HOW ZENER DIODE WORKS

Zener Diode Working Function

Zener diodes are normal P-N junction diodes operating in a reverse biased condition. Working of the Zener diode is similar to a P-N junction diode in forward biased condition, but the uniqueness lies in the fact that it can also conduct when it is connected in reverse bias above its threshold / breakdown voltage.

 These are among the besic types of diodes used frequently, apart from the normal diodes.

Principle behind Zener diode Working :

As stated above the basic principle behind the working of a zener diode lies in the cause of breakdown for a diode in reverse biased condition. Normally there are two types of breakdown- Zener and Avalanche.

   

Zener Breakdown :

This type of breakdown occurs for a reverse bias voltage between 2 to 8V.  Even at this low voltage, the electric field intensity is strong enough to exert a force on the valence electrons of the atom such that they are separated from the nuclei. This results in formation of mobile electron hole pairs, increasing the flow of current across the device.  Approximate value of this field is about 2*10^7 V/m.

This type of break down occurs normally for highly doped diode with low breakdown voltage and larger electric field. As temperature increases, the valence electrons gain more energy to disrupt from the covalent bond and less amount of external voltage is required. Thus zener breakdown voltage decreases with temperature.

Avalanche breakdown

This type of breakdown occurs at the reverse bias voltage above 8V and higher.  It occurs for lightly doped diode with large breakdown voltage.  As minority charge carriers (electrons)flow across the device, they tend to collide with the electrons in the covalent bond and ,cause the covalent bond to disrupt.As voltage increases, the kinetic energy (velocity) of the electrons also increases and the covalent bonds are more easily disrupted, causing an increase in electron hole pairs. The avalanche breakdown voltage increases with temperature.So there is a relation between tempareture.

Zener diode applications :

Voltage regulator :

The load voltage equals breakdown voltage V-Z of the diode. The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.

Zener diode in overvoltage protection :

If the input voltage increases to a value higher than the Zener breakdown voltage, current flows through the diode and create a voltage drop across the resistor; 

this triggers the SCR and creates a short circuit to the ground. The short circuit opens up the fuse and disconnects the load from the supply.

Zener Diode Clipping Circuits :

Zener diodes are used to modify or shape AC waveform clipping circuits.

The clipping circuit limits or clips off parts of one or both of the half cycles of an AC waveform to shape the waveform or provide protection.

Zener Diode Reference
	Here is a handy zener diode list if you work with these critters.  This list is far from complete, but these are some common numbers we encounter.