By John Papiewski Updated Mar 24, 2022
A metal‑oxide varistor (MOV) is a compact, cost‑effective component that safeguards electronics from voltage transients—such as those caused by lightning strikes—by absorbing excess energy and preventing damage to downstream circuitry. When the line voltage exceeds its rated threshold, the MOV clamps the surge, protecting sensitive devices. In addition to its voltage rating, an MOV has a maximum energy rating; exceeding this can cause the device to fail like a fuse.
Begin by reviewing the schematic to determine the operating voltage of the section where the MOV will be installed. For an AC circuit, divide the RMS voltage by 0.707 to obtain the peak value (e.g., 110 V AC ÷ 0.707 ≈ 156 V). For a DC circuit, use the specified voltage directly. Ensure you identify the exact point in the circuit where the MOV will be mounted.
Multiply the working voltage by a factor of four to set the MOV’s clamping voltage. This is the voltage at which the MOV begins to absorb energy; below it the component has no effect on normal operation.
Estimate the energy rating in joules— the total energy the MOV can safely absorb before failure. Use the formula: peak surge voltage × surge current × surge duration. For example, 1,000 V × 0.1 A × 0.01 s = 1 J. Typical 110‑V AC equipment uses MOVs rated between 10 and 200 J; small DC devices may need only a few joules.
Consult the distributor catalog. MOVs are found in the “circuit protection” section, sorted by manufacturer, voltage, energy rating, and package type. Selecting the correct part ensures reliable surge suppression.
You can combine MOVs in series or parallel to tailor clamping voltage and energy handling. Two 500‑V MOVs in series clamp at 1,000 V; two 100‑J MOVs in parallel can absorb 200 J.
