.

Under normal condition voltage clamping device offers high impedance and acts as if it is not present in the circuit.

08․ What is used to protect the SCR from over current ?
CB and fuse.
Heat sink.
Snubber circuit.
Voltage clamping device.

CB and fuse sense the high current through the circuit and breaks the circuit to protect the SCR.

09․ CB used for over current protection of thyristor operates when the fault current is
of long period.
of short duration.
both (A) and (B).
neither (A) nor (B).

As tripping time of CB is generally large .

10․ When a large surge current of very short duration flows through a thyristor then which one of the following device will operate to protect the thyristor ?
Snubber circuit.
Voltage clamping device.
Fast acting current limiting device (FACL fuse).

FACL fuse operates very fast.