A variable auto transformer, also known as a variac or variable voltage autotransformer, is an electrical device used to provide variable voltage output by tapping into a single winding of an autotransformer. It allows for continuous adjustment of the output voltage within a specific range.

Here's how a variac or variable auto transformer typically works:

Single Winding: Unlike a conventional transformer with separate primary and secondary windings, a variac has a single winding with a movable contact called a brush that can be adjusted along the winding's length.

Tapping Points: The winding is divided into multiple tapping points, usually spaced at regular intervals. The brush's position on the winding determines the voltage output by selecting a specific tapping point.

Voltage Adjustment: By moving the brush along the winding, the effective turns ratio between the input and output changes, allowing for variable voltage output. This adjustment is typically done manually using a knob or lever.

Output Stability: As an autotransformer, the variac shares a common portion of the winding for both the input and output voltage. It provides voltage step-up or step-down capability, but the output voltage is not isolated from the input voltage.

Variable auto transformers have several applications, including:

Testing and Calibration: Variacs are commonly used in laboratories, workshops, and industrial settings for voltage testing, calibration of electrical equipment, and simulation of voltage variations.

Lighting Control: In theatrical and studio lighting setups, variacs can be employed to control the intensity or dimming of lights that operate on specific voltage levels.

Voltage Regulation: Variacs can be used to stabilize or adjust the input voltage to certain devices or systems that require specific voltage levels for optimal performance.

Heating Control: Variacs can be used to control the voltage supplied to resistive heating elements, such as in electric furnaces, ovens, and industrial heating systems.

It's important to note that variacs should be used with caution and operated within their specified voltage and current limits to prevent overheating or other safety hazards. Additionally, because variacs do not provide electrical isolation, they may not be suitable for certain applications that require galvanic separation between the input and output voltages.

If you plan to use a variac or variable auto transformer, ensure that you adhere to safety guidelines, follow the manufacturer's instructions, and consult with a qualified electrician or engineer if needed.