Selenium Diode

• The metal oxide varistor or selenium diode is used to protect semiconductor device against transient over voltage or high dv/dt. 
• The MOV is called as voltage dependent Non linear resistor. 
• The resistance of the non linear resistor decreases as the voltage across it increases. 
• It is always connected across the semiconductor device.

Characteristics

• The characteristic of selenium diode is shown in the figure A. 
• The forward break over voltage of this diode is very low but is has comparatively higher reverse breakdown voltage. 
• This diode operates in the reverse biased condition. 
• When over voltage appears across semiconductor, it operates.
• As the selenium diode operates, the voltage across it remains constant but current increases sharply. There are two types of selenium diodes

       ( A ) Polarized

       ( B ) Non – polarized

• The symbol of both types of selenium diodes are shown in the figure A.
• The polarized type diode operates only in one direction whereas non – polarized diode operates in both directions.

Selenium diode for over votlage protection

You may also like : 

Static AC circuit breaker
Types of Conductor : Hollow Conductor, Stranded Conductor
Class E commutaion
Parallel Inverter
Principle of cycloconverter