Luxeon regulation questions from a NooB

[Pokenaut]

I have a MDXL cheapo 3W luxeon close I got on ebay. It seems plenty bright, but is direct driven from 3-AA cells.
I also have a Cateye bicycle headlight that carries 4-AA cells and it drives 3 smaller white LEDs. When I check the voltage coming from the battery compartment, I get 6V (naturally). When I check the voltage on the two posts from each of the three LEDs, I get a tad over 3V, so obviously the small board is regulating the current (and supplying the flashing mode as well for the Cateye).
Can I remove this board, cut off the three small LEDs and wire the board to the head unit of the MDXL that contains the #w close, and then run the 3W Luxeon clone head unit from stacked 4-AA battery holders (will be using 2300mAh NiMH) to increase runtime?
I'm hacking together a cheapo helmet light for mountainbike riding. I've alread gone out with the MDXL at night running direct off of the 3-AA, so will brightness suffer much decreasing the V from 4.5 to 3.2 (regulated)
Did any of this make sense? I am a tinkerer, but new to the whole Luxeon arena

Dan

 

[Pokenaut]

WOW....33 people have read my post and not a single one replies.....I looked around the internet for a location to glean some information about working with Luxeons...and this seemed to be "the place".
Is it because I am not one of the "in crowd" that I get no replies? Is it because I haven't built my own modded 3x5W Star DMag?
What does it take to get a simple answer?

 

[voodoogreg]

[ QUOTE ]
Pokenaut said:
WOW....33 people have read my post and not a single one replies.....I looked around the internet for a location to glean some information about working with Luxeons...and this seemed to be "the place".
Is it because I am not one of the "in crowd" that I get no replies? Is it because I haven't built my own modded 3x5W Star DMag?
What does it take to get a simple answer?

[/ QUOTE ]

I am sure it's not that. sometimes a thread goes for a while then all of a sudden it will be 3 pages long, I would say that no one want's to post unless they know exactly how to help answer your question. Also not everybody know's the working of a light, but are here
just to know about all the better light's discussed here.

Dont take it as a snub, most CPF'er's will go outta the way to help,esp newbie's. I too know nothing about building, just love lights! VDG

 

[chimo]

If the Cateye board is feeding three 5mm white LEDs (say at 20mA each for a total of 60mA) it will almost certainly not supply enough current for your LuxIII. You will want in the order of 500-800 mA to run it nice and bright. If you want to go the regulated route, there are a few venders of driver boards on this site (to drive it from 6V you will need a buck type converter). Otherwise, you may want to go with a direct drive (or resistored) off of three batteries.

Paul

 

[Pokenaut]

Could I run it resistored off of 4-AA @ 6V, and if so, what size resistor would be needed? I ask this because I already have two battery holders that hold 4-AA, and enough 2300mAh NimH AAs to do that.
I am removing the head from the MDXL flashlight and caping it and using it alone as a helmet light

Dan

 

[MoonRise]

4xAA NiMH = 5.6V freshly charged, 4.8V in the majority of the usable run.

4xAA alkaline = 6V fresh, dropping quickly with use.

A LuxIII driven at spec is 700 ma of current, with somewhere between 3-4 V (depending on the bin).

If you want to do a resistored approach with 4xAA NiMH, try 1.75 ohms of resistance (2watt rated). More resistance means less light but also less current and heat from the Lux.

Make SURE that your Lux is well heatsinked! With that much current going to the led, it can quickly overheat and let out the magic smoke unless it is well heatsinked.

Also note that if you put 4xAA alkaline cells in and the LuxIII has a lower voltage bin (say 3.3V), the led will try and draw 1.8A and the resistor will be dissipating nearly 5W!

A safer approach (from the standpoint of controlling the current to the led) would be to use a converter/regulator board. A LuxIII and 4xAA as the power source would need a Downboy or similar. Look over in the Sandwich Shoppe forum area.