Plotting the Zener Diode Characteristics and Finding Breakdown Voltage

Aim

To draw the characteristic curve of a Zener diode and determine its reverse breakdown voltage.

Apparatus Required

A Zener diode, high-resistance rheostat, variable DC supply, voltmeter, multi-range ammeter, one-way key, a resistor, and connecting wires.

Theory

Zener Diode

When a PN-junction diode is reverse-biased and the voltage increases, the reverse current remains very small until a specific voltage is reached. At this point, the current suddenly rises — this is called breakdown voltage.

A Zener diode is specially designed to operate safely in this breakdown region without damage.
The breakdown voltage in a Zener diode is known as the Zener voltage.

Procedure

1. Note the type, number, breakdown voltage, maximum current, and wattage rating of the Zener diode.
2. Make the circuit connections as per the diagram. Ensure positive terminals of both voltmeter and ammeter connect to the positive terminal of the supply.
3. Check the least count and zero error of the voltmeter and microammeter.
4. Keep the rheostat at minimum resistance and insert the plug key to switch on the supply.
5. Slowly increase the reverse voltage across the diode using the rheostat, taking small steps. Record corresponding voltage and current readings.
6. At a specific voltage, the current will shoot up suddenly — this is the reverse breakdown voltage. Note this value.
7. Plot a graph with:
   • Reverse voltage (V) on X-axis
   • Current (I) on Y-axis

Circuit Setup Image

Observations

Precautions

1. Clean the ends of connecting wires with sandpaper to ensure proper connections.
2. Connect the negative terminal of the supply to the p-terminal and the positive to the n-terminal to maintain reverse-biasing.
3. Increase the voltage gradually to avoid damaging the diode.
4. The voltmeter must be connected in parallel and the ammeter in series with the diode.