Characteristics of Zener Diode and Finding the Breakdown Voltage

Experiment No.: 1

Experiment Name: Plot the characteristics of Zener diode and find the breakdown voltage.

Objective:

1. To plot the characteristics of Zener Diode.
2. To find the breakdown voltage.

Theory:

An ideal P-N Junction diode does not conduct in reverse biased condition. A zener diode conducts excellently even in reverse biased condition. These diodes operate at a precise Value of voltage called break down voltage. A zener diode when forward biased behaves like an ordinary P-N junction diode.  A zener diode when reverse biased can either undergo avalanche break down or zener break down.

Avalanche break down: If both p-side and n-side of the diode are lightly doped, depletion region at the junction widens. Application of a very large electric field at the junction may rupture covalent bonding between electrons. Such rupture leads to the generation of a large number of charge carriers resulting in avalanche multiplication.

Zener break down: If both p-side and n-side of the diode are heavily doped, depletion region at the junction reduces. Application of even a small voltage at the junction ruptures covalent bonding and generates large number of charge carriers. Such sudden increase in the number of charge carriers results in zener mechanism.

Circuit Diagram:

Procedure:

Forward biased condition:

1. Connect the circuit as shown in fig (1).
2. Vary V_ZF  gradually steps of 0.1 volts up to 5volts and note down the corresponding  readings of I_ZF.
3. Tabulate different forward currents obtained for different forward voltages.

Reverse biased condition:

1. Connect the circuit as shown in fig (2).
2. Vary V_ZR gradually in steps of 0.5 volts up to 8 volts and note down the corresponding readings of I_ZR.
3. Tabulate different reverse currents obtained for different reverse voltages.

Observation Table:

Forward biased conditions:

Sl. No.	Forward Voltage across the diode VZF (volts)	Forward current through the diode IZF (mA)

Reverse biased conditions:

Sl. No.	Reverse Voltage across the diode VZR (volts)	Reverse current through the diode IZR (µA)

Graph:

1. A graph sheet is taken and divided it into 4 equal parts. Origin at the center is marked of the graph sheet.
2. Now marked   

+ve x-axis as    V_ZF              -ve x-axis as    V_ZR                                         +ve y-axis as    I_ZF                -ve y-axis as    I_ZR                                                                                                         

• The readings tabulated for zener diode forward biased condition in first Quadrant and Zener diodes reverse biased condition in third Quadrant.

Apparatus Used:

Sl No.	Name of Apparatus	Quantity	Specification	Makers name
1.	Zener Diode characteristics trainer kit	1	Zener Breakdown 5.1 V, 6.8 V	M.E.W.
2.	Regulated power supply	1	0 – 30 V DC	M.E.W.
3.	Micro-ammeter	1	0-500 µA, PMMC type

Precautions:

1. While doing the experiment do not exceed the ratings of the diode. This may lead to damage the diode.
2. Connect voltmeter and Ammeter in correct polarities as shown in the circuit diagram.
3. Do not switch ON the power supply unless you have checked the circuit connections as per the circuit diagram.

Inference: 

1. In the forward biased mode the zener diode operates as a p-n diode.
2. In the reverse biased mode zener diode has large breakdown voltage and though the current increases the voltage remains constant. Thus it acts as a voltage regulator.

Result: 

1. The zener diode characteristics have been studied. 
2. The zener breakdown voltage was found to be = ___________

Remarks: