> I have spend a lot of time on the net trying to find a circuit for a
reverse
> phase or trailing edge light dimmer
> for 240vac around the 100 watt mark.
> I would like to make a remote control dimmer using a PIC, I have found
some
> PIC code that I can adapt but I am finding
> it difficult to find any circuits as a starting point.
>
> I am guessing that the circuit would use a Mosfet or IGBT to do the hard
> work.

IF you mean that the power occurs in the last part of each half cycle then
essentially ALL SCR and TRIAC dimmers work this way. This is because they
are turned on by a trigger signal and turned off ("commutated") when current
flow drops below a minimum holding value at the end of the current half
cycle. Traditionally triggering is achieved by crudely integrating the
current half cycle and triggering the switch when a suitable value is
reached. The integration is usually achieved with an RC delay and often a
DIAC breakdown diode to get a sudden energy surge.
If this is what you mean by trailing edge then googling for eg

            triac dimmer diac

should work

Simulation of the integration action in software is feasible but it is
probably easier (and more usual) to detect zero crossing and switch the
device a given period after that. Almost all processor based dimmers work
that way. A trailing edge dimmer would reverse the process - fire at zero
crossing and turn off after a delay.



        Russell McMahon

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