variac

A variac, short for variable autotransformer, is an adjustable transformer that provides a continuously variable AC output voltage. It consists of a single electrical winding on a magnetic core (typically toroidal) with a sliding contact or brush that taps the winding. The mains input is connected to the entire winding, while the output is taken between the brush and one end of the winding, yielding a voltage proportional to the tap position. Because input and output share a common winding, variacs are not isolated from the mains and proper safety measures are essential.

Voltage can be varied from near zero up to the winding’s rated voltage, depending on the unit. For example, a 120 V variac can typically deliver 0–120 V at the output; a 240 V unit can deliver 0–240 V. The current rating is the maximum load current the device can safely supply, with units ranging from fractions of an amp to tens of amps in larger models.

Construction features include a laminated iron core, a continuous copper winding, and a mechanically attached brush that moves along the winding to select the tap. The design provides smooth, continuous voltage control with low noise and without electronic switching.

Common applications include laboratory testing and equipment calibration, power-supply development, motor speed control, and light dimming or voltage testing for electronic devices. Variacs are valued for their ability to apply precise, adjustable voltages without introducing switching noise, but their lack of isolation from the mains requires careful handling and appropriate safety precautions.