# Power Electronics Questions and Answers – Thyristors-2

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Thyristors – 2”.

1. The forward break over voltage is the
a) anode-cathode voltage at which conduction starts with gate signal applied
b) anode-cathode voltage at which conduction starts with no gate signal applied
c) gate voltage at which conduction starts with no anode-cathode voltage
d) gate voltage at which conduction starts with anode-cathode voltage applied

Explanation: It is the forward voltage at which the middle junction breaks down without any gate signal and pushes the device into the conducting state.

2. For a forward conducting SCR device, as the forward anode to cathode voltage is increased
a) the device turns on at higher values of gate current
b) the device turns on at lower values of gate current
c) the forward impedance of the device goes on increasing
d) the forward impedance of the device goes on decreasing

Explanation: Higher the value of anode-cathode forward voltage, lower the gate requirements of the device. Also, the forward resistance of the device is always constant as long as the junction temperature is constant.

3. A thyristor can be bought from the forward conduction mode to forward blocking mode by
a) the dv/dt triggering method
b) applying a negative gate signal
c) applying a positive gate signal
d) applying a reverse voltage across anode-cathode terminals

Explanation: a) & c) are used to turn on the device, b) will damage the SCR.

4. Usually the forward voltage triggering method is not used to turn-on the SCR because
a) it increases losses
b) it causes noise production
c) it may damage the junction & destroy the device
d) relatively it’s an inefficient method

Explanation: In forward voltage triggering the middle junction breaks down without any gate signal and pushes the device into the conducting state. This method can permanently damage the J2 junction and make the device useless.

5. Among the following, the most suitable method to turn on the SCR device is the
a) gate triggering method
b) dv/dt triggering method
c) forward voltage triggering method
d) temperature triggering method

Explanation: d) & b) are unreliable methods, c) can permanently damage the SCR
Gate triggering is simple, reliable & most efficient.
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6. The forward break over voltage is maximum when
a) Gate current = ∞
b) Gate current = 0
c) Gate current = -∞
d) It is independent of gate current

Explanation: Higher the value of anode-cathode forward voltage, lower the gate requirements of the device.

7. For the SCR to remain in the ON (conducting) state
a) gate signal is continuously required
b) no continuous gate signal is required
c) no forward anode-cathode voltage is required
d) negative gate signal is continuously required

Explanation: Unlike the transistor devices, once the SCR is turned on by the gate terminal, the gate terminal losses its control over the device.

8. The value of anode current required to maintain the conduction of an SCR even though the gate signal is removed is called as the
a) holding current
b) latching current
c) switching current
d) peak anode current

Explanation: It is the minimum anode current value required to maintain the conduction of an SCR even though the gate signal is removed. It is a very important parameter when employing an SCR in any circuit.

9. In the reverse blocking mode the middle junction (J2) has the characteristics of that of a
a) transistor
b) capacitor
c) inductor
d) none of the mentioned

Explanation: It is like a capacitor, as the dv/dt voltage triggering turns on the device. The charging current is given by,
IC = CjdVa/dt.

10. ________ are semiconductor thyristor devices which can be turned-on by light of appropriate wavelengths.
a) LGTOs
b) LASERs
c) MASERs
d) LASCRs

Explanation: LASCR stands for light activated SCRs, which can be turned on in made to conduct by firing appropriate light pulses at its gate region.

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