Varistor Metal Oxide Varistor is nonlinear Resistor

Varistor - Metal Oxide Varistor

Varistor is a non linear resistor made of semiconductor and current through which depends non-linearly on the applied voltage across it. The most commonly used form of that is metal oxide varistor or MOV. The current in that device is a function of the applied voltage and given as,


I = kV^α

where k is a constant depends upon the quality of the material and size of the device and α is the non linearity exponent depends upon the quality of the material.
Metal oxide varistor or MOV is a simple voltage dependent resistor, whose resistance becomes suddenly very low after a predetermined value of applied voltage. It absorbs and bypasses surge energy for both negative positive cycle of surge voltage pulses and protect the sophisticated electronic circuit elements. Some time during very heavy surge and lightning strike the varistor may be damaged itself to keep the other costly electronic elements safe.
Although this is a non linear resistor but it is mainly used for economical protection against high voltage transients in different electronic circuits. Transient suppressor diodes are used for same purpose but metal oxide varistor or MOV has some advantages over it. The former absorb much more transient energy and can suppress both positive and negative transients. Actually in this device the electrical resistance varies with voltage across it. Here if voltage crosses a certain predetermined value, then the resistance becomes significantly low. As it is connected across the circuit, the transient is bypassed through the varistor and keep the sensitive costly circuit component safe. Metal oxide varistor is made of non-homogeneous material in such a way that gives a rectifying action at the contact points of two particles. Many series and parallel connections determine the voltage rating and the current capability of the device.The zinc oxide based non linear resistors are primarily employed to protect solid state power supplies from low and medium surge voltages in the supply line.

Non linear resistors are primarily made from silicon carbide obtained by electrically heating a mixture of quartz sand with carbon to a temperature of about 2000^oC. Elements made by baking discs molded from a mixture of silicon carbide, graphite and water glass serve as a negative resistivity material.

Different Characteristics of Varistor

Maximum Continuous Voltage of Varistor

The maximum steady voltage that can be applied across a metal oxide varistor continually without any harm is referred as maximum continuous voltage of varistor.

Varistor Voltage

This is the voltage across the MOV for which 1 mA current flows through it. The measurement should be done for very short period to avoid heat perturbation. The varistor voltage, is a point on the V - I characteristic, utilized for easy comparing of different models and types.

Maximum Clamping Voltage

The maximum voltage across a metal oxide varistor for which the standard pulse current through it rises in 8 µs and decreases in 20 µs (8 µs to 20 µs)according to IEC 60060-2, section 6. The specified current for this measurement is referred as class current of varistor.

Maximum Non Repetitive Surge Current

The maximum current allowed to flow through a varistor is depended upon its pulse shape, duty cycle and number of pulses. In order to determine the capabiity of withstanding pulse currents, it is normally allowed to warrant a maximum non repetitive surge current. This is given for one pulse characterized by the shape of the pulse current of 8 µs to 20 µs following IEC 60060-2, with such an amplitude that the MOV voltage measured at 1 mA does not change by more than 10 % maximum. This is the specified surge current beyond which the device may rupture with expulsion.

Maximum Energy Rating of Varistor

For one pulse current there will be an energy dissipation in the device. This dissipated energy depends upon.
1) The magnitude of the current.
2) The magnitude of voltage across the metal oxide varistor corresponding to its peak current.
3) The rise time of the pulse front.
4) The decrease time of the pulse front.
5) The non-linear characteristics of varistor.