1. The value of the latching current is more than the holding current.
A. True
B. False
Answer: A
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
Answer: A
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 conducting.
3. Calculate the value of the safety factor using the data: tc=12 msec, tq=6 msec.
A. 4
B. 2
C. 3
D. 6
Answer: B
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.
4. For successful commutation circuit turn-off time ______ device turn-off time.
A. <
B. >
C. =
D. ~
Answer: B
For a successful commutation circuit, the 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
Answer: B
The SCR is a semi-controlled device. Turn-on time can be controlled by using a gate pulse but a 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
Answer: B
The holding current of the SCR is less than the latching current. The value of latching current is 2-3 times that of holding current.
7. If the 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
Answer: C
If the 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
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.
9. 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.
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
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.
