Ten Uses for an Autotransformer, or Variac

1) Testing a repaired electronic device.

After performing a repair on a piece of electronic equipment, it is best to slowly power up the device to avoid burning up the replaced parts. You will often discover multiple part failures one at a time. Using the Variac to slowly power up the repaired unit can prevent damaging the replacement parts.

2) Powering up an old radio or amplifier.

When powering up an old radio or amplifier that has not been used for an extended period of time, it is best to apply reduced voltage power to the unit in stages in order to re-condition the electrolytic capacitors.

3) Roasting coffee beans or nuts.

To obtain the freshest coffee possible, coffee connoisseurs prefer to roast and grind their own coffee beans. You can use a Variac to adjust the roasting temperature to the ideal value.

4) Adjusting the temperature of a resistance-type heater.

Resistance-type heating elements can be adjusted by varying the AC power applied.

5) Hot-wire cutting of Styrofoam.

Styrofoam and other types of foam are typically cut to the desired shape using a hot wire cutting system. You can use a Variac to adjust the temperature of the hot wire to the optimal value required for cutting.

6) Dimming incandescent lighting.

Incandescent lights may be dimmed by varying the applied AC voltage. This is not possible with newer LED or fluorescent lamps.

7) Changing fan speed over a narrow range.

The speed of some types of small AC motors may be varied over a narrow range by changing the applied AC voltage. A problem with this application is that the torque of the motor is also reduced which may result in the fan or motor stalling out. Also note that this technique cannot be used with all electric motors.

8) Compensating for line voltage drop in AC power applications.

Long power line runs can often result in voltage drops across the lines, especially in high current draw applications like hot tubs or spas. You can use a Variac to compensate for this voltage loss.

9) High-current unregulated DC power supply.

A high-current adjustable unregulated DC power supply can be constructed using a Variac, high current rectifier, and filter capacitor. Do not attempt this without being aware of the possible safety concerns when using a Variac. Since a Variac is not isolated from the AC power lines, the AC power is directly accessible. The ideal configuration would use an isolation transformer to isolate the AC power mains from the application.

10) Adjusting the voltage of an AC power supply.

The output of an AC power supply, such as an electric train transformer, can be adjusted with a Variac.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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