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From: Fred Bloggs
User-Agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
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Subject: Re: Side effects of potting compound !
References: <3DC40BBB.email@example.com> <firstname.lastname@example.org>
Date: Sun, 03 Nov 2002 01:39:35 GMT
NNTP-Posting-Date: Sat, 02 Nov 2002 17:39:35 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
> Fred Bloggs email@example.com wrote:
>>>For anyone interested here is what I have found following various
>>>of the potting experiments regarding the multiturn trimmer (circular track
>>>Potted in epoxy - affected trimmer position during curing.
>>>Potted in polyeurethane - affected the trimmer position during curing.
>>>Preset entirely covered in silicon then potted in polyeurethane. No effect
>>>Conformal coated then potted in epoxy - affected the trimmer position.
>>>Conformal coated then potted in polyeurethane - affected the trimmer
>>>Grub screw superglued then potted in either epoxy or polyeurethane -
>>>the trimmer position.
>>>Go figure !
>> Your 6% error after curing is, not quite, but very close to the
>>standard wiper non-linearity specifications. It may be that you have
>>luckily discovered the kind of variation possible across fielded product
>>subjected to random shocks, temperature cycling and so forth. You may
>>not be out of the woods yet.
> I can see your point but I don't think so.
> 9 units have now shown around a 6% increase (never decrease) from the desired
> calibration level. But this is 6% change from calibration. *not* 6% variation
> on the preset level. The actual change of the preset setting was *much* higher
> than this. Closer to 40%
So you are saying the pot change was 40% increase...hmmm, major.
> All these units were potted with 2 different types of compound, some had the
> presets conformally coated. Some had the screw superglued.
> 3 other units had the preset covered in silicon sealer then potted. All 3
> remained within calibration. In fact they were identical. No change whatsoever
> (within 0.001 volt on a 6.13 volt calibration).
> This is pretty conclusive to me.
> From this, I'm pretty sure it's something to do with the potting compound
> distorting the case of the preset.
Yes, and I think the potting compound property of interest is the
"non-shrinking" characteristic. Which product did you use, and how does
it compare to this list which specifically calls out protection of
sensitive mechanical parts:
There is also http://www.devcon.com with electronics potting compounds
declared to be absolutely non-shrinking.
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