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Further adventures in bike-light-ology

January 19, 2009

Today’s lesson: Hub generators are current sources.The sidewall generator was starting to tick me off. Fiddly, the mount cracked once, a little noisy, and too slippy in the winter. So, I took my Christmas money, and bought a custom wheel from a local bike shop, with a Shimano 3D71 in the middle.

Hooking this up to the regulator for the sidewall dynamo worked great, till it failed. Simulation of the circuit revealed two unexpected things; the two small capacitors were probably swinging well past their voltage limits, and they were also getting reverse-biased in the process. However, the part that actually failed, was the power transistor in the voltage clamp; it had gotten so hot that the epoxy on it was cooked to a dark brown. Replacing that transistor restored the clamp to its former function, but this time I added an indicator LED (a power indicator LED) to let me know whether the clamp was running part time, or often. Turns out, the light went on solid at about 10mph, and got plenty bright at 15mph.

Given that the clamp is supposed to only turn on at 23.5 volts, and sinks 350mA, that the hub was delivering somewhere in the neighborhood of 10 watts of power (it’s rated for 3).

It seems a shame to convert all that useful power into heat and transistor flambé. Plan B, is to string all the power LEDs I’ve got kicking around into one big load, and either switch that on with the clamp, or maybe just use that for the clamp. Eight LEDs would be a little pricey, but wow, think of the light.

Or maybe I could run a stereo.

2 Responses to “Further adventures in bike-light-ology”

  1. wsanders Says:

    Use all the excess power from the front hub to power a small electric motor on the rear hub. There, fixed that for you!

    My rig is working OK, and sunset is early enough that I don’t have to ride home in the dark anymore. The biggest mistake was using a yellow LED as a front light. Even though it’s rated at more lumens, it just doesn’t seem to be as effective as white. And it seems to confuse people – a couple of times, people out on the trail actually started walking right into me as I got closer. I’ll fabricate a new front light this summer.

  2. dr2chase Says:

    Hey Wiley,

    If you need a white, I recommend either the CREE XRE Q4 (cool white, 100 lumens, sinks up 1000 mA) or the CREE XRE Neutral White. I’d swear that I bought the 80 lumen P4 neutral white (700 mA), but what’s listed now at the place I bought it is the P3 (74 lumens, maybe a little too green).

    The Q4 is a death ray. It’s great.

    I’m using a string of LEDs as a power dump, and it really is kicking out that much power. My temptation is to bump up the current to a full amp, and stick it to a Luxeon Rebel that I bought to see what it would be like (answer: welding-arc blue, and 10 watts on a fingernail-sized puck needs some serious heat sinking — which is exactly what you get on a hunk of aluminum at the front of bicycle).


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