Velleman SCR100 - Manual (Page 9)

Atlas SCR User Guide January 2008 – Rev 3
Page 9
Notes on SCRs and Triacs
Triacs
Triacs are a well established technology, designed specifically for the solid-
state control of AC loads. They offer bi-directional load current switching (for
both half cycles of the mains supply) and also bi-directional gate current
capability to trigger the device. The combinations of the load currents and gate
control currents are referred to as “quadrants”. These quadrants are shown
below:
The main load current is
handled by the connections
MT1 and MT2. The device is
switched to the on state by a
current into (or out of) the
gate terminal with respect to
the MT1 terminal.
Once triggered, main load
current will continue to flow
until a zero-cross in the
mains cycle is encountered,
at this point the device
switches off. Therefore, if the
device is to be kept on, a
continuous gate current must
be provided or at least a gate
current pulse that occurs
immediately after each mains zero-cross.
Many triacs however are only capable of operating
reliably in 3 of the 4
quadrants. (In particular, Q1, Q2 and Q3). Quadrant 4 (negative gate current
and negative load current) can be troublesome with some triacs, suffering from
poor gate sensitivity and slow response.
MT1
MT2
Quadrant 1
GT
MT1
MT2
Quadrant 2
GT
MT2 Positive (positive half cycle)
MT2 Negative (negative half cycle)
MT1
MT2
Quadrant 3
GT
MT1
MT2
GT
Quadrant 4