MCQOPTIONS
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MCQOPTIONS
This section includes 554 Mcqs, each offering curated multiple-choice questions to sharpen your Electronics & Communication Engineering knowledge and support exam preparation. Choose a topic below to get started.
| 401. |
When a thyristor is conducting, the voltage drop across it |
| A. | is absolutely constant |
| B. | decreases with increase in load current |
| C. | increases slightly with increase in load current |
| D. | none of the above |
| Answer» D. none of the above | |
| 402. |
The output wave of a cycloconverter consists of a number of segments derived from the input wave. |
| A. | 1 |
| B. | |
| Answer» B. | |
| 403. |
Assertion (A): When the anode current of an SCR starts increasing during turn on process, the anode cathode voltage drop starts decreasing. Reason (R): During turn on the reverse biased junction in SCR breaksdown. |
| A. | Both A and R are correct and R is correct explanation of A |
| B. | Both A and R correct but R is not correct explanation of A |
| C. | A is correct but R is wrong |
| D. | A is wrong but R is correct |
| Answer» B. Both A and R correct but R is not correct explanation of A | |
| 404. |
In a power diode the reverse recovery time is the time from the instant the forward current is zero to the instant when reverse recovery current has decayed to |
| A. | 50% of peak reverse current |
| B. | 25% of peak reverse current |
| C. | 10% of peak reverse current |
| D. | zero value |
| Answer» B. 25% of peak reverse current | |
| 405. |
In a single phase full wave ac regulator the rms value of load current is 20 A load is purely resistive. Then |
| A. | average thyristor current will be less than 20 A but rms thyristor current will be more than 20 A |
| B. | both the average and rms thyristor currents will be less than 20 A |
| C. | both the average and rms thyristor currents will be more than 20 A |
| D. | rms thyristor current will be 20 A and average thyristor current will be 10 A |
| Answer» C. both the average and rms thyristor currents will be more than 20 A | |
| 406. |
A single phase full bridge inverter for R-L loads needs |
| A. | 4 thyristors |
| B. | 4 thyristors and 4 diodes |
| C. | 4 thyristors and 2 diodes |
| D. | 8 thyristors |
| Answer» C. 4 thyristors and 2 diodes | |
| 407. |
Assertion (A): The circuit of figure thyristor will conduct for 180° during positive half cycle if it is continuously fired. Reason (R): A thyristor can conduct only when it is forward biased. |
| A. | Both A and R are correct and R is correct explanation of A |
| B. | Both A and R correct but R is not correct explanation of A |
| C. | A is correct but R is wrong |
| D. | A is wrong but R is correct |
| Answer» E. | |
| 408. |
A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45°, the speed is likely to be |
| A. | about 350 rpm |
| B. | about 250 rpm |
| C. | about 175 rpm |
| D. | about 125 rpm |
| Answer» E. | |
| 409. |
A thyristor has its gate current zero. To start conduction the forward voltage must exceed |
| A. | about 0.5 V |
| B. | forward breakover voltage |
| C. | on state voltage |
| D. | peak working off state forward voltage |
| Answer» C. on state voltage | |
| 410. |
Assertion (A): A TRIAC is a bidirectional SCR.Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions. |
| A. | Both A and R are correct and R is correct explanation of A |
| B. | Both A and R correct but R is not correct explanation of A |
| C. | A is correct but R is wrong |
| D. | A is wrong but R is correct |
| Answer» B. Both A and R correct but R is not correct explanation of A | |
| 411. |
The terminals of a power MOSFET are called |
| A. | emitter, base, collector |
| B. | source, gate, drain |
| C. | source, base, drain |
| D. | emitter, gate, drain |
| Answer» C. source, base, drain | |
| 412. |
The commutation method in an inverter, is |
| A. | line commutation |
| B. | forced commutation |
| C. | either (a) or (b) |
| D. | none of the above |
| Answer» D. none of the above | |
| 413. |
Which of these devices have four layers? |
| A. | UJT |
| B. | SCR |
| C. | TRIAC |
| D. | SCR and TRIAC |
| Answer» C. TRIAC | |
| 414. |
Which converter is used in HVDC transmission? |
| A. | 3 pulse |
| B. | 6 pulse |
| C. | 12 pulse |
| D. | 24 pulse |
| Answer» D. 24 pulse | |
