When the Zener diode is in reverse biased mode or reverses break down region then we can operate the Zener diode as a voltage regulator.
V-I Characteristics of Zener Diode
The V-I characteristic curve of the Zener diode is shown below.
From V-I characteristic curve we can observe that the Zener diode has a region in its reverse bias characteristic where a constant negative voltage appears regardless of the value of the current flowing through the diode.
The voltage remains constant even with a large change in current. This ability of a Zener diode to control itself can be used to regulate or stabilize the voltage source against supply or load variation. Due to this property Zener diode being operated in the breakdown region can be used as a voltage regulator.
Illustration of Zener Diode as a Voltage Regulator
Consider a given circuit,
Here, Vs=Source voltage
Rs= Resistor to limit the reverse current through Zener
Vz= Zener voltage and zener current
IZ= Zener current
When the voltage across the load RL is less than the breakdown voltage Vz then the Zener diode does not conduct any current.
Here, voltage Vs and resistor Rs are selected in such a way that the Zener diode operates in the breakdown region.
For load voltage (Vout) greater than Zener voltage Vz the Zener diode will operate in the breakdown region.
Now, the supply current Is is divided into Iz and IL.
For a Zener diode operating in a breakdown region voltage, Vz remains almost constant even though the current Iz may change significantly.
With the increase in Vs, Is increases. Since RL is constant and Vout being equal to Vz, IL is also constant.
So, from equation (i) we can observe that increase in Is will be compensated by an increase in Iz.
Now the Zener voltage becomes,
When Vs increases IsRs also increases this makes Zener voltage Vz almost constant. As the voltage across RL is equal to Vz. So, Vout also remains constant. Hence, Zener diode operating in the breakdown region acts as a voltage regulator.