zener diode voltage regulator

Zener Diode as a voltage regulator

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Zener Diode Shunt Voltage Regulator circuit

A Zener diode is a special purpose p-n junction diode that is designed to operate in the reverse direction. They are heavily doped p-n junction diode. Hence, it has very thin depletion region near the junction. When reverse voltage applied across the Zener diode a small reverse current flow through the device called reverse leakage current.

As the reverse voltage is increase continuously, at a certain value of reverse voltage, the junction will be breakdown and drastically a large reverse current flow through the device. This breakdown is called Zener breakdown. The voltage at which this breakdown occurs is called Zener voltage ( Vz ). To protect the diode from this large reverse bias current a resistor (Rs) is connected in series with diode. This resistor is called current limiting resistor . This reverse bias property of Zener diode is used in voltage regulation.

Symbol of Zener Diode
V-I Characteristic of Zener Diode

Read more: Zener Diode

What is a voltage regulator?

Voltage regulator is a circuit that has an ability to maintain constant output voltage across load either input voltage is varying or load current is varying. 

Consider the following circuit of simple shunt Zener regulator.

Shunt Voltage Regulator Circuit

To understand the working more effectively some important points must be remembered-

  1. To get effective voltage regulation, the input voltage Vi must be greater than the Zener voltage Vz.
  2. The value of series resistor or current limiting resister Rs must be such that in no-load condition, it must protect the Zener diode from over current, as in this condition, I=Iz since IL=0.
Reverse Bias characteristic of Zener Diode

Check out Hindi video on how Zener diode work as Voltage Regulator?

Mathematical

There are 3 currents in the circuit, these are I, Iz and IL.

I=Iz+IL… using Kirchhoff’s Current Law

Iz =(Vi−Vz)/R… using Ohm’s law

IL=Vo/RL=Vz/RL…

because, Vz= Vo as Zener diode is connected in parallel with the load resistor RL

Working of Zener voltage regulator circuit

Find three possible conditions for the given circuit

  1. When input voltage Vi and load resistance RL both are constant.
  2. When input voltage Vi is constant but load resistance RL is varying.
  3. When input voltage Vi is varying but load resistance RL is constant.

All three cases are explained as follows:

Cases #1: Let us assume that Vi and RL are constant:

We know that,

                        I = Iz + IL… using Kirchhoff’s Current Law

In this condition, I and IL are both are constant, so Iz is also constant. So, the circuit remains in steady state condition. However, this condition is considered only for understanding the working. But practically, this condition may not exist in regulator circuit.

Case #2: Now assume that Vi is constant, but RL is varying:

We know that,

                        I = Iz + IL… using Kirchhoff’s Current Law

In this condition, input current I is constant, but IL is varying, since RL is also varying. So, there are two possible conditions here.

  1. Let us assume  RL increases. Then IL decreases. But input current I is constant, because we have assumed that Vi is constant. And as I=Iz+IL. So Iz increases proportionally to adjust this condition.
  2. Let us assume RL decreases. Then IL is increases. But input current I is constant as Vi is constant. And as I=Iz+IL so Iz decreases proportionally to adjust this condition again.

Important Point:  Here RL=0 condition is not assumed, because if RL=0, then IL=∞ so Iz=0 and then the regulation process will collapse.

Case #3: Assume that Vi is Varying, but RL is constant: 

We know that,

                        I = Iz + IL… using Kirchhoff’s Current Law

In this condition, input current I is varying, but IL remains constant, since RL is constant. So, there are two possible conditions here.

  1. Let us assume Vi increases. Then I increase. But load current IL remains constant, because we have assumed that RL is constant. And as I=Iz+IL so Iz increases proportionally to adjust this situation.
  2. Let us assume  Vi decreases. Then I decrease. But load current IL remains constant, as RL is constant. And as I=Iz+IL so Iz decreases proportionally to adjust this situation, again.

Important Point: Here if RL=∞ i.e., load is removed from the circuit, then IL=0 so Iz=I. Hence at the starting I have already told, that the value of series resistor R must be properly selected.

In this way, in any possible conditions of the circuit, i.e. when input voltage and load resistance both are changing, the above conditions will work effectively. Thus in all cases, the zener diode maintains constant output voltage across the load RL.

Important Note: All the given conditions are applicable only when Vi >> Vz.

Frequently Ask Questions FAQ’s –

What is voltage regulator?

Voltage regulator is a circuit that has an ability to maintain constant output voltage across load either input voltage is varying or load current is varying. 

What is Zener Diode?

A Zener diode is a heavily doped PN junction diode that is designed to operate in the reverse direction.

What is Zener Breakdown?

The Zener breakdown mainly occurs because of a high electric field. When the high electric-field is applied across the PN junction diode, then the electrons start flowing across the PN-junction. Consequently, expands the little current in the reverse bias.

What is difference between Zener Diode and normal P-N junction diode?

The main difference between a normal P-N junction diode and Zener diode is normal P-N Junction allows current to flow only in one direction while Zener diode allows current to flow in both directions.

Applications of Zener Diode

Zener diodes are used for voltage regulation, as reference elements, surge suppressors, and in switching applications and clipper circuits.


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