| 415. |
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 |
| Answer» D. none of the above | |
| 416. |
An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is |
| A. | 16% maximum |
| B. | 40% of maximum |
| C. | 60% of maximum |
| D. | 84% of maximum |
| Answer» C. 60% of maximum | |
| 417. |
In a single phase converter using a centre tapped transformer and with a highly inductive load, the firing angles is a. Each thyristor conducts for a period |
| A. | p |
| B. | a |
| C. | p - a |
| Answer» B. a | |
| 418. |
The phenomenon of overlap in converter operation due to |
| A. | source resistance |
| B. | source inductance |
| C. | both source resistance and source inductance |
| D. | high value of firing angle |
| Answer» C. both source resistance and source inductance | |
| 419. |
In the below figure, the load is highly inductive. The firing angle is a for each of the thyristors. The periods of conduction of Th1 and Th2 respectively are |
| A. | 0 to p and p to 2p |
| B. | a to p and p + a to 2p |
| C. | a to p + a and p + p to 2p + a |
| D. | a to p - a and p + a to 2p - a |
| Answer» D. a to p - a and p + a to 2p - a | |
| 420. |
Which of the following devices has two gates? |
| A. | SCR |
| B. | SCS |
| C. | SUS |
| D. | SBS |
| Answer» C. SUS | |
| 421. |
A single phase half wave rectifier circuit has a free wheeling diode. The free wheeling diode will conduct only if |
| A. | load is purely resistive |
| B. | load is purely inductive |
| C. | load is combination of R and L |
| D. | load is purely inductive or combination of R and L |
| Answer» E. | |
| 422. |
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 |
| Answer» D. in either electrical or thermal contact with heat sink | |
| 423. |
A bridge type single phase cycloconverter changes the frequency f to f/3. Then one half wave of output contains |
| A. | 3 full waves of input |
| B. | 3 half waves of input |
| C. | 6 full waves of input |
| D. | 6 half waves of input |
| Answer» C. 6 full waves of input | |
| 424. |
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 |
| Answer» C. both AC and DC circuits | |
| 425. |
In a single phase full wave controlled rectifier using centre tap transformer, the voltage across each half of secondary is Vm sin ωt. The peak inverse voltage is |
| A. | 2 Vm |
| B. | Vm |
| C. | 0.5 Vm |
| D. | 0.25 Vm |
| Answer» B. Vm | |
| 426. |
In a modified series inverter |
| A. | the maximum output frequency is less than frequency of damped oscillations |
| B. | the output frequency can be even higher than the frequency of damped oscillations |
| C. | the output frequency cannot exceed the frequency of damped oscillations |
| D. | any of the above |
| Answer» C. the output frequency cannot exceed the frequency of damped oscillations | |
| 427. |
In single phase half wave regulator, the average current over one full cycle |
| A. | is always positive |
| B. | may be positive or negative |
| C. | may be negative |
| D. | is always negative |
| Answer» E. | |
| 428. |
When an SCR is in forward blocking mode, the applied voltage |
| A. | appears across only one junction |
| B. | is equally shared by two junctions |
| C. | is equally shared by three junctions |
| D. | may appear across one or more junctions |
| Answer» B. is equally shared by two junctions | |
| 429. |
Assertion (A): A high dv/dt may cause triggering of a thyristor.Reason (R): A high dv/dt results in charging current in the thyristor. |
| A. | Both A and R are correct and R is correct explanation of A |
| B. | Both A and R correct but R is not correct explanation of A |
| C. | A is correct but R is wrong |
| D. | A is wrong but R is correct |
| Answer» B. Both A and R correct but R is not correct explanation of A | |
| 430. |
Ripple factor RF and form factor FF are related as |
| A. | RF = FF2 - 1 |
| B. | RF = (FF2 - 1)0.5 |
| C. | RF = FF - 1 |
| D. | RF = (FF - 1)0.5 |
| Answer» C. RF = FF - 1 | |
| 431. |
The number of doped regions in a SCR is |
| A. | 2 |
| B. | 3 |
| C. | 4 |
| D. | 5 |
| Answer» D. 5 | |
| 432. |
As compared to a p-n junction diode |
| A. | a schottky diode has lower cut in voltage and lower reverse saturation current |
| B. | a schottky diode has higher cut in voltage and higher reverse saturation current |
| C. | a Schottky diode has lower cut in voltage and higher reverse saturation current |
