Most lumens per ºC? Rebel, Cree MC-E or ???

[potsked]

This is a bit of a followup to another topic on these lights (below), but I think I've narrowed down my question a bit better.

Given my aversion to heat, I've decided to underdrive a nice power LED with a metal core PCB to avoid a heatsink altogether. My question is, which power LED is going to give me the most light while being driven low enough to not need a heatsink? rephrased, which power LED is the most heat efficient?

.....

As a followup to potential answers, just how high can you drive given LEDs without needing a heatsink. The PDF below (from Cree) suggests 350ma, is that realistic?

http://www.cree.com/products/pdf/XLampThermalManagement.pdf

 

[gillestugan]

You just need too look at the lumen/W ratings, as all electric energy that doesn't convert to light becomes heat.

Your question about how high current you can use is very hard to tell. You can calculate it if you know the thermal conductivity of your PCB, its area and the ambient temperature, but it is quite complicated.
Easiest is probably to test it by rising the current step by step while measuring the temperature. The finger-probe works quite well. If it burns, its hot.

 

[R33E8]

Have you thought about mounting the mbpcb onto a metal plate? Only using the MCPCB as a heatsink(which is not what is was made for) will only allow like .5W of reliable light or 1W of light that may fade over a few months time.. Mounting on a piece of aluminium from homedepot or something will allow higher brightness or lifetime.. Have you thought about constructing it out of sheet metal and painting it white? If you do that then you could run the LED's even brighter and use the fixture as a heatsink...

 

[snarfer]

I agree with R33E8. Also I would suggest using multiple LEDs. The more you spread the heat out the better the results will be.

Regarding maximum current without a heatsink, I think you would be better served to think in terms of watts and temperature. How many watts of power can you apply before the light reaches maximum allowable temperature? Surely there is a limit to how hot you can allow this light to get. One limit is in the LED datasheet, but there are others, such as the limit on the temperature of the insulation on the wires feeding the LEDs, or maximum temperature that a typical painted wall can withstand before discoloring. You might consider a thermal feedback circuit that would keep the temperature below a certain preset limit no matter what.