What Is A Varistor?

Posted on August 19, 2022 John Smith What Is A Varistor?

Whether from internal causes like switching surges, or natural causes such as lightning strikes, overvoltage can seriously damage a circuit. To account for this risk, many electrical components have been created to manage or suppress the flow of current in the event of a power surge. A varistor, also called “Voltage- Dependent Resistor” (VDR), is one such device which has an electrical resistance that varies with applied voltage. Due to their ability to adapt to the voltage of a current, varistors are mainly used to clamp the voltage when a circuit is subjected to overvoltage and to absorb excess current to protect sensitive devices. The material of the resistor body is a semiconductor, meaning it will go from being an insulator to a conductor when subjected to a specific voltage.

How Do Varistors Work?

As with any semiconductor, varistors have a voltage threshold, below which, the current flowing through the resistor will be extremely small. At this level of current, the varistor is equivalent to an off-state switch. When the voltage applied to the varistor exceeds its threshold, the current flowing through it increases sharply so that there is very little resistance. In other words, when the voltage applied to it is higher than its threshold, a varistor is equivalent to a closed state switch.

What Are the Functions of Varistors?

As a result of their capacity to act like a switch, varistors are used to control the voltage passing through a circuit. However, they are unique for their ability to adapt to overcharge. This is because, when the voltage exceeds the threshold, the current flowing through it surges and the impact on other circuits does not change much. This adaptability is useful in applications where the surrounding circuitry requires a specific range of voltage and may be damaged if exposed to overvoltage. This protective ability is useful in a wide range of equipment, including modern televisions.

Varistors are mainly used for overvoltage protection in a circuit, but because of their volt-ampere characteristics similar to a semiconductor Zener, they also have a variety of circuit element functions. For example, the varistor is a kind of DC high voltage, small current, voltage stabilizing element with a stable voltage of thousands of volts or more, which cannot be reached by silicon Zener. For this reason, varistors can be used as a voltage fluctuation detection element, a DC level shifter bit element, a fluorescent starting element, a voltage equalizing element, and more.

What Is a Metal Oxide Varistor?

The most common type of varistor is a metal oxide varistor (MOV), which consists of a ceramic block made of zinc oxide particles and a small number of other metal oxides and polymers, sandwiched between two metal sheets. A diode effect is formed at the junction between particles and oxides in the ceramic block, meaning that a current can pass through it in one direction. Due to the large number of messy particles, it is equivalent to a large number of back-connected diodes. These back diodes only allow a small flow of current at low voltage to pass through them. When a high voltage occurs, the diode reverse collapses due to hot electrons and tunneling effect, and a large current flows. Therefore the device will exhibit high resistance at low voltage and low resistance at high voltage. Metal oxide varistors are currently the most used voltage clamping devices because their use of metal oxides make them very effective in absorbing short-term high-voltage currents.


Varistors are a useful device for protecting electrical systems and equipment from experiencing damage due to overvoltage in the circuit. As a result of their composition, they can accept a voltage surge without affecting its normal flow into surrounding instruments. On AFR Enterprises, we offer a wide range of aviation parts, IT hardware parts, connector parts, and other electronic components including high quality varistors. Moreover, our team is dedicated to providing the best procurement service on the market. We make sure that our vast inventory of parts is easy-to-navigate, that prices are competitive, and that shipping is always quick and convenient. Get started today by submitting an Instant RFQ on our website to receive a quote on any of the items we offer in just 15 minutes or less!

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