What is a Varistor?

Varistors protect circuits against excessive voltage by acting as spark gaps.
Varistors come in many shapes and sizes.
A nearby lightning strike may damage a varistor.
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  • Written By: L. S. Wynn
  • Edited By: L. S. Wynn
  • Last Modified Date: 19 October 2014
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A varistor is a type of resistor with a significantly non-ohmic current-voltage characteristic. The name is a portmanteau of variable resistor, which is misleading since it is not continuously user-variable like a potentiometer or rheostat, and is not a resistor but in fact a capacitor. Varistors are often used to protect circuits against excessive voltage by acting as a spark gap.

The most common type of varistor is a Metal Oxide Varistor (MOV). This contains a mass of zinc oxide grains, in a matrix of other metal oxides, sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, causes by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.


If the size of the transient pulse (often measured in joules) is too high, the device may melt, or otherwise be damaged. For example, a nearby lightning strike may permanently damage a varistor.

Important parameters for varistors are response time — how long it takes the device to break down —, maximum current and a well-defined breakdown voltage. When used in communications lines (such as phone lines used for modems), high capacitance is undesirable since it absorbs high frequency signals, thereby reducing the available bandwidth of the line being protected.


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Post 4

Re.: Question of mega ohms/ kilovolt when testing insulation. A standard or passing result is actually what is needed or required to make the product or equipment safe and reliable. I tend to think 20 meg ohm /kv is good, but that is only relative to the level that is needed. Most likely 2-5 meg would be minimum and nearly anything would be safe with 100-150 meg.

As an example; in a lightning strike that put 20,000 volt surge into a line, you would be very safe with 2.5 Billion ohms which is 125meg ohm/kv. Hope this helps, macgyver.

Post 2

when performing an insulation resistance test to a medium or high voltage equipment, what is the standard or the passing result required in MegaOhm/kV?...

Post 1

What's the physical meaning of "The boundary between each grain and its neighbour forms a diode junction".

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