MCQs on Power Electronics

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01․ How can we protect SCR from thermal conditions ?
Use of snubber circuit.
Using heat sink.
Using CB and fuse.
Using equalizing circuit.

Excess amount of heat can be dissipated quickly by mounting the SCR on a heat sink.
02․ Which of the following is used in heat sink?
Iron.
Aluminium.
Carbon.
Silver.

Aluminium is a good conductor of heat. Silver is the best conductor of heat but it is not used due to its high cost.
03․ Materials used in heat sink should have
high thermal conductivity.
large surface area.
high melting point.
All of these.

A heat sink material should have all these characteristics to radiate heat efficiently and quickly.
04․ What is used to protect a thyristor from high di / dt conditions?
Fuse.
Snubber circuit.
Inductor.
Voltage clamping device.

Inductor does not allow sudden change in current through it.
05․ Which of the following is used in SCR to protect from high dV / dt?
Snubber circuit.
Fuse.
Equalizing circuit.
Circuit breaker.

Capacitor in series with the resistor is called Snubber circuit which does not allow sudden change in dV / dt.
06․ What happen due to high di / dt?
Breakdown of junction.
Local hot spot.
Insulation failure.
None of these.

High di / dt causes localized heat generation.
07․ What may happen high dV / dt?
Unwanted turn ON.
Breakdown of J2 junction.
Both A and B.
Anyone of these.

High dV / dt will breakdown the J2 junction and trigger the SCR.
08․ Why resistor is used in Snubber circuit ?
To minimize the loss .
To minimize the charging current.
To minimize the discharging current.
All of these.

Capacitor discharges through the resistor. Resistor limits this discharging current.
09․ Snubber circuit is used with SCR
in series.
in parallel.
either series or parallel.
anti parallel.

Snubber circuit is used in parallel with SCR to limit high dV / dt across the SCR.
10․ Delay time is defined by the interval when
gate current increases from 90 % to 100 % of its final value.
anode current reaches 10 % from forward leakage current.
anode voltage drops from 100 % to 90 % of its actual value.
all of these.

Delay time is the interval between which gate current rises from 90 % to 100 % of its final value at the same time anode forward leakage current rises to 10 % of its final value as well as anode forward voltage falls to 90 % of its initial value.
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