SCR MCQ | Thyristor MCQ

1. In a thyristor

  1. Latching current IL, is associated with the turn-off process and holding current IH is associated with the turn-on process.
  2. IH is associated with the turn-off process and IL is associated with the turn-on process
  3. Both IL and IH are associated with the turn-off process
  4. Both IL and IH are associated with the turn-on process
Answer
Answer. b

2. Turn off time of an SCR is measured from the instant

  1. Anode current becomes zero
  2. Gate current becomes zero
  3. Anode voltage becomes zero
  4. Anode voltage and current become zero at the same time
Answer
Answer. a

3. The purpose of connecting the snubber circuit across an SCR is to

  1. Suppress dv/dt
  2. Increase dv/dt
  3. Decrease dv/dt
  4. Keep transient overvoltage at a constant value
Answer
Answer. a

4. For normal SCRs, The turn-on time is ________ the turn off time.

  1. Less than
  2. Equal to
  3. More than
  4. About half of
Answer
Answer. a

5. SCRs are connected in order to meet the SCRs

  1. High voltage demand
  2. High current demand
  3. Both high voltage and current demand
  4. Protection against high di/dt
Answer
Answer. a

6. Which one of the following statement is correct?

  1. Holding current is the minimum gate current below which the thyristor stops conducting.
  2. Latching current is the minimum on-state gate current required to keep the thyristor in the on-state.
  3. Holding current is the minimum value of anode current below which the thyristor stops to conduct
  4. Latching current is the minimum current below which the thyristor stops conduction.
Answer
Answer. c

7. In high di/dt protection technique of a thyristor

  1. An inductor is connected in series with SCR
  2. Snubber circuit is provided across SCR
  3. Heat sink is provided in SCR
  4. Varistor is connected across SCR
Answer
Answer. a

8. Which of the following conditions leads to continuous in thyristor?

  1. High value of resistance
  2. High value of inductance or capacitance
  3. High value of inductance
  4. High value of capacitance
Answer
Answer. c

9. In Reverse blocking mode of an SCR

  1. J1, J2 and J3 are reversed biased
  2. J1and J2 are forward biased but J3 is reverse biased
  3. J1 and J3 are reverse biased but J2 is forward biased
  4. J1 is forward biased while J2 and J3 are reverse biased
Answer
Answer. c

10. Which of the following condition must be used for avoiding local hot spots in SCR?

  1. Applying a reverse voltage across SCR
  2. Applying anode current below holding current
  3. Using low (di/dt) value
  4. Applying higher gate current during turn-on time
Answer
Answer. c

11. SCR is a switch of the following kinds?

  1. DC switch
  2. AC switch
  3. Square wave switch
  4. Rectangular wave switch
Answer
Answer. a

12. What is the reason behind considering SCR, a semi-controlled device?

  1. SCR can be turned ON but not OFF with a gate pulse
  2. SCR can only be turned ON during one-half cycle of an alternating current
  3. SCR can be turned off but not ON with a gate pass
  4. SCR can only be turned off during one-half cycle of an alternating current
Answer
Answer. a

13. For an SCR which of the following increases when a resistor is connected across its gate and cathode?

  1. dv/dt rating of SCR
  2. Holding current of SCR
  3. Noise immunity of SCR
  4. Turn off time of SCR
Answer
Answer. c

14. In SCR, if gate current is made half, how will it affect anode current?

  1. Anode current will be half
  2. Anode current will get doubled
  3. Gate current has no effect on anode current
  4. Anode current will become zero
Answer
Answer. c

15. Why does an SCR fail to turn off even when it is in conducting mode and a reverse voltage is applied between anode and cathode?

  1. Positive voltage is applied to the gate
  2. The reverse voltage is small
  3. The anode current is more than the holding current
  4. Turn off time of SCR is large
Answer
Answer. c

16. In SCR, surge current is

  1. Repetitive current with sine wave
  2. Non-repetitive current with rectangular wave
  3. Non-repetitive current with rectangular wave
  4. Repetitive current with rectangular wave
Answer
Answer. c

17. Holding current for an SCR is best-described as

  1. The minimum current required for turn-off
  2. The current required before an SCR will turn on
  3. The amount required to maintain conduction
  4. The gate current required to maintain conduction
Answer
Answer. c

18. Turn on time of an SCR can be reduced by using a

  1. Rectangular pulse of high amplitude and narrow width
  2. Rectangular pulse of low amplitude and wide width
  3. Triangular pulse
  4. Trapezoidal pulse
Answer
Answer. a

19. During forward blocking state, the SCR has

  1. Low current, medium voltage
  2. Low  current, large voltage
  3. Medium current, large voltage
  4. Large current, low voltage
Answer
Answer. b

20. If the gate current of an SCR is increased, the forward breakdown voltage VDR will

  1. Increase
  2. Decrease
  3. Not be affected
  4. Infinite
Answer
Answer. b
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