Heatsinking questions and random stuff

[KrisP]

Hi,

I'm in the process of making a lamp (or 2) for my bedside tables. I've temporarily mounted two Cree warm white (7A) and a one Cree P4 (no idea of tint, came out of a Ultrafire C2) onto a large heatsink and have them running via a dimmable circuit that can vary from 5mA to 1A. I've found this wide range very useful as 5mA is not blinding when you wake up in the middle of the night and need to navigate around the room and 1A is bright enough to read from. The reason for mixing the LEDs is because I found the 7A too yellow but mixed with one "white" LED it looks much nicer, like a very white incandescent.

The heatsink is temporarily sitting on top on my old lamp shade with the LEDs facing the ceiling. I've tried placing a diffuser and various other items over the LED's but found it makes the light glare too much from the sides, so i've decided to use an "uplight".

Now to my questions...

What is more efficient as a heatsink; solid aluminium, finned heatsink, solid aluminium with many holes drilled in it? I have a heap of finned heatsinks from home amplifiers and plasma TV circuit boards, but if I mount the LED's on the top of the heatsink, the fins will be facing down and the heat will collect under the heatsink. The heatsinking needs to be shallow so that the LEDs are not raised so high that they are visible from the side of the light.

With the current heatsink at 1A it gets quite hot... I can hold my finger on it, but it's not comfortable. It's about 4" x 8" and finned. I've been thinking of running 9 LEDs at a maximum of 350mA rather than 3 LEDs at a maximum of 1A... Would the 9 produce less heat than the 3?

I had more questions but i've forgotten them after writing all that!


Any help would be much appreciated, thanks

 

[snarfer]

The 9 would probably run cooler than the 3 because: (1) they would be running more efficiently at 350 mA, and thus more of their output, proportionally, would be light, as opposed to heat and (2) with 9 heat sources, instead of 3, the heat will be distributed more evenly to the heatsink, so the heat will dissipate more easily.

As for your heatsinking question, I think there are too many unknown variables to come up with any kind of comprehensive answer. For heatsink without fan I think a lot of people are using pin type heatsinks, as opposed to fins. Not sure how much holes will help. I guess it depends on total surface area of the heatsink, air circulation, and so forth.