Power Electronics Question and Answers – Diode Circuits-1 and Answers

Power Electronics Multiple Choice Questions on “Diodes Circuits- 1”.

1. An ideal diode has _________ &__________
A. some forward voltage drop, some reverse recovery time
B. high switching losses, high reverse voltage drop
C. no forward voltage drop, negligible reverse recovery time
D. no reverse recovery time, high leakage current
Answer: C
Clarification: An ideal diode has no losses and negligible reverse recovery time.

2. A diode circuit is so arranged that when the switch is open it’s KVL gives
Ri+ 1/C ∫i dt = 0
When the switch is closed,
Ri+ 1/C ∫i dt = Vs
Vs is the dc supply voltage.
The diode is so connected that it is forward biased when switch is closed
The circuit is mostly likely be a
A. diode in parallel with Vs, switch, R & C
B. diode in series with R, than parallel with Vs & C
C. diode in series with the switch, R, C & Vs
D. diode in series with R,C & Vs with the switch connected in parallel across Vs
Answer: C
Clarification: Examine the equation, the same current flows through R & C. Also when switch is open, the equation R.H.S is 0. Hence, all the elements are in series.

3. A circuit is so formed such that the source-R-C-diode-switch are in series. Consider the initial voltage across the C to be zero. The diode is so connected that it is forward biased when the switch is closed. When the switch is closed,
A. the current will decay exponentially & the voltage will increase exponentially
B. the current will increase exponentially & the voltage will increase exponentially
C. the current will fall to zero & the voltage both will decay exponentially
D. the voltage and current both remain constant
Answer: A
Clarification: Instant switch is closed, the current is maximum Vs/R than starts to reduce, whereas voltage starts to increase from 0 to Vs (Capacitor Charging).

4. The time constant of a series RC circuit (τ) is given by
A. R/C
B. C/R
C. RC
D. 1/RC
Answer: C
Clarification: For a series RC circuit, τ = RC.

5. A circuit is so formed such that source-R-L-diode-switch are all in series. Consider the initial current in L to be zero. The diode is so connected that it is forward biased when switch is closed.
When the switch is closed,
A. the current will decay exponentially & the voltage will increase exponentially
B. the current will increase exponentially & the voltage will decay exponentially
C. the current will fall to zero & the voltage both will decay exponentially
D. the voltage and the current both remain constant
Answer: B
Clarification: Instant switch is closed, the current is minimum zero than starts to increase till it reaches a constant value Vs/R, whereas voltage starts to reduce from Vs to 0 (Inductor Charging).

6. In the figure shown below,

As the switch (shown in the green) is pressed, the voltage across the diode(ideally)
A. increases to Vs/R
B. increases to Vs
C. decreases to zero
D. remains Constant
Answer: C
Clarification: As the switch is pressed, current starts to flow & the whole supply voltage (ideally) appears across the load R & voltage across the diode is zero.

7. For a diode circuit the voltage across the capacitor is given by
Vc(t)= Vs(1-e^(-t/RC.)
Then the initial rate of change of capacitor voltage is given by
A. 0
B. ∞
C. Vs x RC
D. Vs/RC
Answer: D
Clarification: Find d(VC./dt and put t = 0.

8. In the circuit show below,

The initial current through the inductor is zero. When the switch (shown in green) is closed, then the current through the inductor
A. decreases from Vs/R to 0
B. increases from zero to Vs/R
C. decreases from Vs/L to 0
D. increases from zero to Vs/L
Answer: B
Clarification: Current increases from zero to maximum value gradually due to the L nature. The KVL when switch is closed gives,
Ri + Ldi/dt = Vs
Solve for i(t). Maximum value comes out to be Vs/R.

9. In the figure shown below,

When the switch is open, the voltage across the diode (ideally)
A. is Vs/R
B. is Vs
C. is zero
D. none of the mentioned
Answer: B
Clarification: When the switch is open, the diode experiences all the supply voltage.

10. The time constant of a series RL circuit (τ) is given by
A. R/L
B. L/R
C. RC
D. 1/RL
Answer: A
Clarification: The time constant τ for a series RL circuit is R/L.