Zener diodes have major application for being used as a voltage regulator for providing stable reference voltages for use in power supplies, voltmeters, and other instruments. In this article, you will see how the Zener diode maintains (regulate) a nearly constant dc voltage under the proper operating conditions.
Diodes are used in many applications like half-wave, full- wave rectifiers, clippers, clampers, as voltage multipliers, and one of the most important and usable applications of a diode is voltage regulation, the behavior of the diode to regulate the voltage, due to which it has gain new name known as ZENER DIODE.
What is Zener Diode:-
The Zener diode is a special type of diode that is designed to work in the reverse breakdown region. In simple words, it regulates (Maintain) the voltage across its terminal when it is in reverse biased.
- It is consisting of two terminals one is an anode and another is cathode (kathode).
- In Zener diode, current flows in both direction anode to cathode or cathode to the anode.
Symbol of Zener Diode:
The symbol of Zener diode is shown in below figure.
It is similar to the normal p-n junction diode, but with Z-shape on the vertical bar.
Why We Use Zener Diode?
A Simple p-n junction diode does not operate in breakdown region (reverse bias) because the diode will get large current which will permanently damage the diode. Due to this reason for operating a diode in reverse bias we need special purpose diode which can be operated in reverse breakdown voltage for that, we mostly use Zener diode because it can handle large breakdown voltage.
Working of Zener diode?
- Zener diode acts as normal PN junction diode in forward bias.
- Zener diode is heavily doped as compared to the normal PN junction diode, hence it has very thin depletion layer, therefore, it allows more current than the normal PN junction diode.
- Zener diode allows current just like a simple PN junction diode when it is forward bias but it also allows current when its reverse bias if the applied reverse voltage is greater than the Zener voltage (Zener diodes have particular Zener voltage which indicates that this diode will regulate voltage up to this limit).
V-I Characteristics of Zener Diode:
In forward bias Zener works as an ordinary diode but when it is operated in reverse bias its shows the specialty of voltage regulation, it must be noted that in fig given below. The reverse voltages are increasing but reverse current extremely small (up to Vz) and remains constant till reverse voltages are reached at knee voltages (the voltages at which current rapidly increasing Is known as knee voltages) after the knee voltages reverse current is rapidly increasing its known as Zener current, (the Zener current is the effect of Zener resistance or we can say Zener impedance which decreased when Zener current rapidly increased, above the knee Zener breakdown voltages are essentially constant).
How is voltage across Zener Diode constant?
A Zener Diode is a special type of diode which is designed to operate in reverse bias because it has very thin depletion region that makes this diode to handle large reverse breakdown voltage.
- In forward bias It behaves like ordinary PN junction diode above 0.7v the diode conducts current in one direction.
- In Reverse bias it behave like voltage regulator because when the reverse voltage is applied in the reverse bias condition Zener diode do not conduct until the reverse voltages reaches to the breakdown voltage (Knee voltages) of Zener then the voltage across load is constant up to Knee voltages and the current increases in reverse direction as shown in V-I characteristics of Zener.
How Zener diodes turn ON?
When the voltage across the Zener diode is greater than the breakdown voltage of Zener it breaks the barrier potential and Zener starts to conduct with a fixed voltage and this condition is known as turn ON condition.
it will not conduct until the source voltage is greater than the Zener voltage.
It will regulate voltage up to its Zener voltage value. Every Zener diode has particular Zener voltage (Knee voltages) in that they can regulate voltage.
Zener Equivalent Circuits
- It is like constant voltage source.
Effect of changing load on voltage regulation:-
There are two cases:
- When load resistance is to be zero (Conducting State)
- When load resistance is to be infinite (Open State)
In the first case when the load resistance is to be zero maximum current flows through the load and Zener diode is working as a voltage regulator.
In the second case when the load resistance is to be infinite no current goes to load because of high resistance so the all of the current flows through the Zener diode, because of the heavy current diode may be destroyed in this condition hence, this condition is mostly not desirable.
- It has very high Power dissipation capacity.
- More accuracy
- It has Small size
- Cheap voltage regulators because these diodes are cheap in price.
Practical Applications of Zener Diode:
In many electronic applications, it is desired that the output voltage should remain constant regardless of the variations in the input voltage or load. In order to ensure this, a voltage stabilizing device, called Zener voltage stabilizers is used.
- It is used in rectifiers’ circuits to get constant voltage at the output.
It is used as a voltage reference.
They are used in voltage stabilizers or shunt regulators.
They are used in switching operations
They are used in various circuit protections.
Zener diodes are used in Ammeters and voltmeters.