A Variac is a device for varying the supply voltage to any appliance depending on the requirements. It is an autotransformer with wires wound over grain-oriented steel laminations. An autotransformer is different from the normal two-winding transformer in which the input and output windings are separated with a common core.

In the case of an autotransformer, the same winding is used for both input supply and output connection to the user device. By varying the number of windings in the circuit, the output voltage is regulated to the required level. Small Variacs are used in electrical laboratories and as a part of electrical and electronic devices, mostly where speed variation of motor is needed.