Analog Circuits Questions and Answers – Voltage Regulators – 1

This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Voltage Regulators – 1”.

1. What is IC 723?
a) A voltage regulator
b) A full-wave rectifier
c) A half-wave rectifier
d) A clipper

Explanation: The IC 723 is a voltage regulator, which can act as both a low voltage regulator as well as a high voltage regulator. Output can be set between 7-37 volts. 7 volts is the reference starting voltage.

2. Consider the circuit shown below where the breakdown voltage of the diode is 5V. Source voltage varies between 6V to 12V.

Find the maximum current through the R2, given that R1=2kΩ and R2=5kΩ.
a) 3.5 mA
b) 1 mA
c) 1.4mA
d) 0.2 mA

Explanation: Source current is I = VS-VZ/R2
Thus here I is maximum when VS is maximum.
I = 12-5/5000 = 1.4 mA.

3. For a Zener diode shunt regulator, the source current is IS, the Zener diode current is IZ and the load current is IL. The source voltage is VS, Zener voltage is VZ and load voltage is VL. The load resistance is RL. What is the correct option for the safe operation of the diode?
a) IS = IZ + IL
b) IS =< IZmax + IL
c) IS =< IZmin + IL
d) VL = VZ

Explanation: For proper operation, the current through RS should be at least equal to the sum of IZmin a specified load current. IS >= IZmin + IL
For safe operation of the diode, IS =< IZmax + IL.

4. What is line regulation?
a) The process of keeping Zener diode voltage constant inspite of changes in AC supply
b) The process of keeping load voltage constant irrespective of the fluctuation in AC supply or the line voltage
c) The process of keeping load voltage constant irrespective of fluctuation in load current
d) The process of keeping Zener current constant irrespective of fluctuation in AC supply

Explanation: Line regulation is the process of keeping the load voltage constant, irrespective of fluctuation in AC supply or the line voltage. In line regulation, the load current is considered constant.

a) The process of keeping the load voltage constant irrespective of any change in AC supply
b) The process of keeping the load voltage constant irrespective of variations in load current
c) The process of keeping load voltage constant irrespective of variations in source current
d) The process of keeping load current constant irrespective of variations in AC supply

Explanation: Load regulation is the process of keeping VO constant irrespective of variations in load current. The line voltage is taken to be constant during load regulation.

6. Find the power rating of the diode in the given circuit. The breakdown voltage of the diode is 5V.

a) 200 mW
b) 125 mW
c) 250 mW
d) 300 mW

Explanation: Source current IS = 15-5/200 = 0.05A = 50 mA
The power rating of the diode is the maximum power it can dissipate which occurs when the load is disconnected because then the whole current flows into the diode.
Hence maximum power rating is P = VZxIZ = 5×50 = 250 mW.

7. The following circuit is provided.

Given that V1 varies from 20V to 50V, the diode breakdown voltage is 5V, the knee current is 1mA and the current is 9 mA across R1, find the maximum value of R2.
a) 4500Ω
b) 1500Ω
c) 2000Ω
d) 5000Ω

Explanation: IS >= IZmin + IL
IS >= 10mA
V1-5/R2 >= 10mA
R2 =< V1-5/10-2
Thus R2 =< 1500Ω when V1 = 20V
And R2 =< 4500Ω when V1 = 50V
Hence R2 =< 1500Ω.

8. In the figure below, the voltage V1 ranges from 15 to 35V. The load current is 20mA. The diode knee current is 2 mA, the breakdown voltage is 5V and power rating is 200 mW.

Calculate the range of R2 for the circuit to behave properly as a regulator.
a) 400Ω =< R2 =< 500Ω
b) 450Ω =< R2 =< 500Ω
c) 454Ω =< R2 =< 500Ω
d) 454Ω =< R2 =< 600Ω

Explanation: IS >= IL + IZmin
IS >= 22mA
IZmax = 200mW/5 = 40 mA.
V1 is minimum, IS >= IZmin + IL
V1-5/R2 >= 22mA
R2 <= 454 Ω
IS =< IZmax + IL when VS is maximum
V1-5/R2 =< 60mA
R2 >= 500 Ω
Hence 454Ω =< R2 =< 500Ω.

9. Which of these is a not drawback of Zener diode shunt regulator?
a) The output voltage is fixed
b) The output voltage can vary with temperature
c) Variation in load current needs to be minimal
d) It is difficult to design

Explanation: The Zener diode shunt regulator is a simple voltage regulator circuit that provides a fixed output voltage. However, due to the presence of the Zener diode, the output is temperature dependent because the breakdown voltage of Zener diode depends on temperature. When load current varies a lot, then Zener current also varies which causes a change in output voltage.

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