Triac

The Triac is comparable to two thyristors connected in
antiparallel. It has one gate electrode and two anodes.
The component is called a triac since it has three(TRI) terminals
and is used like a thyristor but for AC; TRIAC

         | A1
      .--*--.  __
      |     | /  |
     _|_   -+-   |
     \ /   / \   |
     -+-   ---   |
 G  / |     |    |
 --*--|-----|----'
      `--*--' 
         | A2

Like the thyristor it is triggered with a by sending a trigger pulse in the gate terminal. It will stay triggered until the current falls below the minimal holding value. This means that it must be re-triggered 100(at 50Hz) times or 120 (at 60Hz) times a second, when used in AC circuits(once for each positive and once for each negative half-period). The most common use for triacs is dimmers and driver circuits for electric motors. In dimming ciruits for normal light bulbs, the mean power is controlled by selecting how late the triac is triggered(the later the trigging occurs the dimmer the light bulb will be);

Waveform over light bulb, no dimming

                __
              .-  -.
             /      \
 --+--------+--------+- 0
    \      /
     `-__-'

Bulb power output:
      ____     ____
    -"    ""--"    "--


 --+--------+--------+--

Waveform over light bulb, 50% of cycle

                 _
                | -.
                |   \
 --+--------+--------+- 0
       |   /
       |_-'

Bulb power output:          
         _        _
   _    | "-_    | "-_
    \--/-----\--/ ----- Mean    
     `.       `.        power
 --+---|----+---|----+-

The flickering that occurs due to the dimming will not be visible, since it has a higher frequency than the eye can detect(100Hz/120Hz). The most used triggering source in a triac dimmer is a diac triggering-circuit. The triac is available for currents from between 0.5A and up to several tousand amperes.

Triac symbol:

   __  A1
  G  \_|__
     \/ /\  
     ""T""
       A2