A portmanteau of variable resistor, the varistor is the active semiconductor in many surge protection devices that stops power surges. It is a voltage dependent resistor (VDR), meaning it operates in a non-conducive (or standby) mode until a higher than normal flow of electricity passes through it.
The higher the voltage entering the varistor the less resistance the varistor has to current flow and vice versa.
The type of varistor used in surge protectors is a Metal Oxide Varistor (or MOV). It looks like a small, colored ceramic disc with two needle-like prongs used as plugs. The ceramic shell houses tiny grains of zinc and other oxides clasped between two plates (which act as electrodes) to create what is called a diode through which current is allowed to flow in one direction only.
During a surge, the varistor heats to a point where it is switches to a highly conducive state (meaning it turns on) and the direction of current changes (clamps) to the direction of the ground area. This response time is instant, less than a nanosecond.
The change in power flow is dependent on the amount of surge power entering the MOV. So, regular current will continue on to power your electronics, but excess will be drawn into the MOV and redirected to the grounding area once the amount of current exceeds normal levels. This is referred to as clamping voltage.
Once the threat of surge power has ebbed, the varistor cools and returns to its non-conducive state.
The only problem with varistor technology is that each surge will degrade its ability to suppress a surge and eventually the semiconductor will fail to respond. In addition, the lifespan of a MOV, which is measured by its joule rating, varies so true value is indeterminable.
The delayed degradation depends on how quickly the MOV can disperse excess heat and return to a normal temperature. Several factors contribute to this, environment being number one.
In a perfect scenario the MOV will never encounter a surge and will therefore retain its effectiveness indefinitely. However, surges are common in every household and so the next best combatant to slow degradation has been for manufacturers to install up to three metal oxide varistors in a single device.
Other factors include design of the surge protector in which the varitors are encased and how well varistor manufacturers construct their product.
What you, the consumer, can do to lengthen a surge protector’s lifespan is to keep your device in as well ventilated an area as you can.
Nonetheless, if you live in an area where electrical storms are prevalent, the lifespan of your MOV-based surge protection device may be shortened.
As a side note, if you’ve ever wondered why some surge protector manufacturers charge high dollar for MOV-based devices, this should make you wonder even more: A standard MOV costs somewhere in the neighborhood of 50 cents.
While some would argue you get what you pay for, quality cannot always be measured by how expensive something is.