Dimmer, Autotransformer

Also known as a variable autotransformer or Variac. This type of Dimmer is now virtually obsolete. It consists of a single winding autotransformer and a carbon brush on a rotating shaft. This allows dimming the input voltage while maintaining the sine wave of the line. These devices are bulky and heavy, and not suited to remote control except via cumbersome fixed-speed motor drives.

A common variant of autotransformer also allows boosting the input line voltage to about 140V by placing the input tap on the transformer between the maximum end-of-travel of the carbon brush and the 0V neutral tap. This type of device was used on the Broadway production of "A Chorus Line" to boost the voltage to the FEL Lamps in the Strong Trouperette III followspots, to gain additional brightness.

Superior Electric (Luxtrol) and Ward-Leonard were the primary manufacturers of autotransformer dimmers for the theatre.