In this blog I am going to discuss about Zener diode and its applications.
The name Zener is derived from the name of American physicist Melvin Zener. He discovered the Zener effect.
Zener diode definition
Zener diode is a special PN Junction diode that is designed to operate in reverse biased condition. It conducts the flow of current in both forward and reverse biased condition. In forward biased condition it acts as normal diode.
It can also be defined as a semi conductor device made using PN junction that conducts the flow of current in reverse biased condition when a desired / specified voltage is reached.
Zener diodes are made by doping high concentration of P-type And N-type material. The concentration of ion in material is kept higher than normal diode. Hence, it has very thin depletion region.
The specified voltage above which Zener diode conducts the flow of current in reverse biased condition is known as breakdown voltage or Zener voltage.
Normal diodes gets damaged / blown in reverse biased condition.
Zener diode Symbol
The symbol of Zener diode is shown in figure below. It has two terminals identified as anode and cathode.
Breakdown in Zener diode
Zener diodes have two types of reverse breakdown.
- Zener breakdown occurs in heavily doped diodes. When the reverse voltage applied to diode is increased. The thin depletion region generates strong electric field.
- When applied reverse voltage reaches close to Zener voltage. The electric field in the depletion region gets strong enough to pull electrons from valence band. The valence electrons gains enough energy from electric field that it breaks the bonding with parent atom.
- The valence electrons that break out are now free electrons. These free electrons carry current. At Zener breakdown region even a small increase in voltage results in high increase of current.
- Avalanche breakdown occurs in both normal PN junction diodes and Zener diodes but at very high reverse voltage.
- When reverse voltage is very high. The free electrons ( that are minority carriers ) gains huge amount of energy and gets accelerated to greater velocity.
- These high speed free electrons collides with the atoms and extract out electrons. Further these electrons again gets accelerated and collide with other atoms. This way a very large number of free electrons are generated.
- These free electrons results in rapid increase of current inside the diode that destroys it.
VI Characteristics of Zener Diode
The VI characteristics of Zener diode is shown in figure below.
When a forward biased voltage is applied to Zener diode, it acts as normal diode.
However, when a reverse voltage is applied. Zener diode allows a small leakage current until Zener voltage is reached.
When the applied voltage reaches Zener voltage, it allows large amount of current to flow. Now even a small increase in applied voltage will rapidly increase the current.
This sudden rise in electric current causes breakdown, known as Zener breakdown. However, Zener diodes are made to handle these kind of breakdown without getting damaged.
The amount of doping determines the Zener breakdown voltage.
Zener breakdown occurs at low reverse voltage when it is heavily doped. Whereas when the diode is lightly doped, the breakdown occurs at high reverse voltage.
Points to remember –
- Zener breakdown occurs because of thin depletion region.
- For a Zener diode, breakdown region id normal operating region.
- Avalanche breakdown occurs at high reverse voltage whereas Zener breakdown occurs at low reverse voltage.
Applications of Zener diode
Zener diodes are widely used in all kinds of electronic circuits.
Applications of Zener diode are –
- Over voltage protection
- Voltage regulator
Over Voltage Protection
Let us understand with an example.
Suppose the circuit requires 6 volt. So we put a Zener diode that has 6 volt of breakdown voltage. When an over voltage, suppose 10 volt try to rush through circuit that needs 6 volt, the circuit may get blown.
Here, Zener diode comes in action. Above 6 volt, Zener diode starts conducting current and provides constant output voltage of 6 volt to the circuit.
Zener diode as voltage regulator
The most wide application of Zener diode is its use as voltage regulator. It can input a wide range of voltage to provide a constant and accurate output voltage.
When the Zener voltage / breakdown voltage is reached the output voltage goes constant.
To limit the excess flow of current through Zener diode, a resistor is added in series.
Advantages of Zener diode
The advantages of Zener diode are as follows –
- Zener diodes are cheap.
- They are small in size
- Output is highly accurate
- It can handle high power
Zener diode specifications
Some common specifications of Zener diode are –
- The range of reverse breakdown voltage in Zener diode ( Vz ) lies between 2.4v to 200v.
- The minimum current required for Zener breakdown is between 5 mA to 10 mA.
- The general power rating of Zener diodes are 400 mW, 500 mW, 1 W & 5 W.
- The tolerance voltage of Zener diode is ± 5 %.
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