# Online Electronics and Communication Engineering Test - Power Electronics Test

Instruction:

• This is a FREE online test. DO NOT pay money to anyone to attend this test.
• Total number of questions : 20.
• Time alloted : 30 minutes.
• Each question carry 1 mark, no negative marks.
• DO NOT refresh the page.
• All the best :-).

1.

The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, v becomes

A.
B.
C.
D.

Explanation:

v ∝ (i)1/4 ∝ (100)1/4 ≈ 3.

2.

The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode)

A.
 limits charging current
B.
 limits discharging current
C.
 limits both charging and discharging currents
D.
 none of the above

Explanation:

Discharge current flows through Re .

3.

For a BJT a = 0.98, then β =

A.
 0.02
B.
 0.5
C.
 4.9
D.
 49

Explanation:

4.

The rated current of a thyristor is 40 A. The holding current will be about :

A.
 20 A
B.
 1 A
C.
 100 mA
D.
 10 mA

Explanation:

The holding current is much less then 1% of rated current.

5.

A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling diode across the load and firing angle is a. During one cycle of input voltage, the total duration of conduction of freewheeling diode is

A.
 a
B.
 2 a
C.
 4 a
D.
 0.5 a

Explanation:

Freewheeling diode conducts for a° in each half cycle.

6.

A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ωt. The thyristor will conduct

A.
 from 0 to 180°
B.
 when v > 50 V
C.
 from 0 to 90°
D.
 When v < 50 V

Explanation:

Thyristor can conduct only when it is forward biased. This occurs only when v > 50 V.

7.

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

Explanation:

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

8.

Assertion (A): When an SCR is conducting, the voltage drop across is about 1 V.

Reason (R): When an SCR is conducting, the junction temperature is about 200°C.

A.
 Both A and R are correct and R is correct explanation of A
B.
 Both A and R correct but R is not correct explanation of A
C.
 A is correct but R is wrong
D.
 A is wrong but R is correct

Explanation:

Junction temperature is about 120°C. Hence R is wrong.

9.

A chopper cannot be line commutated.

A.
 True
B.
 False

10.

If 4 quadrant operation is required we need

A.
 dual converter
B.
 full converter
C.
 semi converter
D.
 none of the above

11.

In a parallel inverter

A.
 each thyristor is turned on twice during each cycle
B.
 each thyristor is turned on once during each cycle
C.
 each thyristor is turned on either once or twice in each load cycle
D.
 each thyristor may be turned on upto four times in each load cycle

12.

In thermal ohm's law, temperature and power loss are analogous to

A.
 V and I respectively of ohm's law
B.
 I and V respectively of ohm's law
C.
 R and V respectively of ohm's law
D.
 I and R respectively of ohm's law

13.

The output frequency of a cycloconverter can be changed by changing the firing angle.

A.
 True
B.
 False

14.

The difference between resonant commutation and self commutation is

A.
 both these methods are based on entirely different principles
B.
 in resonant commutation, the commutating elements carry load current but in self commutation, the commutating elements do not carry load current
C.
 resonant commutation uses L and C but self commutation uses only capacitor
D.
 none of the above

15.

Assertion (A): A fully controlled bridge converter can operate in first and fourth quadrant.

Reason (R): A semi converter is cheaper than a full converter.

A.
 Both A and R are correct and R is correct explanation of A
B.
 Both A and R correct but R is not correct explanation of A
C.
 A is correct but R is wrong
D.
 A is wrong but R is correct

16.

The output wave of a cycloconverter consists of a number of segments derived from the input wave.

A.
 True
B.
 False

17.

A TRIAC can be turned on :

A.
 by applying a positive signal to gate
B.
 by applying a negative signal to gate
C.
 by applying either positive or negative signal to gate
D.
 none of the above

18.

In auxiliary commutated chopper, the peak value of thyristor current is

A.
 equal to peak value of load current
B.
 equal to peak value of capacitor current
C.
 sum of peak value of load current and capacitor current
D.
 none of the above

19.

Which chopper circuit operates in quadrants 1 and 4?

A.
 A
B.
 B
C.
 D
D.
 E

20.

A single phase cycloconverter with centre tapped input transformer requires

A.
 2 thyristors
B.
 4 thyristors
C.
 8 thyristors
D.
 either 2 or 4 thyristors