Semiconductor Switches and Power Controllers for AC

Abstract

In single-phase and three-phase ac circuits the current passes through zero and thereby changes its polarity after each half-wave of the mains cycle. If one introduces a semiconductor switch into a single-phase ac circuits, it must be designed to carry current in both directions; for instance it could consist of two thyristors in antiparallel (see Fig. 2.2). In such a semiconductor the current begins to flow as soon as the thyristors are triggered. The current half-cycles of different polarity are carried alternately by the two thyristors. So that a continuous alternating current can flow the thyristor taking over the current must be retriggered after each current half-cycle. If the thyristor is not triggered, the alternating current is extinguished as it passes naturally through zero.