What Is A Zener Diode And How It Is Used As A Voltage Stabiliser

What Is A Zener Diode

When the reverse bias voltage on a normal crystal diode is increased, a point known as Knee point is reached at a  voltage known as breakdown voltage, when the  reverse current increases sharply to a high value. This breakdown voltage is also known as  Zener Voltage and the sharp increase in current is known as Zener current.

The breakdown voltage of a diode depends on the amount of doping. In a heavily doped diode, the depletion layer is thin, so the breakdown of the junction occurs at a lower reverse bias voltage. On the other hand, a lightly doped diode has a higher breakdown voltage. When an ordinary crystal diode is properly doped so as to have a sharp breakdown voltage ,it is known  as  zener diode. Fig(1) shows the symbol of a zener diode.

A zener diode is always used in reverse biased state. It  has sharp breakdown voltage known as Zener voltage  V_Z. When forward biased ,it behaves just like ordinary crystal diode. Fig(2)  shows the characteristics of a zener diode.

Fig. 2

Equivalent Circuit of  a Zener Diode

Case 1. On State

When the reverse voltage across a zener diode is equal to or higher than the breakdown voltage V_Z , the current increase very sharply.

Fig. 3

In this state, voltage across the zener diode is constant at V_Z irrespective of the change in current though it.

Therefore, in breakdown region an ideal zener diode acts as a battery of voltage  V_Z , and the zener diode is said to be in “ON” State.

Case 2.OFF  State:

When the reverse voltage across the zener diode is less than V_Z but   greater than 0v, the zener diode is in the “OFF”State. Under such state, the zener diode can be replaced by an Open circuit. 

Fig. 4

Zener Diode as Voltage Stabiliser

A zener diode can be used as a voltage stabiliser or voltage regulator to provide a constant voltage from a source whose voltage may vary over a particular range.

Fig. 5

The circuit connection is shown in fig(5).

The zener diode of zener voltage  V_Z is connected reversly across the load resistance  R_L across which constant output voltage E_O is required.

The series resistance R is used to absorb the output voltage fluctuations, so as to maintain constant output voltage across R_L .

When the circuit is properly designed, the output voltage E_O remains constant even though the input voltage  E_i and load resistance R_L may vary over a wide range.

Case 1: E_i Variable  and R_L constant

Fig.6

Suppose the input voltage E_i increases, since the zener is in the breakdown region, the zener diode is equivalent to a battery of voltage V_Z and the output voltage remains constant at V_Z ( E_O =V_Z).

The excess voltage is dropped across R. This will cause an increase in the value of total current I. The zener will conduct the increase of current in I, while the load current remains constant.

Hence the output voltage remains constant irrespective of the change in input voltage E_i

Summary(Case 1)

Suppose  E_i  increases

Since zener is in breakdown region, V_Z remains constant

E_O = V_Z

So  E_O   remains constant

Excess voltage drops across R

So I increases, since   I_L remains constant, I_Z increases

So the excess current is conducted by the zener diode and E_O remains constant irrespective of the change in E_i

Case 2: E_i Constant  and R_L Vaiable

Now suppose the input voltage E_i is constant but R_L decreases

Since the zener diode is in breakdown region , voltage across it will remain constant at   V_Z.

As the output voltage E_O is equal to the zener  voltage, So E_O will also remain constant at V_Z.

When R_L  decreases ,in order  to maintain E_O constant, current through the load resistance  I_L  will increase.

Since E_i is constant, total current  I is also constant. So the increase in load current I_L will come from a decrease in zener current I_Z  .

Voltage drop across R =   E_i   – E_O

Current through R , I = I_Z + I_L