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'[PIC]: effect of high voltage on output pin'
2001\08\22@142940 by Mike Mansheim

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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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2001\08\22@143744 by Spehro Pefhany

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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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2001\08\22@150103 by Steven Bakaletz

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Look at
www.microchip.com/Download/appnote/category/rdesigns/40171a.pdf
it should answer any of your questions.

{Quote hidden}

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2001\08\22@160522 by Olin Lathrop

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> - what deleterious effect was the original design having on the chip?

Running excessive current thru the protection diodes, possibly destroying
them and also possibly causing the PIC to try to short its power supply
sometimes.

> - was that likely the cause of the strange behavior?

Yes.

> - 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?

Yes.  Not that you likely have access to.

Just toss them and move on.  It would be silly to risk $1000s in wasted time
to possibly save $10s on parts.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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2001\08\23@055156 by Ken Walker

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I recon,,,correct me if I'm wrong..but your not turning the triac on long
enough.....and at the correct time in the mains cycle.



Kenny baby,
One of Two in a vast wilderness.


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