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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.
| 451. |
Figure shows a chopper feeding RLE load, The free wheeling diode conducts when |
| A. | thyristor is on |
| B. | thyristor is off |
| C. | both when thyristor is on and thyristor is off |
| D. | partly when thyristor is off and partly when thyristor is on |
| Answer» C. both when thyristor is on and thyristor is off | |
| 452. |
In brushless excitation system of modern alternators, the main exciter is |
| A. | dc generator |
| B. | ac generator |
| C. | either dc or ac generator |
| D. | induction generator |
| Answer» C. either dc or ac generator | |
| 453. |
In a three phase series inverter, the firing frequency is |
| A. | three times output frequency |
| B. | six times output frequency |
| C. | twice the output frequency |
| D. | equal to output frequency |
| Answer» C. twice the output frequency | |
| 454. |
A single phase fully controlled bridge converter is feeding an RLE load. Input voltage is Vm sin ωt and firing angle is a. If average current is I0, then |
| A. | cos a = RI0 + E |
| B. | cos a = RE0 + E |
| C. | cos a = E0 - RI0 |
| D. | cos a = E - RI0 |
| Answer» B. cos a = RE0 + E | |
| 455. |
Which statement is correct? |
| A. | BJT and MOSFET are current controlled devices |
| B. | BJT is voltage controlled and MOSFET is current controlled device |
| C. | BJT and MOSFET are voltage controlled devices |
| D. | BJT is current controlled and MOSFET is voltage controlled device |
| Answer» E. | |
| 456. |
In a single phase half wave ac regulator feeding a resistive load, the rms load current |
| A. | is always positive |
| B. | may be positive or negative |
| C. | is always negative |
| D. | can be even zero |
| Answer» B. may be positive or negative | |
| 457. |
Thyristors are suitable for dc circuit breakers but not for ac circuit breakers. |
| A. | 1 |
| B. | |
| Answer» C. | |
| 458. |
In rectifier circuits, the rms value of ac component of output voltage is equal to |
| A. | Vrms |
| B. | 0.5 Vrms |
| C. | (Vrms - Vdc)0.5 |
| D. | (Vrms2 - Vdc2)0.5 |
| Answer» E. | |
| 459. |
In integral cycle control of ac regulator, the input power factor is equal to |
| A. | duty cycle a |
| B. | a2 |
| C. | a |
| D. | (a)1/3 |
| Answer» D. (a)1/3 | |
| 460. |
If a step down chopper is fed by a dc voltage V with Ton and Toff are on and off periods then is |
| A. | average output voltage |
| B. | rms output voltage |
| C. | average or rms output voltage depending on load |
| D. | neither of the above |
| Answer» B. rms output voltage | |
| 461. |
The total number of leads in SUS, SBS and SCS respectively are |
| A. | 3, 3, 4, |
| B. | 3, 3, 3 |
| C. | 2, 3, 3 |
| D. | 3, 3, 5 |
| Answer» B. 3, 3, 3 | |
| 462. |
A cycloconverter cannot give a frequency higher than input frequency. |
| A. | 1 |
| B. | |
| Answer» C. | |
| 463. |
McMurrary full bridge inverter uses |
| A. | auxiliary commutation |
| B. | complementary commutation |
| C. | natural commutation |
| D. | any of the above |
| Answer» B. complementary commutation | |
| 464. |
A single phase half wave controlled rectifier circuit has an R-L load. A freewheeling diode is also in the circuit. When freewheeling diode is conducting the SCR |
| A. | is forward biased |
| B. | is reverse biased |
| C. | may be forward biased or reverse biased |
| D. | forward biased initially but reverse biased afterwards |
| Answer» C. may be forward biased or reverse biased | |
| 465. |
Assertion (A): A class C chopper can operate in first and second quadrants Reason (R): A class B chopper operates only in second quadrant. |
| 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» C. A is correct but R is wrong | |
