Re: Proper group for heatsink question?


k wallace wrote:
> >
> > Oh. Homework. Should've said so. Try sci.electronics.design.
> >
> > Good luck
> > Chris
> >
> They've given a fairly large grant for this to be considered 'homework'.
> The fact that I work at a university rather than in industry does not
> automatically imply homework. I am simply the ME on the team. I don't
> know as much about some of this as the 3 EE's on the team, regarding the
> electronics. My work is in thermal-fluid sciences, so that's what
> they've got me for.
>
> I know more specifics on this device, but am constrained from discussing
> some of them. Therefore, any vagueness or confusion due to such is
> completely my fault.
>
> Thought I could get some general suggestions here for a thermal compound
> that needs to function in an enclosed space of prev. posted size at a
> T_amb, enclosure of 60C, T_amb, external of 20 C, T_j of 100C, etc. -
> that's all I was looking for. I did get a few good suggestions, but on
> the whole, it appears that this was not the right group to query. Will
> take your (generic) advice, shut up, and go elsewhere.
>
> regards
>
> kw

No good deed goes unpunished -- didn't really say that at all. First,
I gave you quite a bit of _very_ specific advice for a newbie or
hobbyist, which is the assumed audience for s.e.b., and could have been
assumed given your lack of specificity in your original post.
Something vague like your original post is charitably assumed to be the
result of lack of knowledge rather than lack of effort. If you'd spent
more than two minutes on your original post (from further posts, there
obviously was quite a lot more you _could_ have said without bumping up
against non-disclosure), or read the first couple of paragraphs of my
first response, you'd see that and not get offended.

Second, your problem extends a little beyond just heat sink compound (5
watts dissipated by an 0.5" X 0.5" X 4.5" heat sink in free still air
keeping t(j) below 100C and t(sink) below 60C). This seems to be a
very ambitious engineering puzzle, for which there is no easy answer.
Several engineers at s.e.d. have described things they've done with
heat sinks that approach witchcraft, and getting their advice might be
a good idea.

Also, there happened to be some good advice on the heat sink compound
to use in my first post. If you'll look at the website I suggested,
you'd see the Ultrastick phase change product, which has thermal
conductivity ratings better than the others, and looks like it might be
a good starting point (and will probably be cost competitive with the
higher-priced silver-loaded stuff). With something like this, it might
be best to start with the best, then see what kind of margin you have
to play with (unfortunately, I have the suspicion that your margin is
negative already, and getting a really good heat sink compound will
just eliminate that as a source of dither). The manufacturer also will
provide samples, especially if they're talking to someone who is a
potentially good customer.

http://www.aavidthermalloy.com/products/options/greases.shtml

Nobody told you to shut up, although I still feel you might get better
advice elsewhere. Sorry you feel that way.

(By the way, look closely at your mathematical modelling, check your
assumptions, and make sure you can actually accomplish what you want
first. I'm not too sure even a perfect heat sink of the size you
specify could do the job you need in non-computer-modelled free still
air.)

Good luck
Chris

.

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