How to Use a Variac

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Variable autotransformers, or variacs, are handy, inexpensive devices used to adjust the AC voltage for electronic devices and lamps. A technician will use it to safely monitor equipment’s response to AC power. A performing musician may run into poor voltage at a venue; plugging a guitar amp into a variac will compensate for this. A cinematographer would use a variac to adjust the lighting on a movie set. Efficient, sturdy and easy to use, variacs are necessary tools.

    Calculate the variac’s maximum power-handling capacity. Multiply its amp rating by 84.8. For example, a 10 amp rating x 84.8 = 848 watts.

    Choose a lamp rated at less than the value you obtained in step 1.

    Make sure the variac’s switch is off. Make sure its dial is set to zero.

    Plug the lamp’s power cord into the variac’s electrical outlet.

    Plug the variac’s power cord into a wall outlet.

    Turn the lamp and variac power switches on.

    Turn the variac knob clockwise to increase voltage; rotate it counterclockwise to turn it down. Note that the lamp becomes brighter and dimmer as you turn the knob.

    Things You'll Need

    • Variac
    • Floodlamp with incandescent bulb


    • R.M.S. power is a conservative way to figure equipment’s power-handling capacity. It’s .707 times the peak power. In the case given here, 120 volts times the current rating times .707 equals 84.8 times the current rating. If you use less than the calculated R.M.S. power, the variac won’t be overloaded.

      Variacs usually have a fuse. If the variac is not working, check the fuse.

      You can use a lower-rated fuse in a variac (say, 5 amps instead of 10) if you’re concerned your equipment has a short circuit or other wiring problem. If the fuse blows, you’ve verified that a problem exists without putting excess strain on other equipment. Never use a fuse with a higher amp rating than the variac’s.

      If your variac has voltage markings, these will only be as accurate as your household voltage. It will output a voltage that’s proportional to that of the incoming voltage. For example, if your local voltage is 90 volts, the variac will output slightly less voltage than what’s marked on the dial.


    • A variac is not a motor speed control; don’t attempt to use it on an AC motor.

      Most variacs, at the maximum knob setting, will output over 120 volts. Use caution when making adjustments; high voltages can damage equipment.

      Some older variacs lack a grounded outlet. Use caution when plugging equipment into these.