At 01:28 PM 8/22/01 -0500, you wrote:
>Had some trouble with what seemed like a simple application:  using a
>12C509A to control a triac that passed generator AC to a light bulb.  I'm
>the software guy, so I apologize if this description is not adequate.
>Two of the pic pins were connected thru 270 ohm resistors to the triac
>(a Teccor Q2008L4).  The pins were driven low for 150us to turn the triac
>on.  Two pins were used to ensure adequate drive current for the triac.
>The pic was very inconsistent - it seemed to run properly some of the
>time, but had problems most of the time.  The hardware guy said that the
>way this circuit was arranged, Vdd + 1 was present on the load side of
>the 270 ohm resistors whenever the triac was on.  Since this would feed
>current back into the pins, and the 1V exceeded the device spec, and the
>pic was behaving strangely, he added a transistor between the pic and the
>triac (so only one pic pin to turn on the transistor is required now).
>I used a fresh chip for the software, and now everything runs fine.  My
>questions are:
>- what deleterious effect was the original design having on the chip?
>- was that likely the cause of the strange behavior?

Putting current back through protection diodes, it could cause latch-up
or other bad effects. A little-known fact is that triacs have a voltage
on the gate (when on) that is the same polarity as the load current.

>- if this was doing something bad to the chip, are the ones I used before
>  (which were windowed parts) likely to be permanently damaged?  Is there
>  a way to test them?

Probably not any damage if they seem to work ok.

Best regards,

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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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