EDC lab Experiment No. 2 : Plot the V-I Characteristics of Zener Diode

skygyanweb

Object

To study and plot the V-I characteristics of a Zener diode under forward bias and reverse bias conditions and determine its Zener breakdown voltage.

Apparatus Required

  1. Zener diode (5.1 V or 6.2 V)
  2. Regulated DC power supply (0–12 V)
  3. Resistor (1 kΩ)
  4. Digital multimeter
  5. Milliammeter (0–50 mA)
  6. Microammeter (0–500 µA)
  7. Voltmeter (0–10 V)
  8. Breadboard and connecting wires

Theory

A Zener diode is a specially designed semiconductor diode that operates in the reverse breakdown region without being damaged, provided its current is limited within the specified rating.

It has two operating regions:

1. Forward Bias:

  • P-side is connected to positive supply.
  • N-side is connected to negative supply.
  • It behaves like an ordinary PN junction diode.
  • Current increases rapidly after the cut-in voltage (approximately 0.7 V for silicon).

2. Reverse Bias:

  • P-side is connected to negative supply.
  • N-side is connected to positive supply.
  • Initially, a very small reverse leakage current flows.
  • At breakdown voltage (Vz), reverse current increases sharply while voltage remains nearly constant.

Circuit Diagram

A. Forward Bias Circuit

B. Reverse Bias Circuit

Important: The voltmeter is connected in parallel with the Zener diode, while the ammeter is connected in series.

Procedure

Part A: Forward Bias Characteristics

  1. Connect the circuit as shown in the forward bias diagram.
  2. Connect the P-side of the Zener diode to the positive terminal of the supply through a series resistor.
  3. Connect the N-side to the negative terminal.
  4. Switch ON the DC power supply.
  5. Increase the input voltage gradually.
  6. Record the diode voltage (Vf) and forward current (If).
  7. Take 6–8 readings.
  8. Plot the graph between forward voltage and forward current.

Part B: Reverse Bias Characteristics

  1. Connect the circuit as shown in the reverse bias diagram.
  2. Reverse the Zener diode connections.
  3. Connect the N-side towards the positive supply through a series resistor.
  4. Connect the P-side towards the negative terminal.
  5. Increase the supply voltage gradually.
  6. Record the reverse voltage (Vz) and reverse current (Iz).
  7. Take readings before and after the breakdown region.
  8. Identify the Zener breakdown voltage from the graph.

Observation Table

A. Forward Bias Readings

S. No.Forward Voltage Vf (V)Forward Current If (mA)
10.100.0
20.200.0
30.300.1
40.400.3
50.500.8
60.602.0
70.654.0
80.708.0

B. Reverse Bias Readings

Assume a Zener diode with nominal breakdown voltage of approximately 5.1 V.

S. No.Reverse Voltage Vz (V)Reverse Current Iz (mA)
11.00.00
22.00.00
33.00.01
44.00.02
54.50.05
64.80.20
75.01.00
85.15.00
95.210.00
105.315.00

V-I Characteristics Graph

Graph interpretation:

  • Forward region: Current increases rapidly after the cut-in voltage.
  • Reverse region: Small leakage current flows initially.
  • Breakdown region: Reverse current increases sharply at approximately 5.1 V.
  • Zener region: Voltage remains nearly constant despite changes in current.

Result

The V-I characteristics of the Zener diode were studied and plotted under forward and reverse bias conditions. The Zener breakdown voltage was found to be approximately 5.1 V

Precautions

  1. Always use a series current-limiting resistor.
  2. Check the polarity of the Zener diode before switching ON the supply.
  3. Do not exceed the maximum rated Zener current.
  4. Increase the supply voltage gradually.
  5. Use proper ranges of ammeter and voltmeter.
  6. Ensure all connections are tight.
  7. Switch OFF the supply before changing circuit connections.

Viva-Voce Questions

  1. What is a Zener diode?
  2. What is Zener breakdown?
  3. What is the difference between an ordinary diode and a Zener diode?
  4. Why is a Zener diode operated in reverse bias?
  5. What is Zener voltage?
  6. Why is a series resistor connected with a Zener diode?
  7. What is the function of a Zener diode as a voltage regulator?
  8. What is the difference between Zener breakdown and avalanche breakdown?
  9. What happens to the Zener voltage when reverse current increases?
  10. What are the applications of a Zener diode?

Related Posts