| 466. |
In a thyristor the gate lead is connected to the n layer near the p layer to which anode is connected. |
| A. | 1 |
| B. | |
| Answer» C. | |
| 467. |
In a series inverter |
| A. | the commutating elements carry full load current |
| B. | the commutating elements carry upto 50% of load current |
| C. | the commutating elements do not carry load current |
| D. | any of the above |
| Answer» B. the commutating elements carry upto 50% of load current | |
| 468. |
A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about |
| A. | 127 V |
| B. | 120 V |
| C. | 110 V |
| D. | 100 V |
| Answer» C. 110 V | |
| 469. |
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 upto four times in each load cycle |
| Answer» B. each thyristor is turned on once during each cycle | |
| 470. |
Assertion (A): Power transistor has lower switching time than SCR Reason (R): Power transistor has greater switching time than SCR |
| 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. | |
| 471. |
In a single phase dual converter, a1, and a2 are firing angles of the two converters. Then |
| A. | a1 - a2 = |
| B. | a1 + a2 = p |
| C. | a1 - a2 = p |
| D. | a1 + a2 = |
| Answer» C. a1 - a2 = p | |
| 472. |
The rated current of a thyristor is 40 A. The holding current will be about : |
| A. | 20 A |
| B. | 0.041666666666667 |
| C. | 100 mA |
| D. | 10 mA |
| Answer» E. | |
| 473. |
A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be |
| A. | 6 μs |
| B. | less than 6μs |
| C. | more than 6 μs |
| D. | either 6 μs or less |
| Answer» D. either 6 Œºs or less | |
| 474. |
If duty cycle is a and the load is a resistance R, the rms value of output voltage is |
| A. | aV |
| B. | a V |
| C. | a2V |
| Answer» C. a2V | |
| 475. |
A thyristor circuit is feeding an R-L load. The turn on time can be reduced by |
| A. | decreasing R |
| B. | decreasing L |
| C. | increasing L |
| D. | decreasing R and L together |
| Answer» C. increasing L | |
| 476. |
In a multiphase chopper, all choppers operate together. |
| A. | 1 |
| B. | |
| Answer» C. | |
| 477. |
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 |
| Answer» E. | |
| 478. |
A single phase semiconverter is feeding a highly inductive load and has a freewheeling diode across the load. The waveshape of input current is |
| A. | sinusoidal |
| B. | rectangular |
| C. | sinusoidal or rectangular |
| D. | triangular |
| Answer» C. sinusoidal or rectangular | |
| 479. |
A single phase semiconverter has a freewheeling diode. If the firing angle is a and the load is purely resistive, the periods of conduction and freewheeling respectively are |
| A. | p - a and 0 |
| B. | p - a and a |
| C. | p + a and 0 |
| D. | p + a and a |
| Answer» B. p - a and a | |
| 480. |
Assertion (A): Thyristors can be used in controlled heating, excitation systems of alternators, speed control of motors and HVDCReason (R): A static var system using thyristors is very commonly used in high voltage ac systems. |
| 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» C. A is correct but R is wrong | |
| 481. |
In a 3 phase semiconverter, the output voltage becomes discontinuous for |
| A. | firing angle > 30° |
| B. | firing angle > 60° |
| C. | firing angle > 90° |
| D. | firing angle > 120° |
| Answer» C. firing angle > 90¬∞ | |
| 482. |
In a certain inverter circuit each thyristor carries 26.67 A for 60°, 13.33 A for 120° and zero A for 180° of the cycle. The rms current of thyristor is |
| A. | 20 A |
| B. | 13.33 A |
| C. | 6.67 A |
| D. | 3.33 A |
| Answer» C. 6.67 A | |
| 483. |
A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20°C. If thermal resistance is 1.6° C/W, the junction temperature is |
| A. | 114°C |
| B. | 164°C |
| C. | 94° C |
