If a circuit is to
switch a mains voltage, a relay is a simple solution in cases where
switching times are long and high currents are involved. However, at
lower currents, and in particular where rapid switching is required,
such as in sound-to-light systems, a relay no longer ﬁlls the bill.
Electrical isolation is often a requirement, which rules out driving a
triac via a transistor. Here we use the MOC3041 optocoupler, which is
specially designed for such applications, to drive a power triac. The
control circuit therefore remains galvanically isolated from the mains.
The internals of the optocoupler are somewhat more complex than appears
from the circuit diagram. A special zero-crossing detector circuit in
the optocoupler ensures that the connected triac is only triggered when
the alternating mains voltage goes through zero.
This has the advantage of generating less interference compared to
switching the triac at arbitrary phase in a cycle. Indeed, it means that
we can dispense with the suppressor choke at the output that would
otherwise be necessary. If very brief pulses are likely to be present at
the input to the opto-coupler, a 220 nF capacitor should be connected
between the input of the circuit and the emitter of T1 to lengthen the
drive pulses. This ensures that the triac will be triggered even with
very short input pulses, which might otherwise miss the zero-crossing
point of the mains waveform. The triac should be an AW-suffix type.
These types are less sensitive, but have higher dv/dt and di/dt
specifications. The gate resistance must be constructed from two
resistors connected in series, since normal resistors are not suitable
for direct use with mains voltages. It is also necessary to exercise
care around the opto-coupler. In order to guarantee Class II isolation
the solder pads on the input and output sides must be separated by at
least 6 mm. The leads may therefore need to be bent outwards when