13 Jul 2026

V-I Characteristics of a Zener Diode and Voltage Regulation

practical ug-iv electronics zener-diode regulator

Experimental arrangement

Zener test connection: the series resistor limits current and the output voltage is measured directly across the Zener device.

Aim

To plot the V-I characteristics of a Zener diode and study its action as a voltage regulator.

Apparatus

Zener diode, regulated DC supply, series resistor, voltmeter, milliammeter, load resistor, and connecting wires.

Theory

A Zener diode is operated in reverse breakdown. In this region a large change in current produces only a small change in voltage. Hence it can maintain an approximately constant load voltage. The breakdown voltage obtained from the characteristic is the Zener voltage $V_Z$.

Observations

Reverse voltage (V)	Zener current (mA)
4.8	0.2
5.0	0.8
5.1	2.0
5.15	4.0
5.20	6.0

At $V_{in}=9\,\text{V}$, $R_s=470\,\Omega$, and $R_L=1\,\text{k}\Omega$:

Load current (mA)	Regulated output (V)
1.0	5.08
2.0	5.10
3.0	5.11

Result

The Zener voltage is approximately $V_Z=5.1\,\text{V}$, and the circuit maintains an output close to $5.1\,\text{V}$ over the measured load-current range.

Precautions

1. Always use a series resistor to limit Zener current.
2. Do not exceed the rated power of the diode.
3. Increase reverse voltage slowly.
4. Check the polarity before switching on the supply.

Viva Questions

1. Why does a Zener diode regulate voltage? Its reverse voltage changes very little in the breakdown region.
2. What is the purpose of the series resistor? It limits current and protects the diode.
3. What is dynamic resistance? It is the small-signal ratio $\Delta V_Z/\Delta I_Z$ in breakdown.
4. Can a Zener diode be used without a resistor? No, because the current may exceed its safe limit.