Re: LED and constant current supplies

Peter Hucker wrote:
On Wed, 12 Nov 2008 00:59:26 -0000, IanM <Invalid@xxxxxxxxxxxxxxx> wrote:


ehsjr wrote:

yan@xxxxxxxxxx wrote:


I've got a bit of a mystery. I've ordered a couple of these:

<http://www.dealextreme.com/details.dx/sku.4669>

along with a constant current supply:

<http://www.dealextreme.com/details.dx/sku.13553>

The only tidbit of information I can find is that you drive them with
300 ma @1.7V.

I've asked a couple of questions on their forums on how best to drive
2 of these from that one supply; I'm getting conflicting answers.
About half the answers say to drive them in series and the other half
in parallel.

My limited understanding says that you drive LEDs in series so that
each gets the same current.

With a constant current supply, my gut feeling says that driving these
in series should be OK. The PS uses a PT4105 chip, which is a
constant current LED driver. I've looked over the data*** but alas
I'm not an electronics engineer... :-(

<http://www.chipcatalog.com/TI/PT4105.htm>

Also, the vendor says I can drive it with 12-24 V; the data*** says
24V.

I have one PS and two LEDs on the way. How do I start once they get
here?

Thanks,

--Yan


Put the LEDs in _series_

Do NOT put them in parallel.

Ed

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.
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