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