Join Date: May 2005
Location: Sydney, Australia
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Cree XR-E LED x3 homebrew
I've been running a 20watt MR16 for a few years but I need to run a big 7.2Ah SLA to get the runtime I need. My search for greater efficiency/runtime/weight loss without breaking the bank has led to my homebrew light based on three Cree XR-E LEDs.
These LEDs are a really nice piece of kit. Very efficient, small, light, lots of good optics and driver circuits available, and very reasonably priced. They are individually mounted on an aluminium 'star' board, the individual optics mount directly over the LED such that they are perfectly optically aligned. The optics are a two piece arrangement where the clear lens clicks in to the grey outer shell, because its the shell that is glued down, it is possible to replace the lens as easily as popping the old one out and just popping a new one in.
I used a 'Buckpuck' driver. The circuit is simply a current driver. With a small potentiometer you set the maximum current that will be delivered to the LEDs. I wired the 3 LEDs in series and set the pot to 600mA. According to the spec sheet each LED will drop about 3.3v at 600mA so I need to supply at least 9.9v plus whatever the buckpuck drops, which is very small (over 90% efficient). I measured 620mA of current draw at the Yuasa 3.2Ah 12v SLA battery which should give me 4 1/2 - 5 hours runtime, I'll know for sure after testing.
As you can see I didn't spend much time on the asthetics
I'll clean up the cabling some time soon. The buckpuck is mounted in a small black plastic box which sits perfectly in my aerobars, mounted with double sided tape. There's two switches, one is on/off and the other switches from 600mA to 1,000mA to the LEDs. After using it at both settings I decided that I don't need to run the LEDs at full power as the 600mA setting appears to give maybe 80% - 90% of their full brightness (that's my subjective guesstimate). LEDs are actually more efficient at lower power settings.
The light is simply a scrap bit of T sectionm aluminium extrusion I had in the scraps box. A smaller piece is MIG welded to for the mounting bracket. The LED's are glued onto the alu with silver epoxy, which is a 2-part epoxy heat transfer compound. i had a bit left over and those gold anodised small heatsinks so at the last moment decided 'what the heck' and stuck them on. The light gets slightly warm to the touch at 1,000ma and almost imperceptibly above room temp at 600ma so there's more than enough heatsinking.
The light is mounted with a cable tie and double sided tape.
As you can see the battery sits in a waterbottle cage as if the designers at Yuasa designed it for that application, its a perfect, snug fit.
Here's some comparison shots between the LED light at 600mA compared to my friend's 20watt 15degree MR16 pulling 1.9amps from the exact same battery (Yuasa 12v 3.2Ah) he gets less than 2 hours runtime.
In these shots the garage doors are about 40 feet away. There appears to be about the same ammount of light but the color of the LED makes it easier to see details (to me) especially at distances greater than this.
The 20w MR16 halogen...
and the Cree XR-E running at 7watts...
There are 10 types of people in the world - the ones that can count in base 2, the ones that can't count in base 2, and the ones that didn't expect this to be in base 3.