Question

A zener diode of breakdown voltage 10 V is connected to an external voltage of 15 V and a resistance R in series. If power of zener diode is 0.4 W. Find value of unknown resistance R :

• A. 125 $\Omega$
• B. 105 $\Omega$
• C. 130 $\Omega$
• D. 115 $\Omega$

Hint:

In Zener diode circuits, always identify if there is a load resistor. If there is no load resistor, the Zener current is the total circuit current. The voltage across the series resistor is always $(V_{in} - V_{Z})$ as long as the input voltage is greater than the breakdown voltage.

Answer 1

The Correct Option is A

Step 1: Concept used

When a Zener diode is connected in reverse bias without any load resistor, all the current supplied by the source flows through the series resistor and then through the Zener diode.

The Zener diode maintains a constant voltage equal to its breakdown voltage. Any extra voltage appears across the series resistor.

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Step 2: Given data

Zener voltage = 10 V
Input supply voltage = 15 V
Power rating of Zener diode = 0.4 W

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Step 3: Finding current through Zener diode

Power of Zener diode is equal to voltage multiplied by current.

Current through Zener = Power divided by Zener voltage

Current = 0.4 divided by 10

Current = 0.04 ampere

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Step 4: Voltage across the series resistor

The resistor drops the excess voltage after regulation.

Voltage across resistor = Supply voltage minus Zener voltage

Voltage across resistor = 15 minus 10

Voltage across resistor = 5 V

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Step 5: Calculating resistance

Using Ohm's law:

Resistance = Voltage divided by current

Resistance = 5 divided by 0.04

Resistance = 125 ohm

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Final Answer:

The value of the series resistance R is
125 ohm