How to Identify a Varistor

By John Papiewski
You can identify varistors by their shape, color and location.

A metal oxide varistor, or MOV, protects sensitive electronic equipment from voltage spikes in the power line. A gadget may have thousands of transistors, resistors and other components, but only one or two varistors. While it may look similar to a capacitor, you can identify the device by its color, markings and location. The MOV will always be close to the equipment’s power supply and wired to a fuse. Many MOVs have a bright, solid color and are usually coin-sized.

Unplug the equipment from the electrical outlet. Using the screwdrivers, open the case or cover containing the electronics.

Follow where the AC line cord enters the case. The cord will pass through a grommet and will connect to a circuit board. You should see a fuse holder on the board or mounted to the case.

Continue tracing the incoming AC circuit to the power supply. In series with the fuse and in parallel with the power supply, you’ll find one or two MOVs. They will be either block or disc-shaped like a capacitor. Unlike a capacitor, which has capacitance and voltage ratings printed on it, the MOV will have a simple code or voltage. Like a capacitor, the MOV has two leads. Many disc MOVs are black, bright blue, red or yellow so they stand out. Smaller varistors may be oblong blobs on a pair of axial leads.

Close the electronics cover and reinstall the screws.

Tip

Vintage equipment made before the 1970s will not have varistors.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."