As you are detecting the zero crossing, I am assuming you are trying to
turn the Triac just after zero crossing.

The Triac operate on two basic priciple.  When its OFF, you have to supply
TRIGGER current (Igt normally about 10mA to 30mA). Once Triac is turned ON,
HOLD (Ih)current through the Triac will keep it in ON condition. If current
goes below the HOLD current (near the zero crossing) triac will turn OFF.

If the ac line current is 30v and the resistor on MOC3012 say 1k, then Igt
is 30mA and Triac should fire. At this point you can turn the MOC3012 to
OFF state and turn MOC3012 to ON state on the next zero crossing. Power
rating of the resister is 30V*30mA=0.9W.

For a design like yours, I would use say about 300R resister.
        Then the turn voltage, V=30mA*300R=9V+Vgt =10.5V

        Power rating of the Resister = 10V*30mA =300mW

If you need more info check the AppNote AN1007 at Teccor Electronics INc.

Regards,

Jay





At 20:24 14/12/01 -0500, you wrote:
>Hi all. I'm trying to build some PIC-controlled light dimmers for my
school theater. After looking at some schematics from the internet, I
pieced together the attached basic diagram. But if anyone has experience
with this kind of thing, I have a few questions. I'm very confused about
how to calculate the wattages required for the resistors. Using V=IR, and
P=VI, it seems like the 1k resistor between AC supply and the MOC triac
trigger would have to take about 15 watts. Is this correct?
>TIA,
>    Eben Olson
>
>Attachment Converted: "c:\eudora\attach\EEquesti.htm"
>
>Attachment Converted: "c:\eudora\attach\Image6.gif"
>

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