Metal Oxide VaristorMaria Willeth G. Sosa

Shemaiah H. BeguirrasBS ECE IV

Varistor

A varistor/voltage dependent resistor (VDR) is a component which has a voltage – current characteristics that is very much similar to that of a diode. 

Metal Oxide Varistor• MOV is the most commonly used

type of varistor.• made from a mixture of zinc

oxide and other metal oxides like cobalt, manganese and so on and is kept intact between two electrodes which are basically metal plates.• an MOV is basically a huge

number of diodes that are connected parallel to each other.

MOV-Construction

In a metal oxide varistor, the two immediate neighboring  zinc grains will form a diode junction between their borders. Hence a metal oxide varistor can be considered as a collection of huge number of diodes that are connected in parallel. Because of this, when a tiny voltage is applied between the electrodes, the reverse leakage current that appears across the diode junction is very small.

When the applied voltage increases and reaches the clamping voltage, the diode junction breaks due to avalanche breakdown and electron tunneling , and allows a huge current through it.

MOV-How it Works

MOV-Specifications• Clamping voltage

This is the voltage which short circuit the varistor. A lower clamping voltage indicates better protection. But on the other hand the voltage must not be that low, that smaller power changes destroy the varistor. • Energy absorption / dissipation

This rating is given in joules and shows how much energy the varistor can absorb. A higher number indicates greater protection. • Response time

Varistors switch fast but not immediately. There is always a very slight delay as they respond to the power surge. The longer the response time the longer the connected application is exposed to the surges.

MOV-Performance• The varistor is mainly used to perform as a line voltage

surge suppressor. The device does not conduct when the voltage across it is below the clamping voltage.

• When there is an increase in the energy rating, there will be an exponential change in the life of the varistor. Thus, there will be a change in the transient pulses that the device can manage. This increases the clamping voltage when each transient breaks down.

MOV-Applications