Test: Thyristors - Electrical Engineering (EE) MCQ

N = 7 V_string = 3800
V_SCR = 700

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

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.

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and the anode current is above latching current level, there is no need of gate current, hence, gate signal can be withdrawn i.e. gate looses control over the anode current.

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

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.

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

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

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.

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°.