On Tue, 27 May 1997 19:03:09 +0100 David Dobbs <dobbs@INNOTTS.CO.UK>
writes:
>Hi,
>        We are working on a lighting controller but we need some info
>and /
>or code on Zero Crossing, Phase Control and Interfacing Triacs. We
>would
>prefer code for the 16C84 but that doesnt really matter as we can
>convert it.


        I can give you some comments, but my employer paid for the code!

        We used a 16c74a in a 4 channel dimmer (the Dove Systems
Shoebox... see http://www.dovesystems.com/dove ) .  It works like this..

        Zero-crossing detection is done by hanging a current limiting
resistor between one end of the power supply transformer secondary and
the INT pin.  The INT pin is set to watch for a negative edge, which
generates an interrupt a little before the negative zero-crossing.  The
positive zero-crossing is determined by timing from the negative one.
        Phase control is done by using the compare mode of the capture
compare register.  On zero crossing, we look through a table of how far
into the half-cycle we want to turn on the triac, finding the earliest
one.  We put that in the compare register and return.  On getting the
compare interrupt, we turn on that triac and look up the next one.
Actually, we also turn on any others that have a turn on time within 256
clocks of the current timer value to avoid missing any due to interrupt
response time.  This results in a VERY minor interaction between
channels.  I can see it on the scope when running the simulator at 4 MHz,
but can't see it in the product running at 16 MHz.
        Another interrupt is receiving the DMX 250 Kbps signal and
stuffing the appropriate bytes into an array where a table lookup then
determines timer values for the compare register (above).  The main loop
polls the A/D and reads in 4 channels of 0 to 10 volts control, if
present.
        The triacs are driven by MOC3010N opto triacs.  The optos are
driven directly by PIC output pins.

Harold