At 11:21 PM 3/15/2004 +0200, you wrote:
>Hi Russel,
>
>Thanks for your well written response! After building a quick test circuit=
 I
>played with different resistor values and saw that=92s exactly what it did,
>dropping the voltage to a suitable level for the zener. Using your formula=
 I
>will determine the proper value after I know what the current requirements
>will be.

The triac gate will draw a lot of current, more than the PIC most likely.
With an opto-isolator you could probably reduce the size of your regulator
significantly, because you can get them rated for 5mA operation- so perhaps
operate them at 7.5mA.

>The test circuit I built, without a PIC, was just using a resistor to the
>gate of the triac to test it out (I put a 100K resistor in as a dummy load
>for the triac to switch).
>
>To trigger the triac (to switch it on) I had to take that trigger resistor
>to GND in the circuit, is that correct, or did I make a mistake somewhere?
>If this is right, I will have to drive the PIC output pin LOW to turn on=
 the
>triac, and HIGH to turn it off?

Yes.

>I then used a pot for that resitor on the gate of the triac, and noticed
>that I had to lower the resistance to about 330 Ohm before the triac would
>switch on (this is with my test circuit using a 12V AC transformer, I=
 didn't
>have any other ones around). I would guess the function of this resistor is
>current limiting, and the current needed to switch on the triac would=
 depend
>on which triac I'm using. In this case it was a BT139, which is a bit of an
>overkill for this application, but it=92s the only one I had laying around.

That triac: http://www2.eese.qut.edu.au/store/pdf/BT139.pdf requires 25mA
to 50mA depending on grade to drive reliably. More than the PIC can handle
directly unless you have the most sensitive type and then it's on the edge
(you could parallel two outputs or whatever). This is what's guaranteed
at 25=B0C (typical units will be better), but at low temperatures the
trigger current will go up. Note from the data sheet
why I wanted you to use the triac "upside down"- you avoid the ugly
Quadrant IV which can take as much as 100mA to trigger. I'd use the
transistor to drive the triac gate in this circuit.


>Sorry for all the newbie questions and for sticking with me so far! :-)
>Analog electronics is still quite foreign to me.

No problem.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the=
 reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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