Varistor or Metal Oxide Varistor MOVPublished on 1/11/2018 & updated on 3/11/2018
Definition of VaristorVaristor is a variable resistor but its resistance variation depends on the imposed voltage on it. The typical feature of a varistor is that its resistance cannot be controlled manually like potentiometer rather after certain voltage level the resistance of varistor decreases rapidly again below that voltage the resistance gets established. Depending on this feature varistors are widely used for transient overvoltage protection in electrical and electronic circuits.
Characteristic of VaristorIf the voltage applied across a varistor increases slowly it will be observed that there will be no current or very tiny current (reverse leakage current) through the varistor below a certain voltage level in either direction. After certain voltage level, the resistive character of the device suddenly changes and current rises sharply through the device. This is because, at this voltage, the resistance suddenly falls to very low value and the device becomes very much conductive. This voltage is known as varistor voltage. The voltage is specified on the characteristic, with the value when the current through the varistor just crossing 1 mA.The varistor has its own current rating. There is a maximum safe current that a varistor can withstand for a specified pulse duration. The voltage across the varistor which causes this current is known as clamping voltage. This voltage value is also sometimes referred to as discharge voltage. The varistor should be ensured to be operated within this transient overvoltage level otherwise there may be permanent damage in the varistor due to intolerable excess heating.
Current surge is another typical parameter of the device which determines the maximum value of current that a varistor can withstand for a specific pulse duration before it gets damaged. The manufacturers generally recommend replacing the device once it has withstood this surge current.
The peak of operating system voltage must not cross the varistor voltage. Although the maximum operating voltage of a varistor in case of AC rated with RMS value but that value is so chosen that peak of waveform must not cross the varistor voltage level otherwise waveform of the system voltage may be distorted. The maximum operating voltage of the device is taken as 20% above the nominal circuit voltage across which the varistor is connected. The same philosophy is true for both AC and DC system.
At last, we would like to say that the characteristic of a varistor is same for both positive and negative direction of current through it. This is the reason why same varistor can be utilized for both AC and DC systems.
Construction of VaristorVaristors are basically constructed by sintering zinc oxide grains in ceramic substrate. A small disc of material is attached between two metallic discs. Metallic terminals are welded with the metal discs. The body of the varistor is covered with plastic insulation cover as shown below.
Working Principle of Metal Oxide VaristorIn the inner structure of varistor inter zinc oxide grain boundaries behave as PN junction. When a voltage is applied across a varistor some of them become forward biased and some of them become reverse biased. The situation can be visualized as the series and parallel connection of a number of forward and reverse biased PN junction diodes. The entire voltage across the varistor is distributed among series of reverse biased PN junctions. As a result, there will be ideally no current through the varistor. Although a tiny current may flow as reverse saturation current of the diodes. The condition can be viewed as a high resistance offering by the device through itself. Now if the voltage across the device is further increased beyond the threshold value, avalanche breakdown starts occurring on the reverse biased junctions and the current through the varistor is suddenly increased as at that condition the resistance offered by the junctions has become low enough. In this way, the varistor suppresses the overvoltage to reach to the part of the circuit across which the varistor is connected for overvoltage protection.
Applications of Varistors
- MOV is used as main building blocks of lightning arrestor or surge arrestor or surge suppressor in electrical power systems.
- These are also used in low voltage electrical circuits and electronic circuits for overvoltage protection of different circuit elements and also the entire circuit or part of the circuit.
- These are used in dc power circuits as well as dc tiny voltage circuits for the same purpose.
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