MCQOPTIONS
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MCQOPTIONS
This section includes 82 Mcqs, each offering curated multiple-choice questions to sharpen your Power Electronics knowledge and support exam preparation. Choose a topic below to get started.
| 1. |
SCRs with a rating of 1000 V & 200 A are available to be used in a string to handle 6 KV & 1 KV. Calculate the number of series & parallel units required in case the de-rating factor is 0.1. (Round off the fraction to the greatest & nearest integer) |
| A. | Series = 7, Parallel = 6 |
| B. | Series = 6, Parallel = 7 |
| C. | Series = 6, Parallel = 6 |
| D. | Series = 7, Parallel = 7 |
| Answer» B. Series = 6, Parallel = 7 | |
| 2. |
Two parallel connect SCRs have same voltage drop (Vt) having rated current = 2I1. SCR-1 carries a current of I1=2.6 A whereas SCR-2 carries a current of I2=1.4 A. Find the string efficiency. |
| A. | 45 % |
| B. | 77 % |
| C. | 92 % |
| D. | 84 % |
| Answer» C. 92 % | |
| 3. |
Inversion failure in naturally commutated circuits used for motor loads leads to |
| A. | High voltage across thyristor |
| B. | High fault current |
| C. | Low power factor |
| D. | Low fault current |
| Answer» C. Low power factor | |
| 4. |
A unijunction transistor (UJT) is used to generate a ____ wave |
| A. | square |
| B. | sine |
| C. | sawtooth |
| D. | triangle |
| Answer» D. triangle | |
| 5. |
A triac is a _____ switch. |
| A. | Unidirectional |
| B. | Mechanical |
| C. | Bidirectional |
| D. | reverse directional |
| Answer» D. reverse directional | |
| 6. |
Match List-I with List-II and select the correct answer using the code given below the Lists:List - IList – IIA. di / dt rating limits1. Snubber circuitB. dv / dt rating limits2. heat sinkC. i2t limit3. Series reactorD. Junction temperature limit4. Fuse |
| A. | A – 2, B – 4, C – 1, D - 3 |
| B. | A – 3, B – 4, C – 1, D - 2 |
| C. | A – 2, B – 1, C – 4, D - 3 |
| D. | A – 3, B – 1, C – 4, D - 2 |
| Answer» E. | |
| 7. |
_______is a 5-layer bi-directional device which can be triggered into conduction by both, positive and negative voltage at its anodes and with both, positive and negative triggering pulses at its gate |
| A. | SCR |
| B. | TRIAC |
| C. | UJT |
| D. | DIAC |
| Answer» C. UJT | |
| 8. |
A unijunction transistor (UJT) is used to ________ |
| A. | boost the SCR |
| B. | amplify DC |
| C. | boost and trigger TRIAC |
| D. | trigger the SCR |
| Answer» E. | |
| 9. |
An SCR has \({V_{DRM}} = 600\;V,\;{\left( {\frac{{dv}}{{dt}}} \right)_{max}} = 25\frac{V}{{\mu s}},\;and\;{\left( {\frac{{di}}{{dt}}} \right)_{max}} = 30\;A/\mu s\). It is used to energize a 100 Ω resistance load. What are the minimum values for an RC snubber circuit to avoid unintentional triggering? (VDRM means peak repetitive forward blocking voltage) |
| A. | T = 10 μs, C = 2.4 μF and R = 4.5 mΩ |
| B. | T = 24 μs, C = 0.24 μF and R = 0.833 Ω |
| C. | T = 24 μs, C = 2.4 μF and R = 7.0 mΩ |
| D. | T = 10 μs, C = 0.24 μF and R = 7.0 mΩ |
| Answer» C. T = 24 μs, C = 2.4 μF and R = 7.0 mΩ | |
| 10. |
Consider the following statements:SCR can be turned on by1. Applying anode voltage at a sufficiently fast rate2. Applying sufficiently large anode voltage3. decreasing the temperature of SCR4. Applying sufficiently large gate currentWhich of the above statements are correct? |
| A. | 1, 2 and 3 |
| B. | 1, 3 and 4 |
| C. | 1, 2 and 4 |
| D. | 2, 3 and 4 |
| Answer» D. 2, 3 and 4 | |
| 11. |
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. | |
| 12. |
After firing an SCR, the gating pulse is removed. Then the current in the SCR will |
| A. | remain the same |
| B. | immediately fall to zero |
| C. | rise up |
| D. | rise a little and then fall to zero |
| Answer» B. immediately fall to zero | |
| 13. |
