This is a voltage dependent resistor. It is used to protect power supply circuits against voltage spikes.
- Normally its resistance is very high and the varistor has no effect on the circuit.
- At a specified voltage such as 380 Volts, the resistance drops rapidly. (This is a little higher than the highest voltage ever encountered in the AC mains supply in Europe (325 Volts)*).
- At voltages above the varistor's limit, a current (possibly a very large current) flows through the varistor.
- This large current effectively short circuits the spike and the circuit is protected.
- Because the current is momentarily large, the power supply fuse can blow and the varistor itself can be damaged. Both of these components can be replaced at low cost but the remainder of the system is protected.
- The varistor is connected between the live and neutral AC mains connections.
- Surge suppressors contain a varistor and sometimes a low pass filter as well.
*The mains supply in Europe is 230 Volts RMS. The peak voltage is 325 Volts.
