An MOV, or metal oxide varistor, is an electronic device that protects equipment from severe power line surges. It absorbs the energy in a voltage spike, sometimes sacrificing itself in the process like a fuse. To select an MOV, you'll need to know a few things about the equipment it will be protecting, such as its normal working voltage and its environment. Keep in mind that an application may call for several MOVs to protect different paths in a circuit.
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Things you need
- Equipment schematic or specifications
- Varistor distributor catalogue
- Pencil and paper
Examine the equipment specifications and determine its normal working voltage. For DC equipment, simply use the power line voltage given in the specifications. For AC equipment, use the RMS voltage value.
Multiply the normal working voltage by 3 to 4 to obtain a clamping voltage range for the MOV.
Determine the number of MOVs needed. Use one MOV for every incoming wire that has a voltage different from ground, as long as the wire is vulnerable to voltage spikes. For example, use one MOV for a two-wire system, whereas you can have up to three MOVs total, one for each wire pair, on a system having hot, neutral and ground wires.
Determine the MOV's energy dissipation requirements from its environment. MOVs range from a few joules of energy dissipation to thousands.
Determine your packaging and wiring needs. MOVs may have standard leads, screw terminals or surface mount packaging.
Note the operating voltage, clamping voltage, joule requirements and number of MOVs on paper.
Look through the MOV distributor catalogue and find part numbers that match the requirements you wrote down. The catalogue will list working voltage, clamping voltage and joule ratings as well as packaging and recommended use.
Tips and warnings
- Since an MOV is designed to block only severe voltage spikes, its voltage rating will be a few times greater than the equipment's normal voltage. Otherwise, the MOV will fail prematurely.
- To calculate RMS voltage for an AC system, multiply peak voltage by .707. The 120-volt AC power line standard in the U.S. is an RMS value, so you can use that directly.
- You can wire multiple MOVs in parallel to dissipate more energy than a single one.
- A typical, small MOV used in household or office equipment will dissipate 200 to 500 joules of energy. Heavier-gauge power wiring will deliver more energy, including the energy of spikes. An industrial environment will have more electrical noise, requiring MOVs with over 1000 joules of energy capacity.
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