# MCQ in Power Electronics Part 5 | ECE Board Exam

(Last Updated On: August 10, 2022) This is the Multiple Choice Questions Part 5 of the Series in Power Electronics as one of the Electronics Engineering topic. In Preparation for the ECE Board Exam make sure to expose yourself and familiarize in each and every questions compiled here taken from various sources including but not limited to past Board Exam Questions in Electronics Engineering field, Electronics Books, Journals and other Electronics References.

#### Continue Practice Exam Test Questions Part 5 of the Series

Choose the letter of the best answer in each questions.

201. The gate current required to turn on an SCR is

A. few amperes

B. a few mA

C. almost equal to anode current

D. about 50% of anode current

Solution:

202. A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ωt. The thyristor will conduct

A. from 0 to 180°

B. when v > 50 V

C. from 0 to 90°

D. When v < 50 V

Solution:

203. In a 3-phase semi-converter, the firing frequency is

A. equal to line frequency

B. 3 times the line frequency

C. 6 times the line frequency

D. 12 times the line frequency

Solution:

204. A thyristor has its gate current zero. To start conduction the forward voltage must exceed

B. forward breakover voltage

C. on state voltage

D. peak working off state forward voltage

Solution:

205. On load tap changer using thyristors can have a maximum of two taps.

A. True

B. False

C. none

D. all

Solution:

206. In a single-phase full wave ac regulator, the firing angles in the two half cycles

A. are always equal

B. are sometimes equal

C. are never equal

D. may be equal or unequal

Solution:

207. As regards single phase dual converter

A. the change in operation from one quadrant to the other is faster in non-circulating current mode

B. the change in operation from one quadrant to the other is faster in circulating current mode

C. the time response to change in operation from one quadrant to other is the same in both the modes

D. the time response to change in operation from one quadrant to the other may be faster in any of the two modes

Solution:

208. To protect an SCR against high di/dt we connect

A. a capacitance in series with SCR

B. a capacitance in parallel with SCR

C. an inductance in parallel with SCR

D. an inductance in series with SCR

Solution:

209. In a parallel inverter

A. the commutating capacitor C is across the primary of transformer and the load is across the secondary

B. the commutating capacitor and load are connected across the secondary of transformer

C. the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer

D. none of the above

Solution:

210. A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is

A. Vm

B. 0.5 Vm

C. zero

D. 0.5 Vm – voltage drop across inductance

Solution:

211. Which of these commutation methods is used in AC-DC converters?

A. Class A

B. Class C

C. Class D

D. Class F

Solution:

212. When thyristors are connected in series and parallel, it may be necessary to have:

A. current derating

B. voltage derating

C. both current and voltage derating

D. none of the above

Solution:

213. A single phase half wave regulator uses

A. 2 diodes and 2 thyristors

B. 2 thyristors

C. 1 diode and 1 thyristor

D. 2 diodes

Solution:

214. A high frequency series inverter consists of

A. a number of series inverters in series

B. a number of series inverters in parallel

C. a number of parallel inverters in parallel

D. a number of parallel inverters in series

Solution:

215. In an SCR, the anode current consists of

A. only holes

B. only electrons

C. both electrons and holes

D. either electrons or holes

Solution:

216. A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilization factor of 0.35. When a 3-phase full converter is supplying the same load

A. rectification efficiency will be more than 0.6 and TUF will be more than 0.35

B. rectification efficiency will be more than 0.6 but TUF will be less than 0.35

C. rectification efficiency will be less than 0.6 and TUF will be less than 0.35

D. rectification efficiency will be less than 0.6 but TUF will be more than 0.35

Solution:

217. In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current

A. are always similar

B. can never be similar

C. may be similar or dissimilar

D. are similar only if α = 0

Solution:

218. Natural commutation can be used in:

A. DC circuits only

B. AC circuits only

C. both AC and DC circuits

D. none of the above

Solution:

219. In a thyristor the gate lead is connected to the n layer near the p layer to which anode is connected.

A. True

B. False

C. none

D. all

Solution:

220. McMurray Bedford full bridge inverter uses

A. natural commutation

B. auxiliary commutation

C. complementary commutation

D. any of the above

Solution:

221. Which of these commutation methods uses an external pulse for commutation?

A. Class A

B. Class C

C. Class E

D. Class F

Solution:

222. A relaxation oscillator uses

A. tunnel diode

B. UJT

C. both tunnel diode and UJT

D. PIN diode

Solution:

223. In a 3 phase bridge inverter with 180° mode of operation the numbers of thyristors conducting at one time are

A. 1

B. 2

C. 3

D. 4

Solution:

224. A single phase full converter can operate in

Solution:

225. In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle

A. is zero

B. is 60°

C. is more than 60°

D. is more than 90°

Solution:

