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Test: Thyristors - Electrical Engineering (EE) MCQ


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10 Questions MCQ Test - Test: Thyristors

Test: Thyristors for Electrical Engineering (EE) 2024 is part of Electrical Engineering (EE) preparation. The Test: Thyristors questions and answers have been prepared according to the Electrical Engineering (EE) exam syllabus.The Test: Thyristors MCQs are made for Electrical Engineering (EE) 2024 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Thyristors below.
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Test: Thyristors - Question 1

A thyristor switch has voltage rating of 700 V for each and 7 SCRs are connected in series for handling a voltage of 3800 V. The derating factor for this set of arrangement is

Detailed Solution for Test: Thyristors - Question 1


N = 7 Vstring = 3800
VSCR = 700

Derating factor DRF = 1 - η
= 1 - 0.775 = 22.45%.

Test: Thyristors - Question 2

Generally inductor connected at the cathode end of SCR for _________ protection

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Test: Thyristors - Question 3

Assertion (A) : The operation of an SCR can be explained in a very simple way by considering it in terms of two transistors.
Reason (R) : in the two transistor analogy of the SCR, the SCR can be considered as an npn and a pnp transistor, where the collector of one transistor is attached to the emitter of the other and vice-versa.

Detailed Solution for Test: Thyristors - Question 3

In the two transistor analogy of the SCR, the SCR can be considered as an npn and a pnp transistor, where the collector of one transistor is attached to the base of the other and vice-versa as shown below. Thus, reason is a false statement.

Test: Thyristors - Question 4

After firing an SCR, the gate pulse is removed. The current in the SCR will

Detailed Solution for Test: Thyristors - Question 4

Once the thyristor (SCR) is turned-on, and the anode current is above latching current level, there is no need of gate current, hence, gate signal can be withdrawn i.e. gate looses control over the anode current.

Test: Thyristors - Question 5

Thyristor is not made using germanium, because

Detailed Solution for Test: Thyristors - Question 5

The leakage current in silicon is of the order of nA while in Germanium it is in μA.

Test: Thyristors - Question 6

Thyristor is a semiconductor switch which is

Detailed Solution for Test: Thyristors - Question 6

Like the diode, an SCR is an unidirectional device that blocks the current flow from cathode to anode i.e. unilateral. An SCR or a thyristor has three modes of operation namely forward conduction mode, forward blocking mode and reverse blocking mode. Out of these three modes of operation the first two are stable states due to which an SCR is bistable.

Test: Thyristors - Question 7

An SCR having turn on time of 5 ms, latching current of 100 mA and holding current of 38 mA is triggered by a short duration pulse. What will be the minimum width, triggering pulse would require for successful turn ON of SCR

Detailed Solution for Test: Thyristors - Question 7

For successful turn ON i1 + i2 ≥ 100 mA

I = i1 + i2 = (8 + 7.9) (1 - e-20t ) = 100 mA
1 – e-20t = 6.30μ
t = 3.15 × 10-4 s

Test: Thyristors - Question 8

The main reason for connecting a pulse transformer at the output stage of a thyristor triggering circuit is to

Detailed Solution for Test: Thyristors - Question 8

Pulse transformers are often used to couple a trigger pulse generator to a thyristor in order to obtain electrical isolation between the two circuits.

Test: Thyristors - Question 9

Surge current rating of an SCR specifies the maximum

Detailed Solution for Test: Thyristors - Question 9

The surge current rating indicates the maximum possible non-repetitive or surge current, the device can withstand and this may occur due to non-repetitive faults or short-circuits during the life-span of a thyristor.

Test: Thyristors - Question 10

The drawback of R-firing circuit for SCR is

Detailed Solution for Test: Thyristors - Question 10

The R-firing circuit suffers from several disadvantages. One of them is that the trigger angle can be varied only upto an approximate value of 90°.

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