John Walshe @Inpact wrote:
>
> Hi guys,
>     I'm having a bit of a problem driving a triac from a pic.
> The electrical main is 220Vac 50Hz.
> I have the Vdd of the pic connected to the live and Vss is 5v less than
> live.
> I have a PNP transistor: emitter to live, collector to triac trigger,base
> via resistor (1k) to the pic i/o pin. The base is also connected to the live
> via 10k resistor (to prevent false trigger at powerup).
>
> My theory is that with the pic i/o pin high the transistor is off and
> therfore high z to the trigger so no leakage current can trigger it.Thus the
> triac doesn't conduct.
> With the pic i/o pin low the transistor is switched on and the trigger gets
> all the current it needs and the Triac conducts.
>
> The triac I'm using is a BTA12 from Philips (logic level driven device).
>
> My target use is for a small motor but I'm using a lightbulb for testing.
>
> The Problem: The bulb is always on!
>
> I thought I had all the bases covered but obviously I'm missing something.
>
> Has anyone got an idea what this 'something' is.
>
> Thanks in advance
> John

The BTA12 is a pretty high gain device, seems you are relying
in "high z" to stop the triac being gated. I would think
you need some resistor to hold the triac gate "off" rather than
relying on high-z which basically allows leakage or even RF
to activate the traic.

Also check you haven't fried the transistor, what current
limiting (resistor?) are you using in the collector circuit?
-Roman

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu