I believe "reverse phase" dimming control is where the device is turned on
at zero crossing and turned off part way through the half-cycle. This
generally rules out the use of triacs or SCRs (I ugess one could try a
commutating circuit to shut the device down). I think these dimmers are
usually using IGBTs or FETs. The drive circuits for these seem complex and
expensive to me.

Another approach being introduced now is to PWM the AC sine wave, then
filter the output back to a variable amplitude sine wave. Again, drive and
filtering circuits seem complicated.

Harold


>> 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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