| D. | 84° C |
| Answer» E. | |
| 484. |
The total number of leads in SCR, DIAC and TRIAC respectively are |
| A. | 3, 2, 3 |
| B. | 2, 3, 3 |
| C. | 3, 3, 2 |
| D. | 3, 2, 4 |
| Answer» B. 2, 3, 3 | |
| 485. |
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 |
| Answer» D. 4 | |
| 486. |
HVDC transmission systems generally use |
| A. | 12 pulse converters |
| B. | 6 pulse converters |
| C. | 3 pulse converters |
| D. | either 12 or 6 pulse converters |
| Answer» B. 6 pulse converters | |
| 487. |
For a 12 pulse operation, the two 6 pulse units are fed by |
| A. | Y - Y and Y - Y transformers |
| B. | Δ - Δ and Δ - Δ transformers |
| C. | Y - Y and Y - Δ transformers |
| D. | Y - Δ and Y - Δ transformers |
| Answer» D. Y - Œî and Y - Œî transformers | |
| 488. |
The number of thyristors in a single phase fully controlled bridge rectifier is |
| A. | 16 |
| B. | 8 |
| C. | 4 |
| D. | 2 |
| Answer» D. 2 | |
| 489. |
Lead mounting for SCR is used for |
| A. | rating less than mA |
| B. | rating about 1 A or less |
| C. | rating of 20 A or less than 20 A |
| D. | rating 100 A or less |
| Answer» C. rating of 20 A or less than 20 A | |
| 490. |
A single phase full wave regulator feeds R-L load. The best gating signal is |
| A. | short duration pulses |
| B. | long duration pulses |
| C. | pulse train |
| D. | either (a) or (b) |
| Answer» D. either (a) or (b) | |
| 491. |
Thyristors are not suitable for logic circuits. |
| A. | 1 |
| B. | |
| Answer» C. | |
| 492. |
A single phase half wave converter is feeding a resistive load. Output dc power is Pdc and total power delivered to load is Pac then |
| A. | Pdc = Pac for all values of firing angles |
| B. | Pdc = Pac if firing angle is less than 180° |
| C. | Pdc < Pac for all values of firing angles |
| D. | Pdc < Pac if firing angle is less than 180° |
| Answer» E. | |
| 493. |
When a power diode is conducting, the voltage drop across it is about |
| A. | I V |
| B. | 5 V |
| C. | 20 V |
| D. | 50 V |
| Answer» B. 5 V | |
| 494. |
The specified maximum junction temperature of thyristors is about |
| A. | 50°C |
| B. | 80°C |
| C. | 120°C |
| D. | 200°C |
| Answer» D. 200¬∞C | |
| 495. |
The average load current supplied by a thyristor depends on |
| A. | firing angle |
| B. | firing frequency |
| C. | magnitude of gate current |
| D. | all of the above |
| Answer» B. firing frequency | |
| 496. |
The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, v becomes |
| A. | about 100 times |
| B. | about 10 times |
| C. | about 3 times |
| D. | about twice |
| Answer» D. about twice | |
| 497. |
A fully controlled bridge converter is feeding an RLE load. The load current is I0. The input voltage is Vm sin ωt. The KVL equation to find firing angle a is |
| A. | cos a = E + I0R |
| B. | cos a = E + I0R |
| C. | cos a = E - I0R |
| D. | cos a = E - I0R |
| Answer» B. cos a = E + I0R | |
| 498. |
A DIAC has two terminals, anode and cathode. |
| A. | 1 |
| B. | |
| Answer» C. | |
| 499. |
A single phase half wave rectifier is feeding a resistive load. As firing angle is increased |
| A. | dc output voltage and rms output voltage decrease in the same proportion |
| B. | dc output voltage and rms output voltage decrease |
| C. | dc output voltage and rms output voltage increase |
| D. | dc output voltage and rms output voltage increase in the same proportion |
| Answer» C. dc output voltage and rms output voltage increase | |
| 500. |
In a single phase full wave ac regulator using two thyristors only, the gate circuits of the two thyristors |
| A. | must be isolated |
| B. | may or may not be isolated |
| C. | should be preferably, isolated |
| D. | should not be isolated |
| Answer» B. may or may not be isolated | |