Power Electronics MCQ Questions & Answers

31

Consider the following statements:
The turnoff time of an SCR can be reduced by
1. Quick withdrawal of the gate voltage.
2. Reducing lifetimes by doping with gold.
3. Applying a negative voltage pulse to the gate of these statements:

A) 1, 2 and 3 are correct B) 1 and 2 are correct C) 1 and 3 are correct D) 2 and 3 are correct

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Answer & Solution

Answer: Option A

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32

An SCR can be brought to forward conducting state with gate circuit open when the applied voltage exceeds

A) Forward breakover voltage B) Reverse breakdown voltage C) 1.5 V D) Peak non-repetitive off-state voltage

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Answer & Solution

Answer: Option A

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33

Which of the following statements about the working of SCR is INCORRECT?

A) To open the SCR, reduce the supply voltage to zero B) Applying small positive voltage to the gate is the normal way to close an SCR because break over voltage is usually much greater than supply voltage C) There are two ways to turn on the SCR. One of the methods is to keep the gate open and make supply voltage lesser to the break overvoltage D) An SCR has two states i.e., either it does not conduct, or it conducts heavily. Therefore, SCR behaves like a switch

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Answer & Solution

Answer: Option C

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34

Which of the following components convert fixed DC to variable DC?

A) Rectifiers B) Choppers C) Cycloconverters D) Inverters

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Answer & Solution

Answer: Option B

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35

In a DIAC the breakdown voltages are related with the following equation:

A) $${V_{B{R_1}}} = {V_{B{R_2}}} \pm 0.1{V_{B{R_2}}}$$ B) $${V_{B{R_1}}} = 2{V_{B{R_2}}}$$ C) $${V_{B{R_1}}} = \frac{{{V_{B{R_2}}}}}{2}$$ D) $${V_{B{R_1}}} = \frac{3}{2}{V_{B{R_2}}}$$

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Answer & Solution

Answer: Option C

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36

Which one of the following statements is correct about SCR?

A) SCR is constructed using an npn and pnp transistor by connecting base of one transistor to collector of the other transistor B) To switch off an SCR, gate current must be reduced below certain threshold value C) Higher levels of gate currents in SCR cause it to conduct at lower anode-to-cathode voltages D) The higher the gate current in SCR, the higher the holding current to switch off

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Answer & Solution

Answer: Option C

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37

Two Thyristors of same rating and same specification

A) Will have equal turn on and off period B) Will have equal turn on but unequal turn off period C) May have equal or unequal turn on and turn off periods D) Will have unequal turn on and turn off periods

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Answer & Solution

Answer: Option C

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38

Forward voltage drop during SCR turn-on is 1.5 V this drop remains

A) Remains constant and is independent of load current B) Increases slightly with load current C) Decreases slightly with load current D) Varies linearly with load current

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Answer & Solution

Answer: Option B

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39

Identify A and B current limiting techniques in a.c. to d.c. power supplies respectively, in the graph below.
Power Electronics mcq question image

A) Current limiting mode, Constant Current mode B) Re-entrant limiting mode, Current limiting mode C) Current limiting mode, Fold back limiting mode D) Fold back limiting mode, Re-entrant limiting mode

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Answer & Solution

Answer: Option B

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40

When a thyristor is negatively biased,

A) All the three junctions are negatively biased B) Outer junctions are positively biased and inner junction is negatively biased C) Outer junctions are negatively biased and inner junction are positively biased D) The junction near the anode is negatively biased and the one near cathode is positively biased

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Answer & Solution

Answer: Option C

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Power Electronics MCQ Questions & Answers | ECE