Switching Subjects

Q. I have taken the lesson on SCRs and UJTs. But where I work we have equipment with things called triacs. I've been told that they are like SCRs. What are triacs and how do they work?

A. SCRs only conduct in one direction. When they are reverse-biased they don't conduct. Triacs can conduct in either direction, so they make good AC switches.

A triac has two terminals that carry the load current. They also have two other terminals called the "main terminals". There are used to turn the triac on. They function like the gate of an SCR, except that they can work with either polarity of the AC cycle. Triacs will not conduct until a signal is applied to their main terminals.

Their conduction can thus be timed to start early or late in each half-cycle of the AC. This way, they can be used to control the amount of power delivered to the load. The timing of a pulse applied to the main terminals is simply varied in order to vary the power.

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