Bike light #2
November 9, 2007
Generator, BuckPuck, 4 LEDs, 2 diodes, 3 caps, 4 lens, wire, solder, aluminum, and glue(s)
This is a bigger bike light, intended to make me really truly be visible commuting at night.
However, with the benefit of experience, I’ve moved on to hub generators and a homemade circuit to manage the power. Sidewall generators are a toy.
I bought a 12-volt “generator” (expensive! and it is really an alternator, and rain killed its internal electronics!), which helps cut diode losses (because the synchronous rectifier did not work as designed; back to the drawing board). This is fed through a simple rectifier circuit, which runs an adjustable BuckPuck which is used to drive 4 Luxeon III LEDs (white, yellow, yellow+red in parallel) at 600 milliamps. (I bought the generator from Peter White Cycles; he is also the source of the tires you see in the generator picture, as well as the snow tires I’ll be using later this year.)
The rectifier circuit, if unloaded, could develop a voltage of 2.8 times the listed RMS voltage of the generator. For this circuit, that is 33.6 volts, which is a hair over the maximum voltage (32) of the BuckPuck. So, if you build one of these, do not install a switch between the light and the BuckPuck because if the BuckPuck draws no load a damaging voltage will result.
The storage capacitors inside the Altoids tin serve to accumulate charge from the alternating current. The circuit is designed to charge them alternately, so that they are more discharged before charging, yet not completely discharged. 10 mph is 176 inches per second, and the circumference of the alternator wheel is about 3 inches, so the alternator frequency is about 60Hz at 10mph (it may be double that, depending on the construction of the alternator). The capacitors are 1800 microFarads each, and I = C dv/dt, or dv/dt = I/C = 0.6/1.8×10-3 = 333 V/s, or .333V/mS. At 60Hz that is equivalent to a 5.5 volt decline between peaks. However, this picture suggests that I am cutting things pretty close (in particular, at high voltages the BuckPuck will draw less current) or that I do not exactly understand the output of the generator. I am reluctant to make the capacitors smaller, because the generator drive is rough below 10mph (this also suggests that I do not exactly understand the output of the generator, because the voltages in the graph below are more than adequate to drive the BuckPuck at 10mph, and probably at half that — what’s not at all clear, is the speed at which the nominal 12V output is actually developed).
The current was chosen to dump about 5 watts into the LEDs, so that (allowing 10% in the regulator, 5% in the diodes, and 5% to bad luck) about 6 watts would be drawn from the generator. At 600mA, the white LED voltage is about 3.7 V, the yellow is 2.4 V, and at 300mA (the parallel LED current) the yellow and red voltage is 2.2V, for a total of 8.3, giving 5 watts.
The taillight is wired in parallel for two reasons. First, for purposes of visibility, running 300mA is bright enough, through a 10×80 lens. Second, that long run of wire is vulnerable, and if it were series, and if it snagged, it might disable the lights completely, and also expose the unloaded BuckPuck to an overvoltage. By wiring it in parallel, if the wire accidentally pulls loose, the headlights continue to work.
The lenses were chosen for visibility (the 10×80 wide angles, front and rear, to satisfy the law (white, in a wide-forward pattern), and to get a bright, efficient light that was not too hard on anyone else’s eyes (yellow spot). The combination is not at all bad; the wide angles illuminate a little to the side, the spot paints reflectors on other bikes and joggers at quite a distance, and the yellow-white near combo is pretty good. I think the white must be irritating to other people; it was irritating enough to me that a added a shield so it would not be constantly in my eyes. I should probably get a larger shield, with a proper reflective underside, and extend it forward a little to cut off the light from other cyclists’ and pedestrians’ eyes.
The connection to the taillight, on the underside of the snapdeck:
Close-up, showing JB-Weld-mounted socket, and strain relief on plug (made from plug, plus silicone aquarium tubing, plus heat-shrink).
This is not the final word — I need to do a better job at sizing the storage capacitors, and it might even make sense to switch between them depending on the voltage, and I think I need to tinker with the white light somewhat (would it make more sense to put the white on the wide-angle? would that be less annoying?) and it would be good to get the synchronous rectifiers working right. A little storage would be good, too — when the rear wheel locks up, out go the lights.