DealExtreme is one of my favorite online stores. It's a distributor of inexpensive electronic gadgets based in China. I'm always finding something new there. The latest treasure is this little-but-very-bright bare LED:
DealExtreme lists it as a 10 watt LED (SKU 5876). Unbelievably it's just under $12 with shipping included!
Looking at the die shows that it is 9 discrete high-powered white LEDs in a single package. DealExtreme is bad about specs, but the comments in the DX forum seem to suggest that 700 mA at 12 volts is a reasonable spec for this LED. This would yield 8.4 watts.
(I'm wondering though if it isn't 3 x 350 mA @ 3.5 serial LEDs in a 3 parallel strings, which would be 1050 mA @ 10.5 volts. But for now, I'll run it at 700 mA).
DealExtreme lists it as 500-600 lumens @ 6500K color temperature.
As with most LEDs, you need a good current regulated driver circuit since you can't just run these things off a resistor. I decided that the easiest and simplest driver would be one based off the amazingly versatile LM317 chip.
As before, these sites have good javascript based circuit diagrams for calculating LED driver circuitry:
- http://diyaudioprojects.com/Technical/Voltage-Regulator
- http://www.reuk.co.uk/LM317-Current-Calculator.htm
Plugging my values (700 mA) into them yielded the need for a 1.8 ohm resistor with my LM317. Here's the schematic that I designed around those figures (courtesy of ExpressPCH):
Bodged together and plugged into a li-ion pack from my model helicopter and voila, an amazing amount of light. I'm thinking of using it on the headlight of my Piaggio (which currently uses a 3-watt LED) or to replace the bulb on my old 15-watt Niterider headlight, which has seen happier days.
(More photos and photometric testing after the jump)
| Here I have the LED clipped onto a pretty solid aluminium heatsink (with some heatsink paste of course) ready for testing. I'm powering it using a 3 lithium-ion cell battery (11.7 volts @ 1100 mAh nominal) using the aforementioned LM317 current regulating circuit.
The heatsink gets quite hot, around 60-70°C after about half an hour. When I mount this for real, I may need to use a bigger heatsink and/or think of active cooling (i.e., a fan). We normally think of LEDs being "cool" light but that's because they are cool compared to incandescents. However, the efficiency of LEDs is only around 5-40%, which means that of the 9 watts getting pumped into this LED, around 60-95% is being expelled as heat. I don't think this is a totally bad LED but I still need a heatsink that can handle the worst case scenario of around 7 watts of heat. |
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This is a close-up shot of the current regulator (LM317) with the 1.8 ohm resistor mounted on a small PCB. The LM317 is being heatsinked but I found that it doesn't get nearly as hot as the LED. With an input voltage of 12 volts, LM317 is dissipating around 1~2 volts @ 700 mA so that would be around 0.7 to 1.4 watts. Not too much.... I think... With this setup, I measured a voltage drop of 9.38 volts across the LED. |
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My Minolta color meter tells me that the LED is 7800 Kelvins right at the start, which is quite a bluish hue. Oh well, that's what you get for a cheap LED! I don't know how many lumens (spec is 500) but it is certainly blindingly bright. Spec color temperature is 6500 Kelvins according to DX. In reality, the LED seems to settle down a little bit as it warms up, but it seems to stop at 7500 Kelvins. (Note to the nitpickers that the color temperature was tested then the display was "locked" on that value. So it's not showing my ceiling light temp!). |
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Even more: Note that this isn't even the brightest one on their website. They even have a brighter one, a massive 50 watt sucker with 1700 lumens for $60. The heatsinking for this must be intense!
And as noted on the comments below, there are single emitter LEDs such as the SSC P7 that give out 800 lumens @ 3.7 volts and so are brighter than this at just a bit more bucks.... :(
Additional fine print: I do make a small amount of points from an affiliate link to DealExtreme. But (I think) that it doesn't affect my review of their products.
With this setup, I measured a 9.38 volt voltage drop over the LED. So you might want to keep this in mind when doing calculations.
Karen
Browsing the DX site, I perhaps should have also thought of using the SSC P7 which is a 3.6 volt 800 lumen emitter. It's just a bit more expensive ($15) but it would be easier to heatsink: http://www.dealextreme.com/details.dx/sku.12721~r.42921440
LM317 is not recommended for new designs! WTF
Also the 317 is an adjustable regulator with about 2.5v drop-out voltage so if you apply +12v in you'll habe to set it at 9.5v or give it 14.5.
Either way DO NOT use the LM317 with A device of such high eff. IT MAKES NO SENSE not to use a switcher.
Google "12v switching converters"
the 317 just burns off the excess voltage in heat.