226. The two-transistor model of a thyristor consists of two transistors:

A. one n-p-n and other p-n-p

B. both p-n-p

C. both n-p-n

D. one n-p-n and other UJT

Solution:

227. In a thyristor, the forward breakover voltage

A. is constant

B. may be constant or may depend on gate current

C. decreases as gate current is increased

D. increases as gate current is increased

Solution:

228. The gate lead, in a thyristor is welded to:

A. p layer to which anode is connected

B. n layer nearest to the anode terminal

C. p layer nearest to the cathode terminal

D. outside n layer

Solution:

229. In a 3-phase full converter, the firing angle is less than 60°. The instantaneous output voltage

A. will have positive part only

B. will have both positive and negative parts

C. may have both positive and negative parts

D. will have negative part if load is inductive

Solution:

230. In a stud mounted SCR, the stud is

A. in thermal contact with heat sink

B. insulated from heat sink

C. in electrical and thermal contact with heat sink

D. in either electrical or thermal contact with heat sink

Solution:

231. In a full bridge inverter, the waveshape of load current depends on

A. relative values of load parameters R, L and C

B. relative values of load parameters R and L

C. relative values of load parameters L and C

D. gating frequency

Solution:

232. The characteristics of which device has peak point and valley point

A. SCR

B. PUT

C. SCS

D. Shockley diode

Solution:

233. In an ordinary thyristor

A. turn on and turn off times are equal

B. turn on time is more than turn off time

C. turn off time is more than turn on time

D. turn on time may be equal, less than or more than turn off time

Solution:

234. Thyristors connected in series need:

A. static equalizing circuit

B. dynamic equalizing circuit

C. both static and dynamic equalizing circuits

D. none of the above

Solution:

235. If 4 quadrant operation is required, we need

A. dual converter

B. full converter

C. semi converter

D. none of the above

Solution:

236. Which of the following devices exhibit negative resistance characteristics?

A. UJT

B. PUT

C. Both UJT and PUT

D. UJT, PUT and SBS

Solution:

237. As compared to BJT, MOSFET has

A. lower switching losses and higher conduction losses

B. higher switching losses and higher conduction losses

C. higher switching losses and lower conduction losses

D. lower switching losses and lower conduction losses

Solution:

238. A single phase bridge type cycloconverter uses eight thyristors.

A. True

B. False

C. none

D. all

Solution:

239. In the basic series inverter circuit, the source supplies load

A. only in positive half cycle

B. both in positive and negative half cycles

C. only in part of positive half cycle

D. in positive half cycle and part of negative half cycle

Solution:

240. In a parallel inverter

A. each thyristor is turned on twice during each cycle

B. each thyristor is turned on once during each cycle

C. each thyristor is turned on either once or twice in each load cycle

D. each thyristor may be turned on up to four times in each load cycle

Solution:

241. The specified maximum junction temperature of thyristors is about

A. 50°C

B. 80°C

C. 120°C

D. 200°C

Solution:

242. A half bridge inverter

A. needs a 3-wire dc supply

B. may use a two wire or 3 wire dc supply

C. has higher output voltage if fed from 3 wire dc supply

D. has higher efficiency if fed from 3 wire dc supply

Solution:

243. A 3 phase series inverter has

A. 12 thyristors

B. 6 thyristors

C. 3 thyristors

D. 3 or 6 thyristors

Solution:

244. Surge current rating of an SCR is

A. the maximum repetitive surge current in which the SCR can withstand

B. the maximum non-repetitive surge current in which the SCR can withstand

C. the maximum repetitive or non-repetitive surge current in which the SCR can withstand

D. the maximum surge current in which the SCR can withstand if both surge current and steady current exist together

Solution:

245. Which of the following devices has metal-silicon junction?

A. General purpose power diode

B. Schottky diode

C. SCR

D. MOSFET

Solution:

246. In a series inverter

A. the commutating elements L and C are in parallel

B. the commutating elements L and C are in series with load

C. the commutating L and C may be in series or parallel with the load

D. none of the above

Solution:

247. In thermal ohm’s law, temperature and power loss are analogous to

A. V and I respectively of ohm’s law

B. I and V respectively of ohm’s law

C. R and V respectively of ohm’s law

D. I and R respectively of ohm’s law

Solution:

248. A string of series connected thyristors needs equalizing circuit to

A. ensure proper firing

B. ensure proper current division

C. ensure proper voltage division

D. none of the above

Solution:

249. The minimum gate current which can turn on SCR is called:

A. trigger current

B. holding current

C. junction current

D. breakover current

Solution:

250. Which of these devices can conduct in both directions?

A. SCR

B. SUS

C. SBS

D. UJT

Solution:

### Complete List of MCQ in Electronics Engineering per topic

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