# 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

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

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

a) forced commutated at ωt = π
b) forced commutated at ωt = 2π
c) naturally commutated at ωt = π
d) naturally commutated at ωt = 2π

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

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

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.

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

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

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

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

Explanation: The terminals are Emitter, Base1 & Base 2.

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

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

Explanation: When the C starts to discharge through the resistance with time constant RC the UJT turns on & hence pulses are generated.

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

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

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