| D. | a schottky diode has higher cut in voltage and lower reverse saturation current |
| Answer» D. a schottky diode has higher cut in voltage and lower reverse saturation current | |
| 433. |
In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally |
| A. | equal |
| B. | different |
| C. | equal or different |
| D. | different but sometimes equal |
| Answer» B. different | |
| 434. |
In a thyristor the gate current is increased, then |
| A. | anode current will increase |
| B. | anode current will decrease |
| C. | anode current will remain constant |
| D. | anode current may increase or decrease |
| Answer» D. anode current may increase or decrease | |
| 435. |
In a step down chopper using pulse width modulation, Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping frequency is |
| A. | 333.33 |
| B. | 250 |
| C. | 500 |
| D. | 1000 |
| Answer» C. 500 | |
| 436. |
A chopper |
| A. | converts constant voltage dc into ac and then into variable voltage dc |
| B. | converts constant voltage dc into variable voltage dc directly |
| C. | converts ac of one frequency into ac of another frequency |
| D. | converts ac to dc |
| Answer» C. converts ac of one frequency into ac of another frequency | |
| 437. |
Two thyristors of the same rating and specifications |
| A. | will have equal on state voltage drops |
| B. | will have equal forward blocking voltage for the same current |
| C. | will have different on state voltage drops |
| D. | may have equal or different on state voltage drops and equal or different forward blocking voltage's for the same current |
| Answer» E. | |
| 438. |
Assertion (A): An ideal rectifier should have unity harmonic factorReason (R): An ideal rectifier should have unity displacement factor. |
| A. | Both A and R are correct and R is correct explanation of A |
| B. | Both A and R correct but R is not correct explanation of A |
| C. | A is correct but R is wrong |
| D. | A is wrong but R is correct |
| Answer» E. | |
| 439. |
An RC snubber circuit is used to protect a thyristor against : |
| A. | false triggering |
| B. | failure to turn on |
| C. | switching transients |
| D. | failure to commutate |
| Answer» D. failure to commutate | |
| 440. |
A 3 phase fully controlled converter is a |
| A. | 3 pulse converter |
| B. | 6 pulse converter |
| C. | 2 pulse converter |
| D. | 12 pulse converter |
| Answer» C. 2 pulse converter | |
| 441. |
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 |
| Answer» C. from 0 to 90¬∞ | |
| 442. |
UJT can be used for |
| A. | firing an SCR |
| B. | turning off an SCR |
| C. | both firing and turning off an SCR |
| D. | none of the above |
| Answer» B. turning off an SCR | |
| 443. |
The terminals of a power BJT are called |
| A. | emitter, base, collector |
| B. | emitter, base, drain |
| C. | source, base, drain |
| D. | source, base, collector |
| Answer» B. emitter, base, drain | |
| 444. |
In a half bridge inverter the gate pulses of the two thyristors have a phase displacement of |
| A. | 60° |
| B. | 90° |
| C. | 120° |
| D. | 180° |
| Answer» E. | |
| 445. |
The total number of leads in MOS controlled thyristor is |
| A. | 2 |
| B. | 3 |
| C. | 4 |
| D. | 2 or 3 |
| Answer» C. 4 | |
| 446. |
A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if |
| A. | load torque is constant |
| B. | load torque increases with increase in speed |
| C. | load torque decreases with increase in speed |
| D. | either (a) or (b) |
| Answer» C. load torque decreases with increase in speed | |
| 447. |
In a semi converter feeding a resistive load, the firing angle is a. Each thyristor conducts for |
| A. | p radians |
| B. | (p - a) radians |
| C. | (p + a) radians |
| D. | (p - 0.5 a) radians |
| Answer» C. (p + a) radians | |
| 448. |
In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is |
| A. | p - a |
| B. | p |
| C. | p + a |
| D. | p - 2a |
| Answer» C. p + a | |
| 449. |
An SCR is triggered at 80° in the positive half cycle, only. The rms anode current is 25 A. If the firing angle is changed to 160°, the rms current is likely to be |
| A. | 25 A |
| B. | 12.5 A |
| C. | less than 25 A |
| D. | less than 12.5 A |
| Answer» E. | |
| 450. |
A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is |
| A. | 20 W |
| B. | 50 W |
| C. | 92 W |
| D. | 128 W |
| Answer» C. 92 W | |