Abbreviation for Metal Oxide Varistor. A varistor works like a special type of variable resistor, hence the name varistor. These are semiconductor devices that can change their resistance to current flow based on the voltage applied to them. Specifically a varistor will rapidly lower its resistance as input voltage increases. This makes them very handy to use for protection of your gear against large surges and spikes on your electrical service. An MOV can be placed across your AC line (basically in parallel with your equipment) such that when a large spike occurs, its resistance drops, and the over voltage gets shunted back to ground instead of being passed on to your equipment. Most surge protection devices use MOV’s to do this dirty work.
On a 120v piece of gear, a MOV is generally positioned between the hot 120v wire and the neutral wire just after the main power switch. With no voltage applied to the MOV it is an open circuit. This means no voltage can flow across it (from one of its two leads to the other). When voltage is applied to the piece of gear and the power switch is turned on, as long as the voltage remains below a certain threshold (let’s say around 160v for a 120v unit) the MOV remains an open circuit. When the voltage supplied to the unit momentarily (nano seconds) spikes above this threshold, the MOV “clamps” or the circuit closes allowing voltage to flow from the hot to the neutral through the MOV. This is in effect a momentary (as long as the spike lasts) short circuit and has the effect of bringing the voltage down, at which point the MOV goes back to it’s open state.
One last thing about MOVs is that they have a limit to the number of spikes or transients they can take before they fail. When they fail they usually burn for a moment. This explains why sometimes a piece of gear will smoke and yet keep functioning. The MOV is not needed for the unit to function. It does however keep the normal everyday transients from getting to the more delicate parts of the circuit board. Without it your gear will not have as long a life. So any time you see or smell smoke come from a unit, get it checked out.