half-cycle of the supply from θ1 to 180° i.e. the shaded portion of positive half-cycle. Similarly, the shaded portion of negative half-cycle will pass through the load. In this way, the alternating current and hence a.c. power flowing through the load can be controlled. Since a triac can control conduction of both positive and negative half-cycles of a.c. supply, it is sometimes called a bidirectional semi-conductor triode switch. The above action of a triac is cer
* For example, the speed of a ceiling fan can be changed in four to five steps by electrical method. ** Although it appears that ‘triac’ has two terminals, there is also third terminal connected to the control circuit.
Power Electronics 579 tainly not a rectifying action (as in an *SCR ) so that the triac makes no mention of rectification in its name.