Variacs

A variac, short for variable autotransformer, is a device used to adjust AC voltage smoothly. It provides a continuously adjustable output by tapping a portion of a single winding on a magnetic core, rather than using two separate windings as in a typical transformer. By turning a dial or moving a brush along the winding, the user selects a different voltage fraction of the input, producing an output voltage that ranges from near zero up to the device’s rated input voltage.

Construction and operation: The core carries one continuous winding with a moving contact that slides along the turns. This makes the device compact and capable of handling relatively high current for its size. The output voltage is proportional to the tap position, and the frequency and waveform are preserved, since the source and load share the same winding.

Electrical characteristics and safety: Because the winding is common to input and output, variacs do not provide galvanic isolation. The current rating limits both input and output, and start-up inrush or transient voltages can be significant. The case can be live if not properly enclosed or insulated, so the unit should be operated within its rated voltage and current with appropriate safeguards and grounding as required.

Applications: In laboratories and electronics work, variacs are used to test equipment across a range of voltages, to compensate for line voltage variations, and to control motor speed or lamp brightness in a controlled manner.