Varistor (MOV / Varistor)
A voltage-dependent protection device that behaves like an insulator below a certain threshold voltage and, above it, drops its resistance rapidly and draws the current onto itself. It clamps transient overvoltage peaks; but it degrades a little with every pulse and its threshold shifts over time.
A metal oxide varistor is a ceramic made of zinc oxide grains. The barriers between the grains become conductive above a certain voltage. The pulse energy then flows through the varistor instead of the circuit to be protected.
- Selection: the rated voltage must be above the system's continuous peak voltage. Because the continuous operating voltage in a 24 V system can rise to 32 V, classes with a continuous DC rating of typically 36 V and above are chosen; a varistor selected too low draws current continuously in normal operation and heats up.
- Ageing: every pulse degrades the grain structure a little. Over the years the leakage current rises and it can eventually fail as a short circuit. That is why it is appropriate to use it together with a fuse in series.
- Speed: the response time is of the order of nanoseconds, but it is slower than a TVS diode and its clamping voltage is less sharp.
In vehicle-mounted installations the varistor is generally the first line of defence: it absorbs the large energy, while the fine clamping is done by the TVS diode on the board. Together they form staged protection.
Context in which this term is used: Installation, redundancy and field continuity