Trying to design a bike light

November 12, 2006

Once upon a time, we were happy to power a 3 watt generator; now, we make do with 80 milliwatt LEDs.

I’ve got some old leftover bicycle alternators (“generators”) that can crank out about a half amp at 6 volts, alternating. I’ve got an expensive LED bike light, but for all its brightness, it doesn’t give me enough light to see road hazards. LEDs are only about 60-80 milliwatts, depending on color. Seems to me that with an old generator, a little EE work, and a lot of LEDs, I could put together a vastly brighter bike light.

I’ve been reading a lot about LEDs, power supplies, components, and the response of the human eye to light. For my purposes, the following things seem to be true about LEDs:

? blue, or blueish, light is not the best choice. This is because we don’t focus well on blue light.
? red light is not the best choice. This is because it is intended to be a front light, and because (while commuting) streetlights and oncoming traffic will ensure that my night vision is not well adapted, so I’ll get no benefit from the preserved color vision.
? white LEDs vary, a lot. Some of them have a spectrum with blue spikes in it, some do not. I am very interested in the 3000K color temperature “warm white” LEDs from LEDTronics , because they have only a tiny blue spike, peak at 565nm, and deliver the bulk of their light as “yellow”.
? white LEDs have one disadvantage; they do fade a bit after 1000 hours of use. However, in practice it appears that I would only want to ride in the dark four months per year, or 126 days, or 18 weeks, two hours per week. That’s 36 hours per year, which gives a more than adequate efficient bulb life (the bulbs also last longer when run in the cold, which correlates nicely with the darker months).
? green LEDs are another choice. For reasons that are unclear, to me, some of these LEDs are incredibly efficient, and also put all their energy into spectrum that we can easily see and focus on. For example, 25000 mcd @ 3V , versus 14000 @3.2V (both with a 16-degree beam). However, I am a little worried about confusing drivers with a green light, and the green LEDs are 40% more expensive.

I am stuck on the power supply issue. It seems to be a standard that the lights should come on at 5.5mph. I need to calibrate the power output of my little generators to see how much they produce at that speed. My old generators are clearly unregulated; I cooked a few headlights coming down hills years ago. LEDs are much less tolerant of over-voltage. So I need a regulator. There are several designs that I am considering.

One is a straight current-source regulator, out of EE textbooks, probably built using a power MOSFET so that I can make the turn-on voltage as low as possible. This has the disadvantage of pulling a constant current from the generator, no matter what the voltage. At high speeds, I would not only be dumping some number of watts into the MOSFET, I would be taxing my own self to do it.

A second choice is a switching power supply. The National Semiconductor LM3478 looks like a good choice, and they have lots of design notes at their site. I’m particularly interested in the SEPIC designs, because they can take a high or low input voltage and regulate it to a constant output. There are four sources of power loss here. One is in the switching diode, which will consume 0.5 volts. The second is in the current sense resistor, which will probably consume 0.1 volts. Switching losses in the MOSFET are at high frequencies. Resistive losses in the power supply during the charging duty cycle also contribute.

A third choice is a cheesy variant on a switching power supply. This would simply use the input voltage to charge a capacitor, and use a MOSFET to gate current into the capacitor, based on current feedback. The inefficiencies here are resistive losses in the MOSFET, resistive losses in the current sense resistor, and resistive losses in the MOSFET during switching.

A fourth choice is not to conserve excess power, but instead to dump it into additional lights. When riding fast, I want as much light as possible, and that could well be more than the light supplied at lower speeds. One easy way to do this to set up multiple simple-switching power supplies for larger and larger stacks of LEDs, run in parallel, and to gate the MOSFETS from the overall input voltage.

One thing I did learn — Schottky diodes are probably my friend. I can save 0.2 V, or 28%, on my forward voltage drop over silicon (if I could find a power Ge diode, that might be even better).

The fourth choice is looking pretty good, except that my transistor design skills are stale, and my FET design skills are really stale.

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