For an SCR, during turn-on and turn-off, the quantities responsible, respectively are |
| A. | gate voltage and gate current |
| B. | holding current and latching current |
| C. | latching current and holding current |
| D. | forward break over voltage and reverse break over voltage |
| Answer» D. forward break over voltage and reverse break over voltage | |
| 14. |
A thyristor can be termed as |
| A. | DC switch |
| B. | AC switch |
| C. | Either (1) or (2) |
| D. | Square wave switch |
| Answer» B. AC switch | |
| 15. |
Identify the symbol |
| A. | SCR |
| B. | PUT |
| C. | LASCR |
| D. | Diac |
| Answer» B. PUT | |
| 16. |
AN SCR requires 50 mA gate current to switch it ON. The driver circuit supply voltage is 10 V. The gate-cathode drop is about 1 V and the voltage drop across diode is also 1 V. The resistive load supplied from a 100 V supply is |
| A. | 66 Ω |
| B. | 73 Ω |
| C. | 80 Ω |
| D. | 100 Ω |
| Answer» D. 100 Ω | |
| 17. |
dV / dt protection is provided to thyristor circuit by using |
| A. | triggering circuits |
| B. | phase shifting circuits |
| C. | commutation circuits |
| D. | snubber circuits |
| Answer» E. | |
| 18. |
Practical way of obtaining static voltage equalization in series connected SCRs by the use of: |
| A. | Resistors connected in series with SCR |
| B. | Resistors of same value across each SCR |
| C. | One resistor across the string |
| D. | Resistors of different values across each SCR |
| Answer» C. One resistor across the string | |
| 19. |
In a thyristor: |
| A. | Latching current is associated with turn-off process |
| B. | Holding current is associated with turn-on process |
| C. | Both latching and holding currents are associated with turn-off proces |
| D. | Latching current is associated with turn-on process |
| Answer» E. | |
| 20. |
Assertion (A): thyristor is a switching device. It is required to turn it ON and OFF according to the requirement.Reason (R): During reverse conduction, gate current is to be supplied through the gate in SCRs |
| A. | Both (A) and (R) are true and (R) is the correct explanation of (A). |
| B. | Both (A) and (R) are true, but (R) is not the correct explanation of (A) |
| C. | (A) is true, but (R) is false |
| D. | (A) false, but (R) is true |
| Answer» D. (A) false, but (R) is true | |
| 21. |
In the forward blocking region SCR is |
| A. | reverse biased |
| B. | in ON state |
| C. | in OFF state |
| D. | at point of breakdown |
| Answer» D. at point of breakdown | |
| 22. |
A triac operating at a voltage of 120 V rms and a frequency of 60 Hz delivers power to a resistive load. The maximum rate of change of current \(\left(\frac {di}{dt}\right)\) expected is 50 A / μs. The required snubber inductor value is |
| A. | 2.4 μH |
| B. | 4.4√2 μH |
| C. | 4.4√π μH |
| D. | 4.4 π μH |
| Answer» B. 4.4√2 μH | |
| 23. |
Consider the following statements:1. In a silicon controlled rectifier (SCR), if the cathode gate is reverse-biased, then the SCR cannot fire at all.2. The turn-on time of an SCR increases with temperature.3. After an SCR is turned on, it can be made to turn off again by reverse biasing the gate.4. Gate recovery time is the minimum time that the anode voltage must be maintained below holding voltage VH to turn off the SCR.Which of these statements are correct? |
| A. | 1, 2, 3 and 4 |
| B. | 1 and 2 only |
| C. | 2 and 4 only |
| D. | 3 and 4 only |
| Answer» C. 2 and 4 only | |
| 24. |
An SCR can be turned off |
| A. | by passing a negative pulse to its gate |
| B. | by removing the gate supply |
| C. | by reverse biasing it |
| D. | by forcing the current through gate to become zero |
| Answer» D. by forcing the current through gate to become zero | |
| 25. |
