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'[PIC]: DAMP! Startup problems?'
2000\09\29@075427 by Graham North

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Dear all,

I have a few circuits that are kept in water proof boxes, but while in
storage (in not the most ideal place in the word) they seem to get damp.

My question is not how to prevent this, as I have sealed the boxes as well
as is humanly possible, and have put sachets of silica gel in the boxes
also.

My question is would a crystal or RC res circuit (in am currently using
cheap resonator) startup more reliabily in these conditions?

Has anyone come across this problem before?

I know electronics should not be subjected to these environmments, but we
all live in the real world don't we!

Thanks for your valuble time and comments,

Regards

Graham North
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2000\09\29@092141 by staff

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Graham North wrote:
{Quote hidden}

Graham, one word; Encapsulate! Set up your hardware to use
ICSP for upgrades, then set the whole lot in a chunk of plastic.
If you do it right you can run the thing submerged in saltwater
for years and it won't care. Microchip make it easy with the simple
ICSP on the flash chips, and I have found you can use the PicstartPlus
for ICSP with no probs, saving some bucks there if that is an issue
for you.

Most big hardware stores have cheap polyester (fibreglass) resin,
play with this until you are comfortable with the whole thing and
your molds etc, then order some higher quality electrical resin
from anyone in the yellow pages. Normally you can "pot" the
device in a cheap plastic hobby box with just the wires hanging
out the top. I was trained in encapsulating elec devices many
years ago in industry, it has numerous adantages including
tamperproofing and mechanical strength. Our current product can
be run-over repeatedly by a car with no affect on performance.
Not many electronic products can claim that! :o)
-Roman

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2000\09\29@103026 by mike

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On Fri, 29 Sep 2000 11:24:12 GMT, you wrote:

>Dear all,
>
>I have a few circuits that are kept in water proof boxes, but while in
>storage (in not the most ideal place in the word) they seem to get damp.
>
>My question is not how to prevent this, as I have sealed the boxes as well
>as is humanly possible, and have put sachets of silica gel in the boxes
>also.
>
>My question is would a crystal or RC res circuit (in am currently using
>cheap resonator) startup more reliabily in these conditions?
>
>Has anyone come across this problem before?
>
>I know electronics should not be subjected to these environmments, but we
>all live in the real world don't we!
>
>Thanks for your valuble time and comments,
If anything, a _little_ leakage can actually help resonator startup,
but a lot would probably cause problems. Leakage will always have some effect on the frequency of an RC osc,
but will probably not actually prevent operation unless R was high and
you have a lot of leakage across the cap. If this is a major problem, conformal-coat the assembly. Remember that moisture on a _powered_ board can cause rapid corrosion.

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2000\09\29@160037 by Andy Howard

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----- Original Message -----
From: "Graham North" <spam_OUTgs_piclistTakeThisOuTspamHOTMAIL.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Friday, September 29, 2000 12:24 PM
Subject: [PIC]: DAMP! Startup problems?


> Dear all,
>
> I have a few circuits that are kept in water proof boxes, but while in
> storage (in not the most ideal place in the word) they seem to get damp.
> My question is not how to prevent this, as I have sealed the boxes as well
> as is humanly possible, and have put sachets of silica gel in the boxes
> also.

Does the silica gel absorb much of the moisture? Enough to change colour? Is
the silica gel on the inside of the waterproof box?

> My question is would a crystal or RC res circuit (in am currently using
> cheap resonator) startup more reliabily in these conditions?
> Has anyone come across this problem before?

Yes, I had a similar problem with equipment at a transmitter site atop a
small mountain. The equipment was only used occasionally and suffered from
internal condensation. We solved the problem by keeping it powered and
fitting a net-like arrangement of resistors in the lid to keep the kit very
slightly warmer than ambient.

If your equipment is in storage then keeping it powered might not be
practical but the principle of keeping it slightly warmer than the ambient
air temp still holds good. Maybe lay along the shelves a run of that
resistance wire which horticulturalists use for soil heating, you don't need
much heat to keep the condensation at bay IME.

Another option is to seal the kit along with  _freshly baked_ silica gel
packs in either heat-sealed plastic or if that's not available you could try
those zip-topped bags sold in supermarkets for storing stuff in freezers.
(Do this in a warm dry environment.)  Once it's completely sealed there's
only the small amount of moisture in the bag and case to be absorbed, which
fresh gel should be able to handle pretty simply.  I did once have a handy
table for silica gel showing how much was recommended for a certain volume
at a given RH but I can't find it just now.

> I know electronics should not be subjected to these environmments, but we
> all live in the real world don't we!

Lance is yer man for waterproofing really. I'm sure he'll have some other
suggestions...













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