# Power Electronics Questions and Answers – Gating Circuits

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Gating Circuits”.

1. If the RC firing circuit used for firing an SCR is to be used to fire a TRIAC then
a) the capacitor should be removed
b) the diode should be replaced by a diac
c) the diode should be replaced by a bjt
d) the diode should be shorted using a resistor

Explanation: The TRIAC is a bidirectional SCR, hence it will need gating in both the directions. This can be achieved by replacing the diode by a DIAC (bidirectional diode).

2. In the thyristor gating circuit, the supply to the pulse amplifier is provided by the
a) zcd
b) isolation transformer
c) synchronizing transformer
d) control signal generator

Explanation: Isolation transformer provides the supply to the amplifier and also provides the necessary isolation for the load and triggering circuit.

3. In the thyristor gating circuit, the ZCD is used to
a) amplify the voltage
b) produce a train of pulses
c) convert AC input the ramp voltage
d) used to step-down the voltage

Explanation: It is used to convert the AC synchronizing input voltage into ramp voltage & synchronizes it with the zero crossing of the AC supply.

4. The firing-angle delay is
a) inversely proportional to the synchronizing transformer voltage
b) inversely proportional to the control signal voltage
c) directly proportional to the synchronizing transformer voltage
d) directly proportional to the control signal voltage

Explanation: If Ec is lowered the firing angle decreases & vice-verse.

5. The pulse gating is not suitable of
d) It is suitable of every type of load

Explanation: It is not suitable of RL load because initiation of SCR conduction is not well defined in these types of loads.

6. In case of a cosine firing scheme, __________ is used to get a cosine wave
a) ic 555
b) a comparator
c) an integrator circuit
d) a differentiator circuit

Explanation: The Sync. Transformer is connected to a integrator to obtain a cosine-wave.

7. If the gating circuits generator negative pulses, then those can be removed by using
a) schmit triggers
b) clippers
c) clampers
d) zener diodes

Explanation: The clippers can be used to clip the negative part.

8. The improved version of the UJT oscillator triggering circuit is the
a) ramp & pedal triggering
b) rc triggering
c) cosine-pulse triggering
d) ramp triggering

Explanation: The ramp & pedal triggering is the improved version of the UJT oscillator triggering circuit.

9. RB1 = 3 kΩ & RB2 = 6 kΩ. Find the intrinsic stand-off ratio (η) of the UJT.
a) 9
b) 1/3
c) 2/3
d) 3

Explanation:
η = RB1/(RB1+RB1).

10. The decaying factor in the wave shape of the output pulses from the pulse transformer is its
a) transformer ratio
b) inductance
c) capacitance
d) resistance

Explanation: L is the decaying factor in the waveform which emerges from the PT.

Sanfoundry Global Education & Learning Series – Power Electronics.

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