My triple XR-E bike light (home job)

[morrisman1]

hi, im new here and probably wont be around much at all, just thought i would share my creation with some like-minded people

i wanted to build a grunty LED light for my bike, so i thought i would use the CREE XR-E as my platform as i was pleased with how it performs in my fenix T1.

One is never enough so i went with three of them. after some research and thinking, i decided to run with the LEDs in series, off a 12v/7Ah Lead acid battery (im cheap and cant afford a Li-Ion pack, plus they aren't as easy to get here in New Zealand) there is a 1.5 ohm resistor and a 5.6 ohm resistor in the circuit and i can decide what one i will direct the current through so theres two brightness levels.

on the 2.7 ohm resistor the LEDs run at approximately 80% of their rated capacity.

as you can tell the casing is an absolute work of art, there's some serious precision engineering happening there! I have put the fan in to keep the resistors and the LEDs cool, i found that the LEDs can get very hot and just to keep things a bit a little more in the safe range i have a 40mm computer fan wired in to keep the air moving. The lens are 2x 25° and 1x 10°

http://nz.youtube.com/watch?v=sKf2vBw937I

heres a pic of inside:

the insides have changed slightly since that photo was taken


your comments are most welcome!

 

[Buck91]

I can't tell what that casing is made of, but those leds appear to have absolutely zero heat sinking. If this is the situation then they are going to fry- fan or not.

 

[ktronik]

ditto, you need a heat sink for the leds...its a MUST!!

 

[morrisman1]

hi, ok thanks.

at the moment the fan is keeping the backing of the LEDs at what i estimate as about 40°C. I will look into some heat sinking to keep the leds a bit cooler again.

any ideas on what i should use for heatsinking?

 

[purduephotog]

A couple of small chunks of aluminum would work as a start.

Perhaps a few old computer heat sinks- take one off an old PII chip (drill it off) and chop it to fit- nice long fins there.

 

[Buck91]

Well, if you can relocate that resistor assembly, maybe you could epoxy (arctic alumina thermal epoxy) a CPU-type heat sink right in there. Otherwise a chunk of alum. will do just fine! Copper also works very very well.

 

[morrisman1]

so i would have to use a special thermal epoxy?

i might find an old computer heatsink off like a geforce2 or something like that and cut it down to fit each led, that would give ample heatsinking

edit: i just saw what purduephotog said. i thought i was being innovative!

 

[R33E8]

I highly doubt the geforce 2 card's heat sink cut up will provide enough heat sinking without air moving over it.. But since you are using a fan it may work..

 

[morrisman1]

how much heat sinking is required?? do you know of those small heat sinks used for memory chips on graphics cards? they would certainly help. the fan is staying in there so i guess the heat sinks can be a lot smaller than what they would need to be if there wasnt a fan.

where do i get this thermal epoxy from??

 

[znomit]

where do i get this thermal epoxy from??

http://www.ascent.co.nz
1 x Arctic Silver AATA-5G, Alumina Epoxy Adhesive, 2 tubes, 5 grams, Thermal Paste @ $20.26

 

[Buck91]

I'd check out 4sevens.com for AA with free shipping... or got mine at Micro Center USA, which is a B&M computer parts store locally.

 

[ellerbro]

A heatsink like this one might work for you. I think it's just the right size.

 

[ellerbro]

I just noticed the video link. You must have the beta version of the Seca 700. Very nice.

 

[R33E8]

A heatsink like this one might work for you. I think it's just the right size.

That one is too small for natural convection... If you check out the data sheet you will see that at 3 watt's the temperature will be >100C over the ambient... I don't know how it will fair with the fan on since I don't know how fast the air is moving..

 

[znomit]

I'd check out 4sevens.com for AA with free shipping... or got mine at Micro Center USA, which is a B&M computer parts store locally.

Its harder to find in NZ. I'm on my second batch, you get enough for 10-20 leds.

I've done a few resistor drive setups and now favour simple LM317 circuits. You get a constant current over the full battery discharge and you can use different resistors to set different currents. OK efficiency from 12V onto 3 LEDs. 317s are a buck or two at Jaycar.

 

[rufusbduck]

Another place to find a heat sink is in the hardware isle. Take a copper cap for 3/4" pipe and cut the end off. Glue one of these to each ofyour stars. You can "unroll" the remainder and fit these pieces to the inside of your housing and solder any points of contact with the copper slugs. This will provide a thermal path from the led to the housing. Just waiting for my "probation" to end so I can upload attachments(pictures).