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From: Mike Harrison
Subject: Re: Side effects of potting compound !
Organization: White Wing Logic
X-Newsreader: Forte Agent 1.9/32.560
Date: Thu, 31 Oct 2002 14:36:44 +0000
NNTP-Posting-Date: Thu, 31 Oct 2002 14:37:00 GMT
Quite high local mechanical stresses may be set up as the resin cures
- you might try a 1-turn pot, as these can sometimes have better
mechanical stability. With a 1-turn, itt might also be worth ensuring
you back off the setting so the screw is in the 'backlash zone', so
small screw movements won't affect it.
Failing that, a softer potting compound may help reduce mechanical
On 31 Oct 2002 13:39:41 GMT, email@example.com (Gibbo) wrote:
>We've just finished a small run of a project for a client. Everything
>completed. At the last minute the client said "can we have them potted please
>I replied that I saw no reason why not but would check a few to make sure.
>Potted 3 yesterday. Came to test them today. Calibration is miles out, by
>almost exactly the same error in each unit (it's a calibrate once only unit).
>Checked the compound.... fine, compound specs.. fine. I spent all this morning
>breaking open the compound to try to find out what had gone wrong. Had it
>affected some caps ? Had it formed some crazy dissimilar metal junction
>somewhere ? Had it absorbed moisture in the curing process ? Had it not cured
>completely and was slightly conductive ? Checked all sorts of ideas.
>There's a voltage reference feeding a resistor divider with a trimmer at the
>junction for fine trimming. A multi turn preset. Voltage at both sides of the
>trimmer are as they should be. Voltage at the wiper is now 6% higher (which is
>actually about 50% of the trimmer range). Overall resistance of trimmer is
>normal. Ratio has changed.
>Potting the PCB in the box using epoxy potting compound has actually moved the
>Anyone come across this before ? Anyone got any ideas how to prevent it ?
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