On 03.09.15 15:28, Gene Heskett wrote:
> Mid afternoon PS: The LM317T is in and working, but that 1.75x3.5" bit
> of 1/8" alu for a heat sink is running just under 150F after about 20
> minutes running. The draw of a 3rd fan will also make it run even
> hotter, so there is a point of vanishing returns to such thoughts. I may
> have to add a 3rd fan, but have no clue what to hang it from. It will
> no doubt get more air with the lid closed, spillage from the end of the
> X motor driver.
>
> The theory was when I built that box was to have enough air moving inside
> it to carry the heat to the outside walls for normal radiant cooling.
If you add Rth:LM317junction-case-insulation-heatsink-air-enclosure-air,
it's a big pile of K/W, even with an internal fan to help with the first
leap across air. A big finned HS for the device, with the fan blowing
along the fins, is a necessity if there are lots of watts to shift. That
small scrap of Al is too skimpy if it's running quite hot.
After that, a fan on the outside would further boost heat transfer,
especially if a few lengths of Al channel or angle from the junkbox were
screwed on inside and out, aligned with airflow, and gooped underneath
with a smear of thermal compound.
**************
But mounting the (TO3?) regulator on a block of Al screwed and gooped to
the inside of the Al enclosure wall, would be nearly as good as a
heat-pipe, eliminating the less effective huffing and puffing with a fan.
If a silicone insulation sheet, twenty times the size of the one now
under the T03 case, insulates the Al block instead, Rth for that step
drops by 95%. OK, that calls for nylon bolts or bolt insulation - both
doable. And if a bit of milling of the block is called for ...
**************
Alternatively, blowing cool air into the enclosure, through a fine dust
filter would also exclude swarf - just not fine spray mist. (And would
wreck the current concept, I fully admit. ;-)
A light spray of flat black paint on the outside will boost radiation a
bit, while not helping convection at all. I'd still plaster vertical
fins on the outside to give some chance of lowering Rth without an
external fan.
If there's no room for an adequate heatsink in the LM317 locality, and
moving it to the enclosure wall isn't an option, then a heat-pipe will
pipe the heat half a foot and through the enclosure wall, for an Rth of
as little as 30 millidegrees/W. (They're out there for not many dollars
a piece, but watch those from RS; the one I looked at used water as
working fluid, and needed 70°C to get up to speed.) A complete finned
heatsink & double heatpipe assembly from a PC is cheap on fleabay. Whack
that through the case wall, and you're done.
Erik
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