TRIAC
Tutorial
TRIACs look deceptively simple to use, however, a
few design guidelines need to be followed.
At first glance you
would think that since a TRIAC is controlling AC, it's two terminals M1 an
M2 would be interchangeable. This isn't the case, if you swap M1 and
M2 the TRIAC will not work. The gate drive must be referenced to
M1. The schematic symbol for the TRIAC always has the gate drawn by
M1, to show the asymmetry of the device.
The load should always be placed on M2 and not M1.
This is important because this will cause the AC voltage to appear across
the gate resistor, and will cause it to overheat and even melt. Yep,
you guessed it - I've made this mistake.
Another factor is the gate drive current. The
data sheet should specify gate trigger current: IGT.
The gate should have a series resistor so that the current to the gate is
greater than the trigger current. The resistor value should be the
drive voltage divided by IGT
+ 20% to give it some margin. If the resistor is too small, the
TRIAC will not turn on or only turn on half the cycle.
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