What is a varistor?

Hello dear friends,

A varistor is a voltage-dependent resistor, whose resistance changes in response to changes in applied voltage. Its name is derived from "VARIable resiSTOR." Varistors are often used to protect circuits against excessive transient voltages, effectively acting as a voltage clamp.

The most common type of varistor is the metal-oxide varistor (MOV). An MOV consists of a ceramic mass of zinc oxide grains in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates, which are the electrodes of the device. When a small or normal voltage is applied across the electrodes, the varistor remains non-conductive. But when a higher-than-specified voltage is applied, the varistor becomes conductive, allowing a lot of current to flow through and thus diverting the excessive voltage away from sensitive components.

What is a Varistor (MOV)? — Live Surge Protection Demo

A Metal Oxide Varistor (MOV) is normally high-resistance and carries no current. When the line voltage exceeds its rated threshold (MCOV), its resistance drops rapidly, diverting current to ground and protecting the connected load from spikes. Once voltage returns to normal, the MOV goes back to its passive state.

Line Voltage (Vrms)

Selected: 230 V

DERELL

MOV State

Passive (High R)

Load Safety

Safe

Indicator

Peak (approx): 325 V

Clamp threshold: ≈430 V_p

Note: An MOV does not normally conduct. When voltage exceeds its rating, it clamps the surge to ground. In practice, it should always be paired with a fuse or thermal protection, since very large surges may permanently damage it.

Key features and uses of varistors:

    Overvoltage Protection: Varistors are frequently used in power supply circuits to protect against voltage spikes (surges). For instance, they can be found in surge protectors to safeguard electronic devices from voltage spikes in the mains power supply.

    Non-linear Response: Varistors exhibit a non-linear voltage-current relationship, meaning their resistance decreases as the applied voltage increases beyond a certain threshold.

    Short Response Time: Varistors can respond to overvoltage events very quickly, often within nanoseconds, providing rapid protection to the circuitry.

    Limited Lifespan for High Energy Events: Each time a varistor responds to a high-energy voltage spike, it undergoes a little wear. If exposed to many such events or a few very large ones, a varistor can fail or degrade over time.

In summary, varistors are essential components used in many electronic devices and systems to protect against transient voltage events. Their primary function is to divert excess voltage away from sensitive components, thus safeguarding the overall circuitry.

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