Three Watts of Light
Last Monday, the tenth of October, I received my first batch of “power” or high-output LEDs. These are similar to the Luxeon everyone may or may not have heard of. Scroll down a few posts to “UFO Sighting”
Each led sits under a little round plastic lens, and is mounted on a slug of aluminum or some aluminum like material. The slug has holes in it, for mounting. These LEDs are rated for a maximum of 350mA but they don’t really specify a voltage. Well, luckily, you don’t really need to know the voltage … as long as you limit the led to 350mA, it will only take the voltage it needs to conduct that current. That voltage varies with color; at 300 mA the reds were around 2 volts, green around 2.8 and blue around 3.3, as measured in-circuit with a digital multi meter.
So I took three of these ‘ufos’ and mounted them on an old 586 heat sink. Why a heat sink you ask? Well, unlike filament and plasma based lighting (light bulbs and neon), LEDs are semiconductors - there is a fragile slice of silicon in there doped with various materials, that emits photons (light energy) when you force electricity through it. But, it is not fond of heat. Heat causes the package to melt into the semiconductor, causes the wire-bonds to come undone, and all sorts of other badness. So, unlike a halogen bulb that runs at 500F, these LEDs like to keep it cool, under 85C. 85C according to google, is 185 degrees Fahrenheit - which doesn’t seem that cool, but, that is the absolute highest temperature the junction (where the P and N type silicon meet) can sustain… and that junction is insulated inside a plastic shell, so the heat can’t escape very well. Basically, if a led feels warm, imagine how hot that little junction must be inside all that plastic. So, I lubed up each led with thermal paste (a bit too much I might add) and screwed them down onto the heat sinks.
Here’s a shot of the heat sink as I’m fitting it with the wire which supplies the current:
You are seeing the backside, the fins the fan blows on in order to deal with the heat of a 20-30 watt 586 processor. For a supply cable, I used stranded cat 5 cable in stylish red.
I jammed the sheath of the cable into the gap between the fins (which used to hold the cpu socket retention spring)… it is a very snug fit.
Using the LEDs themselves as a template, I marked and then drilled out holes for the supply wires and mounting screws. Aluminum is soft luckily, and I just used steel machine screws to hold the led down - they were able to cut their own thread, without benefit of tapping the hole first, although that might have helped them go in easier.
Here is a led lubed up with cheap silicone based thermal interface material. I was a bit over-zealous with the application, and it got me in the end… as some of the silicone oozed up under the lenses somehow.
You can see the little glob of goo near the lens on the “g” led, that glob also made it under the lens, but stopped short of diode thankfully.
The end result, three watts of light … each color blends into something resembling white, but it’s more of a pastle pink. I’m sure with a better diffuser and fiddling with the levels a bit, a nice warm white could be the result… but I have other plans!
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