# Electric Drives Questions and Answers – Solid-State Devices – Thyristors

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This set of Electric Drives Quiz focuses on “Solid-State Devices – Thyristors”.

1. The value of latching current is more than holding current.
a) True
b) False

Explanation: The latching current is the minimum anode current at which SCR starts conducting. Holding current is the anode below which SCR stops conduction. The latching current is 2-3 times more than the holding current.

2. SCR is a current controlled device.
a) True
b) False

Explanation: SCR is a current controlled device. It can be explained using a two transistor model. When the gate pulse is applied, then it will start conduction.

3. Calculate the value of safety factor using the data: tc=12 msec, tq=6 msec.
a) 4
b) 2
c) 3
d) 6

Explanation: The value of the safety factor is the ratio of the circuit turn-off time and device turn-off time. S.F=tc÷tq=12÷6=2.
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4. For successful commutation circuit turn-off time ______ device turn-off time.
a) <
b) >
c) =
d) ~

Explanation: For successful commutation circuit turn-off time must be greater than the device turn-off time. The excess stored charged carriers should be removed from the gate junction in order to avoid false triggering.

5. SCR is a __________ device.
a) Uncontrolled
b) Semi-controlled
c) Fully controlled
d) Voltage

Explanation: The SCR is a semi-controlled device. Turn-on time can be controlled by using a gate pulse but commutation circuit is required to turn off the device.

6. Holding current of the SCR is ___________
a) > than latching current
b) < than latching current
c) = to latching current
d) ≥ than latching current

Explanation: Holding current of the SCR is less than the latching current. The value of latching current is 2-3 times than holding current.

7. If holding current of an SCR is 6 mA then its latching current should be ___________
a) 4 mA
b) 5 mA
c) 12 mA
d) 6 mA

Explanation: If holding current of an SCR is 6 mA then its latching current should be 12 mA. The latching current is 2-3 times more than the holding current.

8. If latching current of an SCR is 10 mA then its holding current should be ____________
a) 5 mA
b) 7 mA
c) 8 mA
d) 4 mA

Explanation: The latching current is 2-3 times more than the holding current. If a latching current of an SCR is 10 mA then its holding current should be 5 mA. Holding current=(latching current)÷2=10÷2=5 mA.

9. Which of the following is used to protect SCR from overcurrent?
a) Heat sink
b) Fuse
c) Snubber circuit
d) Zener diode

Explanation: Fuse and circuit breakers are used to protect SCR from overcurrent. When current exceeds the certain limit they cut down the supply of current in the circuit.

10. Which of the following is used to protect SCR from thermal conditions?
a) Heat sink
b) Voltage clamping device
c) Zener diode
d) Snubber circuit

Explanation: SCR is mounted on a heat sink. When temperature exceeded the rating of the SCR excess amount of heat is dissipated through the help of the heat sink.

11. Which of the following is used to protect SCR from d(i)/d(t) condition?
a) Inductor
b) Capacitor
c) Heat sink
d) Fuse

Explanation: Inductor is used to protect SCR from d(i)/d(t) condition. The inductor does not allow the sudden change in current.

12. Which of the following is used to protect SCR from d(v)/d(t) condition?
a) Capacitor
b) Fuse
c) Snubber circuit
d) Inductor

Explanation: Snubber circuit is used to protect SCR from d(v)/d(t) condition. Snubber circuit consists of a Capacitor and resistor. The capacitor does not allow the sudden change in voltage and resistor resists the discharging of the capacitor.

13. A snubber circuit is used with thyristor in ____________
a) Parallel
b) Series
c) Anti-parallel
d) Series or Parallel

Explanation: A snubber circuit is used with thyristor in parallel. Snubber circuit is used to protect SCR from d(v)/d(t) condition. Snubber circuit consists of a Capacitor and resistor.

14. When maximum power loss occurs?
a) Rise time
b) Delay time
d) Rise time or Delay time

Explanation: Rise time is very low (1-4 μsec). During rising, time anode voltage and anode current are very high. Power loss(V×I) is very high.

15. Calculate the value of Vpeak if crest factor=47 and Vr.m.s=6 V.
a) 282 V
b) 300 V
c) 290 V
d) 286 V

Explanation: The value of the crest factor is Vpeak÷Vr.m.s. The value of Vpeak is (crest factor) × Vr.m.s = 47×6 = 282 V. It signifies the ratio of Vpeak and Vr.m.s.

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