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From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: Side effects of potting compound !
References: <3DC14523.4BC6D5@xympatico.ca> <email@example.com>
Date: Thu, 31 Oct 2002 14:05:31 -0500
NNTP-Posting-Date: Thu, 31 Oct 2002 14:05:48 EST
Organization: Bell Sympatico
> Joseph Legris firstname.lastname@example.org wrote:
> >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
> >> ?"
> >> 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
> >> Checked the compound.... fine, compound specs.. fine. I spent all this
> >> 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
> >> completely and was slightly conductive ? Checked all sorts of ideas.
> >> There's a voltage reference feeding a resistor divider with a trimmer at
> >> junction for fine trimming. A multi turn preset. Voltage at both sides of
> >> trimmer are as they should be. Voltage at the wiper is now 6% higher (which
> >> 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
> >> wiper.
> >> Anyone come across this before ? Anyone got any ideas how to prevent it ?
> >> Gibbo
> >Unless the trimmer is extremely touchy, which is a design issue with or
> >without potting, I am guessing that the epoxy got inside and mucked up
> >the contact between the wiper and the resistance element.
> >Even though it appears quite viscous, epoxy has an incredible ability to
> >flow into the tiniest holes and crevices when it is liquid. Don't get it
> >on any electromechanical parts such as switches, pots, connectors or IC
> >sockets unless they are sealed. You may be able to solve your problem by
> >completely sealing around the pot with hot glue (or conformal coating as
> >Spehro suggested).
> >Best regards,
> >Joseph Legris
> The odd thing is a resistance measurement from end to wiper to end gives the
> right readings. The ratio has actaully changed in the potted ones.
It's worth checking: measure the resistance from the wiper to each of
the other terminals.
You can unpot epoxy using heat. I've used a hot soldering iron as a
chisel to dig away just enough epoxy to place test probes. Do this in a
well ventilated area and avoid breathing the fumes. Don't use a good
soldering iron tip (iron plated) for this - it will get gummed up and
will be hard to clean without using an abrasive.
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