LED Generator light

For many years I relied on an old Union headlight powered by an even older Soubitez bottom-bracket generator. So long as I didn't use it much it was just fine. Lately, however, nocturnal bicycle rides have become the norm on my commute home and better light became a priority. Among bicycle generators nobody offers more watts than the Soubitez, but theledlight offered a ray of hope [sorry, couldn't resist] in the form of Luxeon's Star/O integrated power LED and collimator.

Here are some illustrations of a very simple headlight setup using Luxeon

  Star/O light-emitting diodes with integrated optics. The Luxeons are the 1 watt size arranged in series pairs wired inverse parallel. Here's the schematic:

  The mounting frame is just a piece of aluminum carpet tack strip chosen from the local hardware store. An aluminum channel with the LED's tucked inside would be better to protect the diodes. The frame provides both a heatsink and an adjustable mount: The LED's have some collimation and the soft metal support can be bent to adjust the light pattern. It helps to give the two outermost LED's a bit of upward tilt to compensate for steering stem rake in turns.

One question apt to come up is "Why four LEDs?"

After all, the generator is rated at only three watts, a single 3 watt LED is much cheaper than four one-watters.

The simplest answer is efficiency; the generator makes AC, a single LED will use only half the current unless there's a rectifier, but a full-wave rectifier will waste nearly a volt and a half. Seemingly much better to use two LEDs in inverse parallel, so each one takes a half cycle. No rectifier loss this way. But, there's another fly in the ointment. Perhaps better to say, a bug in the lampshade 8-) Actually, a huge moth, casting a large shadow.

The problem is impedance matching. The generator is designed to drive a load that takes about half an amp at six volts around normal riding speed, say 10 mph. The Luxeon LEDs take only slightly over three volts at one-third of an amp. Splitting the current between two LEDs uses all the current but wastes nearly half the generator's voltage capability.

Putting two LEDs in series pairs comes much closer to the load the generator is designed for and roughly doubles the light output. The AC voltage is clamped at 7.2-7.4 volts, I haven't measured the AC current but Andreas Oehler reports 580 mA for a 6v 3w dynamo. Split between the two diode strings that's well within the 350 mA continuous rating for one watt Luxeons.

But, we've gone from a $20 light to a $60 light. Is it worthwhile? In an environment without competing lights, yes, without question. For the extra light I'll gladly pay and it is useful in total darkness. Riding in traffic is a somewhat different problem. No 3 watt generator light can compete with dazzle caused by streetlights and car headlights. The lack of a standlight is a serious handicap.

Poke-in-the-eye battery lights definitely have their uses.

A DC system allows use of a capacitor (or battery, if the voltages work out right) to store energy and run the light when the bike is stopped or running slowly, as when picking one's way around an uncertain corner.

One could put capacitors in parallel with the two grounded LEDs, reasoning that they'd store a little extra energy to keep at least those two glowing dimly at standstill. Unfortunately this will double the reverse voltage on the top two LEDs, which is probably not good for them.

A dual-mode system is a plausible compromise; efficient AC for forward light, with regulated DC for capacitor or battery backed tail- and sidelights. The wiring will be a bit more complex, but all progress has a price.

Notes on optics

The advertised beam width of the Luxeon Star/O is less than 10 degrees at FWHM (full width half maximum), but my subjective impression is that the beam spread is significantly more. This isn't entirely bad, it's relatively easy to read street signs. But, the optics are a limitation. Better collimation would put more light where it's needed.

Mounting features of the Luxeon Star/O

The mechanical design of the Luxeon Star/O is surprisingly friendly to shadetree mechanics.. The four I bought were on a single piece of aluminum scored to let them break apart with a little effort. The aluminum is pre-punched with notches to let two #4 screws bolt the LED to a heatsink. A pair of tiny solder pads provide the electrical connections. The optics are snapped into a plastic collar that appears to be glued to the aluminum. The assembly does not appear to be sealed, so weatherproofing is a matter of some concern. However, it's complete and easy to use.

On balance the quad Luxeon configuration is an improvement over a traditional three (actually 2.4) watt halogen bicycle headlight. Whether it can compete with a rectified single-LED setup using a battery for standlight ability is yet to be determined.

One very good puzzle is the performance limits on generators. The 6 volt 3 watt standard is very old, dating from the days when Alnico was the ultimate magnet material and tungsten lamps were the ultimate light source.

Modern permanent magnets and magnetic alloys are much better. Light emitting diodes are more efficient than tungsten filaments. Their forward voltage-current characteristic is much easier to match to an inductive driver.

The light source folks have moved and raised the bar of normal performance. Maybe it's time the generator folks took notice and responded. A three-watt output is pitifully feeble. Especially when compared to the hundred watts of a resting human.

May I have a little more power, please?

bob prohaska