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What is a Varistor and Their Applications?

Electrical resistors are a common electrical component that provide a circuit with electrical resistance. This may be in place to reduce current flow, divide voltages, adjust signal levels, terminate transmission lines, and cater to other operations. Resistors are basic components of electronic circuits, and they may be found in a multitude of electronic devices and networks. Varistors are a special type of resistor, allowing for varying resistance to serve as protective devices against excessive transient voltages. Varistors play important roles in a variety of applications to protect equipment and lines, and they have been prevalent in electronics since the early 1900’s. In this blog, we will provide a short overview on the functionality of varistors, as well as the applications that they benefit.

Varistors are also known as voltage dependent resistors (VDR), and their variability of resistance is provided by a change in voltage. In general, resistance of the varsitor increases as the amount of voltage is decreased, and vice versa. If there is an excessive increase of voltage, the resistance drops exceptionally. Because of this ability, varistors are very beneficial in protecting circuits from surges in voltage. Typically, these spikes in voltage that varistors serve to protect against are caused by electrostatic discharges or lightning strikes.

Varistors were developed and released in the late 1920’s, serving as a new type of rectifier. During the 1930’s, varistors began to replace electron tube circuits within telephonic transmissions. The Western Electric type 500 telephone set of 1949 also brought out a dynamic loop equalization circuit that utilized varistors to adjust transmission and receiving signal levels automatically. Due to their high resistance and low altering of signals, they proved useful. In the 1970’s, Japanese researchers studied the electronic properties of zinc oxide, ultimately leading to them becoming the preferred method for circuit protection.

Varistors may come in various types, and the most common is the metal oxide varistor. Metal oxide varistors are manufactured from zinc oxide grains that are constructed in a ceramic mass. These grains are then placed in a matrix with bismuth, manganese oxides, and cobalt which are all in between two metal plates. In between each grain is a boundary that serves as a diode junction, constricting current flow in a single direction. With this construction, the metal oxide resistor is able to provide high resistance with low voltages and low resistance at high voltages.

Due to the characteristics and properties of varsitor components, they may be used in a variety of devices as a surge protector device. Varistors are common within surge protector power strips, due to their ability to mitigate and protect against the effects of lightning strikes, electrostatic discharges, and inductive charges. Low capacitance varistors may also be used to protect telephone lines, radio communication equipment, cable TV systems, power supplies, microprocessors, electronic equipment, low voltage board level components, car electronics, and industrial high energy AC equipment.

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October 23, 2020

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