--- Ron Amundson <
ron_amundson@...> wrote:
>
> >It's been my experience that epoxy is your friend
> >against harsh environments and climates...
>
> I've used epoxy too, and have filled potting boxes
> with it. However it can
> raise havoc with heat producing parts, as well as
> creating additional
> thermal and conductive paths. However if the
> economic conditions allow it,
> its quite workable except when it comes to service.
Heat was never an issue when the box is in the water.
The entire box acted as a heat sink. Hoever, we also
did it like this when devices had heat issues and the
unit was not submersed in water:
--------------------------------
|epoxy potting |
| ------------ -------
| | ----------| heat |
| | PCB |componet | sink |
| | ----------| |
| ------------ -------
| |
--------------------------------
The heat sink protrudes from the expoxy and dissapates
the heat.
> On key thing with some epoxies is that they can
> cause stress related
> failures, eg op amp Vos drift is exagerated with
> temp, and ferrite beads
> really hate the stuff unless you balance their
> coefficient of expansion with
> the epoxy.
Extruding the heat sink as diagramed above will fix
the op amp problem. Regarding ferrite beads, This is
strange to me. I've hever had an application where
they conducted heat and I cannot think of one.
> The underwater stuff sounds really cool. What did
> you have to do for cases,
> as I imagine the pressure loads were really high.
The expoxy potting ∗was∗ the case. In fact, it was
perfect: Cheap, reliable and never leaked and good to
1000 Feet. Woods Hole Oceanographic Institute still
does it this way. Pressure was never an issue because
epoxy contracts very little under high pressure.
Best regards
Marvin Dickens
Alpharetta, Georgia USA
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