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.