1.In case of a R firing circuit with Vgp > Vgt
a) α = 90°
b) α > 90°
c) α < 90°
d) α = 0°
Explanation: For the values of Vgp great than the gate triggering voltage the firing angle is less than 90°. And for Vgp = Vgt the firing angle is equal to 90°. Α cannot go beyond 90° in case of a R firing circuit.
2. The figure shown below is that of a
a) R firing circuit
b) RC half-wave firing circuit
c) RC full-wave firing circuit
d) UJT triggering circuit
Explanation: The given circuit is a RC half-wave firing circuit.
3. The figure shown below is that of an RC firing circuit.
In case of negative cycle at Vs, the capacitor C
a) charges through D2 with lower plate negative
b) charges through D1 with lower plate negative
c) charges through D2 with lower plate positive
d) charges through D1 with lower plate positive
Explanation: The current flows through Vs+ – C – D2 – Load – Vs.
4. Find the value of R in case of an RC firing circuit which is to be turned on with a source voltage of 150 V and the following parameters.
Igt = 2A
Vd = 1.5V
Vgt = 125 V
a) 11.75 Ω
b) 54.25 Ω
c) 96 Ω
d) 5 Ω
Explanation:
R = (Vs-Vgt-Vd)/Igt.
5. 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).
6. 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.
7. 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.
8. 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.
9. The pulse gating is not suitable of
a) R loads
b) RC loads
c) RL loads
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.
10. 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.