Electronics and Communication Engineering - Power Electronics

36. 

When thyristors are connected in series and parallel, it may be necessary to have :

A. current derating
B. voltage derating
C. both current and voltage derating
D. none of the above

Answer: Option C

Explanation:

Thyristors in series do not share the voltage equally and require voltage derating. Thyristors in parallel need current derating.


37. 

A single phase half wave regulator uses

A. 2 diodes and 2 thyristors
B. 2 thyristors
C. 1 diode and 1 thyristor
D. 2 diodes

Answer: Option C

Explanation:

Regulation is possible only in positive half cycle.


38. 

A Shockley diode can be used

A. in place of SCR
B. as a trigger switch for SCR
C. in place of UJT
D. both in place of SCR and as a trigger switch for SCR

Answer: Option B

Explanation:

It is a four layered PNPN device without a gate terminal. Its characteristics is similar to that of SCR with zero gate current. It is acting as a trigger switch for Thyristor.


39. 

An SCR has a turn on time of 4 μs. The gate pulse should have a duration of about

A. 2 μs
B. 5 μs
C. 20 μs
D. 50 μs

Answer: Option B

Explanation:

Duration of gate pulse should be slightly more than turn time of SCR.


40. 

In a multiphase chopper, the different choppers operate in

A. series and all simultaneously
B. parallel and one at a time
C. parallel and simultaneously
D. either (b) or (c)

Answer: Option B

Explanation:

Only one chopper operates at one time.