Power Electronics Questions and Answers – Firing Circuits-3

This set of Power Electronics question bank focuses on “Firing Circuits-3”.

1. For the following RC triggering circuit with R load and a firing angle of α, the voltage across the R load is zero for
The voltage across the R load is zero for ωt = 0 to α & ωt = π to 2π+α
a) ωt = 0 to α and ωt = π to 2π+α
b) ωt = 0 to α
c) ωt = π to 2π+α
d) ωt = α to 2π
View Answer

Answer: a
Explanation: The SCR is triggered at α therefore voltage appears across the load from ωt = α to π only as in case of a half wave firing circuit the SCR is naturally commuted at π.

2. For the following circuit, the SCR (thy2) is
The voltage across the R load is zero for ωt = 0 to α & ωt = π to 2π+α
a) forced commutated at ωt = π
b) forced commutated at ωt = 2π
c) naturally commutated at ωt = π
d) naturally commutated at ωt = 2π
View Answer

Answer: c
Explanation: Circuit is that of a half-wave RC firing circuit where natural commutation takes place at π due to the sinusoidal voltage source.

3. For the following RC triggering circuit with R load and a firing angle of α, the voltage across the SCR (thy2) will be zero for
The voltage across the R load is zero for ωt = 0 to α & ωt = π to 2π+α
a) ωt = 0 to α & ωt = π to 2π+α
b) ωt = 0 to π
c) ωt = α to 2π
d) ωt = α to π
View Answer

Answer: d
Explanation: The SCR is triggered at α therefore voltage appears across the load from ωt = α to π only as in case of a half wave firing circuit the SCR is naturally commuted at π.
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4. In case of an RC full wave firing circuit with R load, the voltage across the load is zero for____________
a) ωt = 0 to α and ωt = π to 2π+α
b) ωt = 0 to α
c) ωt = π to 2π+α
d) ωt = α to 2π
View Answer

Answer: b
Explanation: The SCR is not triggered(turned-on) until α, hence no current flows until α.

5. For an RC full wave firing circuit the empirical formula for calculating the value of RC is
a) RC = 157/ω
b) RC = 157 x ω
c) RC = ω/157
d) RC = 157 x ω2
View Answer

Answer: a
Explanation: RC = 157/ω, Where ω is the angular frequency.
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6. Pulse triggering can be only used by the _____________ type of triggering circuit
a) R
b) RC
c) UJT
d) RLC
View Answer

Answer: c
Explanation: R & RC produce prolonged pulses which increases the gate power dissipation. There is no RLC firing circuit.

7. The UJT terminals are
a) E, B1 and B2
b) E1, E2 and B
c) E, G and C
d) G, S and D
View Answer

Answer: a
Explanation: The terminals are Emitter, Base1 & Base 2.
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8. In case of the UJT firing circuit, when the UJT turns on
a) the capacitor starts to charge
b) the capacitor starts to discharge
c) the capacitor remains unaffected
d) there is no capacitor in a UJT firing circuit
View Answer

Answer: b
Explanation: When the UJT is fired, C starts to discharge.

9. In the UJT firing circuit, the pulses are generated while the
a) capacitor charges
b) capacitor discharges
c) capacitor voltage is zero
d) there is no capacitor in a UJT firing circuit
View Answer

Answer: b
Explanation: When the C starts to discharge through the resistance with time constant RC the UJT turns on & hence pulses are generated.
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10. Find the value of the charging resistor in case of a UJT firing circuit with firing frequency of 2 kHz, C = 0.04 μF, η = 0.72
a) 5.62 kΩ
b) 37 kΩ
c) 4.23 kΩ
d) 9.82 kΩ
View Answer

Answer: d
Explanation: T = 1/f = 1/2 kHz
R = T/C ln(1/1-η).

Sanfoundry Global Education & Learning Series – Power Electronics.

To practice Power Electronics question bank, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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