When Zener is operating in its ‘Zener region’ of characteristics, its output voltage remains constant with variation in amount of current from IZ Min to IZ Max . (Fig a)

Following fig (b) shows circuit of Zener diode shunt regulator since Zener is connected in parallel (or shunt) with load, the circuit is known as shunt regulator.

A resistance, (Rs) is connected in series with Zener to limit current in circuit

Therefore, resistor Rs is also known as series current limiting resistor.

The output voltage is taken (VO) across the load resistor (RL).

WORKING

Case-I : Regulation with varying input voltage (line regulation)

Consider the circuit diagram, shown in fig (c).

Here the load resistance (RL) is kept constant and input voltage (Vin), varies within limits From circuit diagram,

We have

IS= IL+IZ ----(I)

And

VO= VZ= IL.RL ----(II)

If vin increases, then input current IS will increase i.e.

From equation (I), (IL+IZ) increase but, ILis constant (as RL =constant)

ThereforeIZwill increase and keeps output voltage constant at load.

Similarly, when Vin decreases input current IS will decrease. i.e.

(IL+IZ) will decrease from equation(I).

But IZ will decrease and keeps output voltage constant at load.

B) Case(II): Regulation with varying load resistance (load regulation) Consider the circuit diagram, shown in fig (d).

Here input voltage (Vin) is kept constant and load resistor RL is varied When RL increases, IL decreases

AS, RL=Vo/IL =VZ/IL -----From(II)

But IS is constant (vin= constant)

Therefore IZ Will increases -------From(I)

When RLdecreases, IL increases. But as IS is constant, the Zener current will decrease and the output voltage will remain constant.