piclist 2001\08\22\142940a >
Thread: effect of high voltage on output pin
face BY : Mike Mansheim email (remove spam text)

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?
- 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?

Thanks for any help.

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