These converters are used to obtain a variable ac output voltage Vo from a fixed ac source and a single-phase converter with a TRIAC. A TRIAC allows a current flow in both directions. It can be turned on by applying gate voltages at ωt = α for a current flow in the positive direction, and also at ωt = π + α for a current flow in the negative direction. The output voltage is controlled by varying the conduction time of a TRIAC or firing delay angle, α. These types of converters are also known as ac voltage controllers.
