What is a Varistor?

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  • Originally Written By: L. S. Wynn
  • Revised By: C. Mitchell
  • Edited By: L. S. Wynn
  • Last Modified Date: 30 April 2020
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A varistor, also sometimes called a “voltage-dependent resistor,” is a part of electrical circuitry that helps channel and divert volts in order to keep the energy levels that are flowing through it constant. Understanding the specifics of what it does and how it functions can be complicated. On a very general level, though, these components work as internal surge protectors and help keep electrical currents contained within a given device. They are typically made of metal composites that have been specially formulated to absorb and conduct electricity at certain levels, though they aren’t normally considered to be true conductors. Almost all modern electrical appliances contain them, and they’re perhaps most obvious and most important in things like generators and transformers that receive regular surges of high voltage currents.

What it Does

The word “varistor” is a combination of the words “variable” and “resistor,” and in simple terms that is exactly what this component does: it resists electricity based on how much energy is coursing past and that energy’s intensity. Its main role is to act as more or less of a spark gap that protects circuit boards from unchecked voltage. Without them, electrical currents could run freely through a system at high levels. This could be both dangerous to humans and also runs the risk of “frying” or damaging the circuit board itself.

These components are often very small, though their size is usually relative to the sort of device or appliance at issues. In smaller devices like personal computers or home electronics they’re usually no more than about 0.4 inches (1 cm) across, but in generators and transformers they can be much bigger. Most are circular, and they generally have two pins or prods that extend out of them and into the circuit board of whatever it is they’re working to help protect. Many of the smallest examples are circular in shape, and are often said to resemble small coins or tiny batteries, though square-shaped models are common, too. There is a lot of variance when it comes to appearance, and a lot of this has to do with the specifics of the use. Small personal appliances necessarily have different needs than larger, more powerful things like telecommunications hubs and computer servers.

Most Common Uses

Most devices that use electricity have a need for voltage suppression, which means that nearly every electronic device, from alarm clocks to industrial machinery, needs a variable resistor. In small appliances this component isn’t often very noticeable and it often works in conjunction with other voltage checking measures to provide basic surge protection. Surge protectors — long strips of outlets that plug into wall sockets — are some of the most common places to find these resistors around the house. Larger machines are the ones that tend to depend on them more overtly, though.

Things like transformers, telephone switchboards, and large-scale mechanical appliances often depend on powerful resistors in order to prevent total meltdown and dangerous working conditions. Varistors in these settings tend to be a bit more powerful and often have the ability to trigger a shutdown of the entire device in the event that overly high voltages are detected. This usually necessitates immediate repairs and possible replacement of the sensing device, but can in most cases save the device as a whole.

How it Works

On a general level, these components function by shunting currents through a series of ion-charged particles sandwiched between two opposing metal plates in order to create barriers and junctions, effectively channeling the currents into specific places. The most common type is known as the metal oxide varistor, or MOV. Examples in this category use zinc oxide grains to absorb and more or less trap flowing currents, and force those currents to flow in just one direction. When currents double-back on themselves there is a heightened risk for problems.

The grains usually carry an electrical charge in and of themselves, which helps when it comes to both absorbing and directing currents. In most cases it also leads to increased adaptability. Small currents with low to moderate voltages pass across the plates and don’t usually require much action. High voltage currents can overwhelm ordinary diode junctions, though, which kicks the resistors into action. As a result, the MOV can be said to have a high resistance at low voltages but a low resistance at high voltages. In other words, it’s flexible in a variety of settings.

Importance of Energy Containment

The main role of this sort of device is to channel and shunt energy, but it isn’t a failsafe. Overly powerful bursts of energy can damage or even destroy it. Lightening strikes, for instance, often destroy these sorts of resistors or cause them to melt down.

Still, most problems can be anticipated, and in most instances the resistors are able to heed off serious problems. Two of the most important parameters are response time, which is how long it takes the device to break down, and maximum current and defined breakdown voltage, which set caps on acceptable energy levels. Different uses have different capacity needs and specifications.

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Discuss this Article

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 neighbor forms a diode junction".

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