A varistor is a passive, two-terminal semiconductor device used for eliminating the voltage spikes in AC voltage. It protects electrical and electronic devices from overvoltage transients via voltage clapping. Varistors can automatically change their resistance value with the change in voltage across them.
Varistors are considered voltage dependent resistors as the resistance is varied and influenced by the voltage being applied. In the case that voltage spikes occur, the resistance drops, protecting circuits during voltage surges. Voltage surges often happen as a result of lightning strikes and electrostatic discharge. When such occurrences take place, varistors protect circuitry by rapidly changing from high resistance to low resistance.
A varistor is made using silicon carbide or metal oxide crystals which are pressed into a ceramic material. More than that, the electrical contacts are usually coated with silver or copper to improve their conductance. However, the electrical characteristics of the varistor are dependent on the atmospheric conditions and temperature of the operating environment.
It is important to note that varistors cannot be used in places where the temperature exceeds the rated operating temperature due to direct sunlight or other factors. Varistors cannot be utilized in humid and dusty environments, or places exposed to salt-mist or corrosive gasses. Solvents or acetones must be avoided as they can make the exterior covering of varistors deteriorate.
Intense vibrations, shocks, or pressure can also make the assembly crack, so varistors are not optimal for applications subjected to such stressors. Furthermore, peak current exceeding the rated maximum energy should never be applied. They should not be utilized in circuits that have a frequency over 1 Khz as this can damage the varistor’s elements. While perhaps obvious, they should not be installed near flammable substances.
Metal Oxide Varistors and Silicon Carbide Varistors
There are two main types of varistors: metal oxide varistors (MOVs) and silicon carbide varistors. MOVs serve as the most basic types, containing a ceramic mass of zinc oxide grains as well as other metal oxides wedged between two metal plates. The interface between each grain and its neighbor creates a diode junction that allows for unidirectional current flow.
When a small voltage is applied, only a tiny current flows, resulting from a reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to the combination of thermionic emission, electron tunneling, and large current flow. This behavior results in a highly non-linear current voltage characteristic, where the MOV has a high resistance at low voltages and a low resistance at high voltages.
Before MOVs, silicon carbide varistors were used in a myriad of applications. These devices use silicon carbide and find use in high power and high voltage applications. The benefit of these varistors is that they draw significant standby current. As such, they require a series gap to limit the standby power consumption.
The non-linear characteristics of varistors make them good options for use as surge protection devices. That being said, they have a wide range of applications. For instance, they may be used for microprocessors, power supply, and car electronics protection.
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