Re: LED and constant current supplies

On Thu, 13 Nov 2008 20:42:12 -0000, IanM <Invalid@xxxxxxxxxxxxxxx> wrote:

yan@xxxxxxxxxx wrote:

On Nov 12, 12:22 pm, IanM <Inva...@xxxxxxxxxxxxxxx> wrote:

Peter Hucker wrote:

On Wed, 12 Nov 2008 00:59:26 -0000, IanM <Inva...@xxxxxxxxxxxxxxx> wrote:


If the LEDS are going to be left on for more than a few seconds at a
time, they also should be mounted on a heatsink if you dont want to fry
them.

How much of a heatsink should there be? Most commercial applications I've seen only have the circuit board tracks to sink heat.

Not a lot :-) If you looked at the OP's link, the LED dies are supplied
mounted on what appears to be an aluminium backed circuit board. Each
LED is dissipating about 1/2 a watt. They will probably survive for a
while albeit running rather warm in free air, but if fully or partially
enclosed in a non-metallic enclosure I would expect them to fail
prematurely due to overheating. If they are mounted to an aluminium
or copper bar or as you say a circuit board with heavy copper tracks and
there is a reasonable extra surface area to dissipate the heat they'll
do fine. The key thing is to monitor the temperature rise, If they are
running more than 30 deg C over ambient they should probably have a
bigger heatsink.- Hide quoted text -


Well, I plan to use a salvaged aluminum external hard drive enclosure;
that way I get a big heat sink area + a 12VDC supply. And it's free,
or nearly so. :-)

If that's not enough, I will add a couple of old CPU heat sinks and
some cpu heat sink goop.
I seem to remember a rule of thumb for LED heatsinking that was
something like 30 sq inches of Aluminium surface in free air per watt
(rather less with a fan). I'm pretty sure a drive case is big enough.
You should be good to go with just a smear of heatsink grease on the
back and three screws engaging the notches (dont forget a fiber washer
on each under the head so they dont short the LED) or a dab of thermally
conductive epoxy to mount them. If you can keep your finger on them
without getting burnt, they're cool enough.

That is a HUGE area, those figures must be wrong.

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