Electronics notes/Varistors

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✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

A varistor is a short name for a Voltage Dependent Resistor (VDR), (a.k.a. VARIable resiSTOR).

They act not unlike zener diodes, and have applications similar to polyfuses: often used to protect a load against relatively transient high voltages, wasting that energy as heat.

Varistors are often specifically a metal-oxide varistor (MOV), to the point MOV and varistor are near-synonyms to most people.

A MOV responds to voltage: high resistance at low voltages, lowering resistance at high voltages.

But not linearly - find a varistor I-V curve, and you'll note they present a steepish curve over a specific threshold (much like diodes, because its workings are like a diode's, though MOVs are bidirectional so can be used in AC), the MOV's rated voltage is basically that curve's knee, where it starts conducting noticeable current.

This characteristic makes them useful in parallel with a sensitive load, to protect that load against high transient voltages / small surges, effectively shunting them away and wasting them as a heat.

Which is also why they cannot deal with sustained over-voltages; they would basically become the load, heat up, and probably burn - so MOVs used as protection are often combined with other protective elements .