How to Test a Metal Oxide Varistor

A Metal Oxide Varistor (MOV) is an electronic device that protects an appliance's power supply from voltage spikes and surges in the AC power line. Normally, the MOV has very high electrical resistance. If lightning strikes a nearby power line, the high voltage causes the MOV to become a shunt, preventing the electrical surge from harming sensitive equipment. After this happens, the device's fuse will blow. You can test an MOV simply by testing its resistance.

Unplug the device containing the MOV from the wall outlet. Unscrew the case and open it.

Identify the fuse inside the case. Remove the fuse and examine it. If the fuse is blown, set it aside and make a note to replace it. Identify the metal oxide varistor. In most smaller equipment, it is a two-lead, coin-sized device, similar to a disc capacitor with a bright colour, such as red, yellow or blue. Note if the MOV appears to have burn or scorch marks, indicating damage.

Touch the tip of the hot soldering iron to one of the MOV's leads to melt the solder, then draw off the solder with the desoldering pump. Work the lead loose with a long-nose pliers.

Turn the multimeter on and set its function knob to read resistance. Touch the meter probe tips to the MOV's leads and measure its resistance. If it has a resistance much under 100 ohms, it has blown. Desolder and remove the remaining lead and remove the MOV from the equipment. If the part has resistance over one million ohms, it is still good. If it's good, resolder the lead you removed in step three. If it's blown, remove the lead that's still connected.