# Zener diode and its characteristics

## Definition of Zener Diode

Zener diode is a reverse-biased heavily doped P-N junction diode that operates in the breakdown region.

## Biasing Of Zener Diode

### (I) Forward Bias

When the anode of the Zener diode is connected to the positive terminal and the cathode is connected to the negative terminal of the battery then the diode is said to be forward-biased and behaves as the normal forward bias P-N junction Diode as shown in fig below:

### Reverse Biasing Of Zener Diode

When the cathode of the Zener diode is connected to the positive terminal and Anode is connected to the negative terminal of the battery then the diode is said to be reverse biased. Zener Diode in Reverse bias condition act as a voltage regulator.

As we increase the reverse bias, initially a very small reverse current will flow but at a certain reverse voltage the reverse current rises sharply and that voltage is called breakdown Voltage or Zener Voltage and is denoted By Vz and the diode is said to be in Zener region where the Zener voltage remains constant but current changes depending upon the applied voltage.

## Equivalent circuit of Zener Diode

The practical Zener diode is equivalent to a battery of voltage Vz in series with a resistance rz , Resistance rz is known as the dynamic resistance of the Zener diode given as

$r_z\;=\frac{\triangle V_z}{\triangle I_z}$

In the ideal equivalent circuit of the Zener diode, the value of Zener resistance rz is zero. During the voltage from 0 V to Vz in reverse bias, the Zener diode behaves as an open circuit as shown in 9. The Zener diode is said to be “ON” when the reverse bias voltage exceeds Vz as shown in fig 8 while it remains “OFF” if the voltage across the Zener is less than Vz in reverse bias. The “ON” & “OFF” states are shown below:

## Zener Diode application as a Shunt regulator /Voltage regulator.

A voltage regulator is a circuit whose function is to maintain a constant output d.c voltage in spite of the a.c input voltage fluctuations or changes in load resistance values.

The input current is calculated by

${\displaystyle V_s\;=I_s\;R_s\;+\;V_z}$

${\displaystyle I_s\;=\frac{V_s\;-V_z}{R_s}}$

Here    $V_s$= Input d.c unregulated voltage
$V_z$= Voltage across Zener diode.
$V_L=V_z$

## Working of Zener diode Shunt regulator

### (I) Regulation with varying Load

In this case, the load resistance RL is fixed and the input voltage Vs is varied as shown in fig below:

The output VL =Vz is a constant

${\displaystyle I_L\;=\;\frac{V_L}{R_L}=\;\frac{V_z}{R_L}}$ = Constant

and ${\displaystyle I_s\;=\;I_z\;+\;I_L}$

Now if Vs increases then total current Is increases. Hence the current Iz increases to keep IL constant. If Vs decreases then total current Is decreases. Hence the current Iz decreases to keep IL constant.

### (ii) Regulation with Varying Load

In this case, the input voltage is constant and the load resistance RL is varied.

As Vs is constant so the total current Is will also be constant.

Since , Is = Iz +IL = constant

Now if RL decreases, So IL increases, and Iz decreases to keep IR constant. Similarly if RL increases. so IL decreases and Iz increases to keep IR constant. Therefore the load voltage remains constant.

VL = Constant

## Advantages of Zener diode voltage regulator

1. It is a simple circuit
2. It is more reliable
3. It provides regulation over a wide range of current.
4. It is smaller, lighter, and more rugged.
5. It has a longer life.
6. It is cheaper.