Consider the following statements regarding Thyristor:1. It conducts when forward biased and positive current flows through the gate2. It conducts when forward biased and negative current flows through the gate3. It commutates when reverse biased and negative current flows through the gate4. It commutates when the gate current is withdrawnWhich of these statement(s) is/are correct? |
| A. | 1, 2 and 3 |
| B. | 1 and 2 only |
| C. | 2 and 3 only |
| D. | 1 only |
| Answer» E. | |
| 26. |
In an SCR with four layers represented as shown in the given figure, which are the layers that are equally doped? |
| A. | P1 and P2 |
| B. | P2 and N2 |
| C. | P1 and N1 |
| D. | N1 and N2 |
| Answer» B. P2 and N2 | |
| 27. |
In the reverse blocking mode, the middle junction(J2) has the characteristics of that of a: |
| A. | None of the mentioned |
| B. | Inductor |
| C. | Transistor |
| D. | Capacitor |
| Answer» E. | |
| 28. |
Commutation circuitry is an extra circuit used to turn off |
| A. | Line-commutated thyristors |
| B. | Phase-commutated thyristors |
| C. | Forced-commutated thyristors |
| D. | Reverse-commutated thyristors |
| Answer» D. Reverse-commutated thyristors | |
| 29. |
Identify the circuit based on the working principle. |
| A. | Fully-controlled rectifier |
| B. | semi-controlledd rectifier |
| C. | A.C regulator |
| D. | Controlled inverter |
| Answer» D. Controlled inverter | |
| 30. |
In normal operation of an SCR, gate is ________ with respect to cathode. |
| A. | Positive |
| B. | Negative |
| C. | At zero potential |
| D. | None of the above |
| Answer» B. Negative | |
| 31. |
In an SCR circuit, the angle of conduction can be changed by ______ |
| A. | Changing anode voltage |
| B. | Changing gate voltage |
| C. | Reverse biasing the gate |
| D. | None of the above |
| Answer» C. Reverse biasing the gate | |
| 32. |
______ Limits the dv/dt across the switching device during ______. |
| A. | Snubber, turnoff of the device |
| B. | Inductor, turnoff of the device |
| C. | Inductor, normal running conditions |
| D. | Snubber, normal running conditions |
| Answer» B. Inductor, turnoff of the device | |
| 33. |
For low power applications a GTO has: |
| A. | Low on-state gain |
| B. | Low ratio of peak surge current to average current |
| C. | Higher blocking voltage capability |
| D. | Low ratio of peak controllable current to average current |
| Answer» D. Low ratio of peak controllable current to average current | |
| 34. |
A silicon-controlled rectifier (SCR) conducts appreciable current when its _______ with respect to the cathode. |
| A. | anode is negative and gate is positive |
| B. | anode and gate are both negative |
| C. | anode and gate are both positive |
| D. | gate is negative and anode is positive |
| Answer» D. gate is negative and anode is positive | |
| 35. |
Due to low internal generation in GTO, the GTO has |
| A. | Lower latching current |
| B. | Lower holding current |
| C. | Lower latching and holding current |
| D. | Higher latching and holding current |
| Answer» E. | |
| 36. |
For effective turning off of the SCR after the anode current has reached zero value, _________. |
| A. | None of these |
| B. | Chargers are removed by applying reverse anode-cathode voltage |
| C. | Chargers are injected by applying reverse anode-cathode voltage |
| D. | Chargers are injected by applying a gate signal |
| Answer» C. Chargers are injected by applying reverse anode-cathode voltage | |
| 37. |
Presence of gold in thyristor |
| A. | reduces minority carrier life time and increases leakage current |
| B. | reduces minority carrier life time and reduces leakage current |
| C. | increases minority carrier life time and reduces leakage current |
| D. | increases minority carrier life time and increases leakage current |
| Answer» B. reduces minority carrier life time and reduces leakage current | |
| 38. |
_______ anode current waveforms will have the minimum junction temperature. |
| A. | AC |
| B. | 100% DC |
| C. | 25% DC |
| D. | 50% DC |
| Answer» D. 50% DC | |
| 39. |
Calculate the anode current for a two-transistor analogy of SCR.Given: Gain of pnp transistor = 0.4, gain of npn transistor = 0.5, gate current = 60 mA |
| A. | 150 mA |
| B. | 250 mA |
| C. | 200 mA |
| D. | 300 mA |
| Answer» E. | |
| 40. |
Natural commutation means |
| A. | reducing the thyristor current to zero |
| B. | reducing the thyristor current below “holding current” |
| C. | reducing the current to zero and allowing time to recover thyristor junctions |
| D. | none of (1), (2) and (3) |
| Answer» D. none of (1), (2) and (3) | |
| 41. |
Identify the given symbol. |
| A. | Photo thyristor |
| B. | Sidac |
| C. | Triac |
| D. | Diac |
| Answer» D. Diac | |
| 42. |
By using ____false triggering of the SCRs by varying flux & noise is avoided |
| A. | F.A.C.L.F & C.B |
| B. | Shielded cables & twisted gate leads |
| C. | Snubber circuits |
| D. | di/dt inductor in series with the gate terminal |
| Answer» C. Snubber circuits | |
| 43. |
Consider the two-transistor analogy of SCR, if α1 & if α2 are the common-base current gains of both the transistors then to turn-on the device: |
| A. | α1 – α2 should approach zero |
| B. | α1 + α2 should approach zero |
| C. | α1 + α2 should approach unity |
| D. | α1 x α2 should approach unity |
| Answer» D. α1 x α2 should approach unity | |
| 44. |
If the cathode of Thyristor is made +ve with respect to the anode & no gate current is applied, then: |
| A. | Only the middle junction is forward biased |
| B. | Only the middle junction is reversed biased |
| C. | All the junctions are forward biased |
| D. | All the junctions are reversed biased |
| Answer» B. Only the middle junction is reversed biased | |
| 45. |
An SCR consists of: |
| A. | Three PN junction |
| B. | One PN junction |
| C. | Two PN junction |
| D. | No PN junction |
| Answer» B. One PN junction | |
| 46. |
_______ exhibit a regenerative switching characteristic. |
| A. | Thermistors |
| B. | Thyristors |
| C. | Resistors |
| D. | Diodes |
| Answer» C. Resistors | |
| 47. |
How can the 3rd harmonic current be filtered in Thyristor-controlled reactor? |
| A. | By connecting in delta |
| B. | By connecting in star |
| C. | By connecting in star-delta |
| D. | None of the above |
| Answer» B. By connecting in star | |
| 48. |
Match the elements of List I and List II with respect to SCR triggering techniquesList I(Triggering Technique )List II (Operation)a) High Voltagei) A light wave strikes the junction of SCRb) dV/dTii) A positive gate voltage is applied between the gate and cathode terminalsc) Gate Currentiii) Rate of rise of anode-cathode voltage is highd) Lightiv) Anode to cathode voltage is greater than forward break down voltage |
| A. | (a) – (iv), (b) – (iii), (c) – (ii), (d) – (i) |
| B. | (a) – (iii), (b) – (ii), (c) – (i), (d) – (iv) |
| C. | (a) – (ii), (b) – (iii), (c) – (i), (d) – (iv) |
| D. | (a) – (i), (b) – (iv), (c) – (ii), (d) – (iii) |
| Answer» B. (a) – (iii), (b) – (ii), (c) – (i), (d) – (iv) | |
| 49. |
In a GTO, anode current begins to fall when gate current. |
| A. | is negative peak at time t = 0 |
| B. | is negative peak at t = storage period ts. |
| C. | just begins to become negative at t = 0 |
| D. | just begins to become positive at t = 0 |
| Answer» C. just begins to become negative at t = 0 | |
| 50. |
It is preferable to use a train of pulse of high frequency for gate triggering of SCR in order to reduce |
| A. | dv/dt problem |
| B. | di/dt problem |
| C. | the size of the pulse transformer |
| D. | the complexity of the firing circuit |
| Answer» D. the complexity of the firing circuit | |