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What is a varistor and how does it protect against voltage spikes?

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A varistor, short for "variable resistor," is an electronic component used to protect sensitive electronic devices and circuits from voltage spikes or transient voltage surges. Voltage spikes, also known as transients, are short-duration, high-amplitude voltage increases that can occur in electrical circuits due to events such as lightning strikes, switching of inductive loads, or sudden power interruptions.

A varistor is made from a semiconductor material, often zinc oxide (ZnO), with a highly non-linear voltage-current characteristic. This means that its resistance varies significantly depending on the applied voltage. Under normal operating conditions, when the voltage across the varistor is below a certain threshold (often referred to as the "clamping voltage"), the varistor has a very high resistance and effectively isolates the protected circuit from any small voltage fluctuations.

However, when a voltage spike occurs and the voltage across the varistor exceeds its clamping voltage, the varistor's resistance drops dramatically. This low resistance allows the varistor to divert the excess voltage away from the protected circuit by conducting the transient current through itself. This action effectively limits the voltage level reaching the sensitive components downstream and prevents them from being damaged by the excessive voltage.

In summary, a varistor acts as a voltage-sensitive switch that remains in a high-resistance state during normal voltage conditions but quickly transitions to a low-resistance state when voltage spikes occur. This protective behavior helps to safeguard electronic equipment by absorbing and dissipating the energy of voltage transients before they can cause damage.

It's important to note that while varistors provide effective protection against transient voltage spikes, they have limitations. They are designed to handle relatively short-duration events, so they might not provide complete protection against prolonged overvoltages. Additionally, varistors can degrade over time as they absorb multiple transient events, so they might need to be replaced if they become worn out or ineffective. For more critical applications, additional protective measures such as surge protectors, overvoltage crowbar circuits, or combination approaches might be employed to provide comprehensive protection against voltage spikes.
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