After experimenting with these LED flashlight bulbs for a while I decided I wanted to use my own LEDs and drivers. When being driven by a dynamo LEDs don't really require a regulated current source, defeating the purpose of the driver circuit built into these bulbs. The dynamo itself acts mostly like a constant current source nicely limited to 500-600 mA (depending on the hub). Many high power LEDs are available with maximum current limits above what the dynamo can generate. You're really spoiled for choice in this category, so picking one is really just a matter of price. I went for the premium XP-G series for my front light (490lm @ 1.5A), which is step below the XM-L series that can produce a whopping 943lm @ 3A. I am debating between a Cree XP-E (98lm @ 0.7A) or an OSRAM Golden Dragon Plus (118lm @ 1A) in red for the taillight.
|Cree XP-G. 490lm if you can get 1.5A into it (and get rid of the heat!)|
Cree XP LEDs are really, really small (3.45 mm square to be exact) and come in a leadless surface mount package. While small is good, it's also a pain in the butt for prototyping. So, I designed a PCB that I should be able to mount in a variety of lamp housings. I'm not exactly sure of the mounting details yet, but this one fits nicely into the Luxor lamp housing. I used EagleCAD to design the PCB and got this:
|EagleCAD LED disc board|
|LED disc bottom|
|LED disc top|
Ok, now what? Well, with leadless packages you need to reflow; a soldering iron isn't much use. I use the stove top 'frying-pan' method, which is a variation of the hotplate/skillet method (scroll down...). The frying pan method does not employ any temperature control. I just pop the PCB in the hot pan, press it down with tweezers, wait for the solder paste to reflow and then pull it out as fast as I can. This is potentially risky, as most parts have a maximum reflow temperature/time above which you can cause damage. Here is the finished product:
|White XP-G on LED disc board with reflector|
I put a plastic adhesive reflector on it. Not sure if a reflector will be necessary or if the optics of the original lights will suffice. Here's the red XP-E with 400 mA running through it:
Not much to see here except that it is extremely bright. For the rear light, I doubt I'll need optics. The lamp's lens should be enough to diffuse the light. At this point, I think decisions about which LED to use become a bit philosophical. The Gold Dragon Plus can put out 118 lumens of red light at 1A. The XP-E in the above photograph is probably putting out about 50lm and it is blinding. When flashing it will be even more noticeable. It also gets quite warm, so it might not be a good idea to put more current through it. I'm going to have some PCBs made for the Golden Dragon just to try it out, but I doubt the existing design will be sufficient to handle the heat it will produce at full current.