A varistor (also known as a voltage-dependent resistor, VDR) is an electrical component whose resistance depends on the applied voltage. As the voltage increases, the resistance of a varistor drops. Their voltage-resistance behaviour is similar to that of a diode, except a varistor exhibits this behaviour with voltages of both polarities (where diodes are a one-way device).
They can also be called MOVs, but is strictly a type of varistor named after it's inclusion of a metal-oxide chemical.
They are commonly used as a form of circuit-protection against voltage transients, especially in mains and other high voltage applications.
Because their resistance in not fixed, they are called non-Ohmic devices. Note that this does not mean they do not obey Ohm's law, it's just that the resistance is different for each operating point.
This is the voltage at which the varistor starts to “conduct”. The varistor is considered conducting when the current through it exceeds some small threshold amperage (typically 1mA).
The surge current rating is an indication of how much current the varistor can withstand during a voltage transient event.
Typically a one-off (non-repetitive) and repetitive pulse energy rating is given for the varistor. The ratings are normally given for standardised transients, such as the 8/10 or 10/1000.
Comparison With Other Forms Of Circuit Protection
Varistors are preferred over TVS diodes when the product is price-sensitive, higher surge energy absorption is required, when physical space is not a constraint, or when high operating voltage capability is required.