Thyristor Characteristics and Triggering Questions and Answers

Q: In a SCR circuit, the angle of conduction can be changed by changing :

The conduction angle is changed by changing the gate current.An SCR blocks forward voltage until a gate current pulse triggers it, after which it conducts until its current falls below the holding current. Varying the gate current (or the timing of the gate pulse) changes the firing angle at which the SCR turns on in each cycle, and so the conduction angle and the average power delivered to the load. This is phase control.The anode voltage is the supply waveform being controlled. The anode current is set by the load once the SCR conducts. The forward current rating is a fixed device specification, not a control variable.

21.

In a SCR circuit, the angle of conduction can be changed by changing :

A.anode voltage
B.anode current
C.forward current rating
D.gate current

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: D

The conduction angle is changed by changing the gate current.

An SCR blocks forward voltage until a gate current pulse triggers it, after which it conducts until its current falls below the holding current. Varying the gate current (or the timing of the gate pulse) changes the firing angle at which the SCR turns on in each cycle, and so the conduction angle and the average power delivered to the load. This is phase control.

The anode voltage is the supply waveform being controlled. The anode current is set by the load once the SCR conducts. The forward current rating is a fixed device specification, not a control variable.

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22.

When the current through SCR is greater than holding current, then the voltage across the SCR is

A.1.4 V
B.0 V
C.Supply voltage
D.Half the load voltage

Exam Year(s)

UGC NET - 2012,

Answer: A

Once an SCR is conducting with a current above the holding current, it stays on and the voltage across it is the small forward drop, about 1.4 V (option A), a little above one volt at the usual currents.

Zero volts (B) would be an ideal switch, but the junctions of a real SCR keep a drop of a volt or more. The supply voltage (C) appears across the SCR only in the forward-blocking state, before it is triggered, and half the load voltage (D) is not a state of the device.

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23.

The intrinsic stand-off ratio for a UJT is 0.6. If the inter-base resistance is 10 kΩ, the values of \(R_{B1}\) and \(R_{B2}\) are

A.6 kΩ and 4 kΩ
B.4 kΩ and 6 kΩ
C.5 kΩ and 5 kΩ
D.10 kΩ and 10 kΩ

Exam Year(s)

UGC NET - 2012,

Answer: A

The values are \(R_{B1}\) = 6 k\(\Omega\) and \(R_{B2}\) = 4 k\(\Omega\), option (A). The intrinsic stand-off ratio of a UJT is \(\eta = R_{B1}/(R_{B1}+R_{B2}) = R_{B1}/R_{BB}\), where \(R_{BB}\) is the inter-base resistance.

\[ R_{B1} = \eta R_{BB} = 0.6\times10 = 6\ \text{k}\Omega \]

\[ R_{B2} = R_{BB} - R_{B1} = 10 - 6 = 4\ \text{k}\Omega \]

Option (B) reverses the two values. Option (C), 5 k and 5 k, would be the case of \(\eta = 0.5\). Option (D) gives each resistance as the whole inter-base resistance, which is not possible since they add up to \(R_{BB}\).

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