A metal oxide varistor, or MOV, is an electronic component that protects equipment from voltage surges in a power line, such as from lightning strikes. Simple and inexpensive, the MOV absorbs the energy in a surge, preventing it from damaging other devices in a circuit. Excessive voltage triggers the MOV into action, so to size it for surge protection, select a voltage rating suitable for the equipment you want to protect. In addition to voltage, MOVs have a maximum energy rating, above which they will burn as a fuse does. The energy depends on the equipment and environment in which the MOV will work.
Things You'll Need
- Electronics schematic
- Electronics component distributor catalog
Examine the electronics schematic and determine the working voltage of the circuit. For alternating current, or AC, divide the RMS, or root mean square, voltage by .707 to obtain a peak value. For example, dividing 110 volts AC by .707 gives 156 volts. For direct current, or DC, simply use the voltage specified on the schematic. As various parts of a circuit may have different voltages, make sure you identify where in the circuit the MOV will go.
Multiply the working voltage by a factor of 4 to get the MOV's clamping voltage. This is the voltage at which the MOV begins to absorb energy. Below this voltage, it has no effect on normal circuit operation. It only acts when the voltage spikes above a threshold value.
Estimate the MOV's energy rating in units of joules. This is the electrical energy that the MOV can safely absorb at least once before failing. You can calculate the energy by multiplying peak surge volts by amps, and then by seconds of time. For example, 1,000 volts x .1 amps x .01 seconds equals one joule. To estimate, low- to medium-power 110-volt AC equipment may have MOVs rated for between 10 and 200 joules. Small DC-powered devices may need an MOV rated for a few joules.
Find suitable MOVs in the electronics component distributor catalog. They will be located in a section such as "circuit protection," which also includes fuses and circuit breakers. The catalog organizes the MOVs by manufacturer, voltage and energy rating, and physical package type.
Tips & Warnings
- You can combine two or more MOVs for greater flexibility in protecting your circuit from surges. MOVs connected in a series circuit, or one after the other, will multiply the overall clamping voltage. For example, 2 MOVs rated at 500 volts will clamp at 1,000 volts. Connected in parallel, they can dissipate more energy. Two 100-joule MOVs will absorb up to 200 joules of energy.
- Photo Credit Comstock/Comstock/Getty Images
What Is a Varistor?
A varistor is a semiconductor device that is used for line filtering. A line filter is an electronic filter that is used...
What Is a Branch Circuit?
Every home that uses electricity has a branch circuit. Branch circuits send power to outlets, lights, washers, dryers and other appliances. Each...
Do Surge Protectors Go Bad?
A surge protector in a power strip can eventually lose its ability to protect electrical components over time, even though the power...
How to Install a Varistor
Made of metal oxides and granular material, Varistors serve a very important safety function on electrical appliances and components that have a...
How to Read Schematics
An introduction to reading electrical schematics
How to Identify a Varistor
A metal oxide varistor, or MOV, protects sensitive electronic equipment from voltage spikes in the power line. A gadget may have thousands...
How to Test a Varistor
A varistor is an electronic component that protects expensive devices from harmful power surges, much like a shock absorber. For example if...
How to Select an MOV Varistor
An MOV, or metal oxide varistor, is an electronic device that protects equipment from severe power line surges. It absorbs the energy...
How to Calculate Superheat and Subcooling
Air conditioning and refrigeration systems provide cooling (and heating) by circulating a refrigerant through a system containing a compressor, condenser, thermal expansion...
MOV vs. RC Surge Suppression
Surge suppression is a critical element in protecting today's sensitive electronics. This protection comes in a variety of forms, including the use...