21. 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
Answer: A
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.
22. Which of the following is used to protect SCR from overcurrent?
A. Heat sink
B. Fuse
C. Snubber circuit
D. Zener diode
Answer: B
Fuse and circuit breakers are used to protect SCR from overcurrent. When current exceeds a certain limit it cut down the supply of current in the circuit.
23. 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
Answer: A
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.
24. Which of the following is used to protect SCR from d(i)/d(t) condition?
A. Inductor
B. Capacitor
C. Heat sink
D. Fuse
Answer: A
Inductor is used to protect SCR from d(i)/d(t) condition. The inductor does not allow the sudden change in current.
25. Which of the following is used to protect SCR from d(v)/d(t) condition?
A. Capacitor
B. Fuse
C. Snubber circuit
D. Inductor
Answer: C
Snubber circuit is used to protect SCR from d(v)/d(t) condition. The snubber circuit consists of a capacitor and resistor. The capacitor does not allow the sudden change in voltage and the resistor resists the discharging of the capacitor.
26. A snubber circuit is used with thyristor in ____________
A. Parallel
B. Series
C. Anti-parallel
D. Series or Parallel
Answer: A
A snubber circuit is used with a thyristor in parallel. Snubber circuit is used to protect SCR from d(v)/d(t) condition. The snubber circuit consists of a capacitor and resistor.
27. When maximum power loss occurs?
A. Rise time
B. Delay time
C. Spread time
D. Rise time or Delay time
Answer: A
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.
28. 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
Answer: A
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.
29. Which one of the following loads is suitable for load commutation in a Single-phase Half-bridge inverter?
A. R-L load
B. R-L-C overdamped
C. R-L-C underdamped
D. L-load
Answer: C
R-L-C underdamped are leading power factor loads. They do not require any forced commutation technique. Anti-Parallel diodes help in the commutation process.
30. R-L-C overdamped loads are generally lagging power factor loads.
A. True
B. False
Answer: A
R-L-C overdamped loads are generally lagging power factor loads. They require forced commutation. Anti-Parallel diodes do not help in the commutation process.
31. A step-down chopper has an input voltage of 40 V and output voltage of 20 V. Calculate the value of the duty cycle.
A. 0.5
B. 0.2
C. 0.3
D. 0.4
Answer: A
The output voltage of the step-down chopper is Vo = Vin×(D).
The value of the duty cycle is less than one which makes the Vo < Vin. The step-down chopper is used to step down the voltage.
