According to Rashid (2001), " TRIAC is a semiconductor constructed to resemble two SCRs connected in reverse parallel. Ratings from 2 to 50 A and 200 to 800V. Used in lamp dimmers, home appliances, and hand tools. Not as rugged as many other device types, but very convenient for many ac applications."
TRIAC symbols & Example:
Figure 1. TRIAC
How it works?
TRIACs is a 3-terminal devices can be triggered using short duration pulses
to act as an on-off mains switch. Once they are fired (turned on), they stay ON until the
voltage across them is reversed. This will happen every half of an ac voltage cycle. To
accurately control the power we need to know where we are in the
mains cycle, so you can turn the triac on in the appropriate place. To doing this we need a zero
crossing detector (ZCD).