Tuesday, September 27, 2011

Choosing LEDs and batchPCB success!

My main aim is to design a dynamo powered LED lighting system that can be retrofitted into vintage bicycle housings.  My earliest approach was to use T10 Edison screw replacement LED bulbs.  These are LEDs with a driver circuit retrofitted into a standard 10 mm threaded incandescent bulb housing, which were very common in flashlights until the flange base bulb came into vogue.  Almost all of the vintage lamps I've come across use the Edison T10 thread.  In theory (and mostly in practice), this is an easy and reasonable way to upgrade vintage lamps to use modern LEDs.  All you need is a rectified DC output from the dynamo and the LED's driver circuit handles the rest. However, these LEDs aren't the brightest ones available and you are, for better or worse, at the mercy of the built-in optics of your lamp housing.

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!)
Keep in mind that, without some extra driver circuitry, you can't drive these LEDs at their maximum current rating with the hub dynamo, nor would you want to; getting out the heat they produce at high currents requires careful thermal management design.  So, by picking LEDs with maximum current ratings considerably higher than what the dynamo can generate there should be a fairly wide safety margin for overcurrent and overheating conditions.

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
There's a pad to mount the LED and a spot for a through-hole backplane connector.  A via is placed in the thermal pad to connect the top and bottom planes to dissipate heat. There's probably a total of 1.5 square inches or so of PCB that acts as a heatsink.  Hopefully this will be enough!  I also have a couple of holes for mounting a reflector.  The holes are spaced for this particular reflector from DealExtreme, although I've yet to drill and tap the pilot holes in the reflector body.  I used BatchPCB to get a few of these made. For $30 I got 8 boards shipped, but they wound up sending me 16! It takes about a month, so not great if you're in a hurry!

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.

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