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Power Supplies of Bicycle Light: Dynamo Systems

Bicycle lighting serves to enhance safty and the ability of the rider to see in dark conditions and at the same time to increase the visibility of the rider to others, e.g. to increase the rider's conspicuity.
Batteries, either rechargeable or disposable, are often used to power electric bicycle lights. For bicycle touring, commuting or if managing the batteries is just too much of a nuisance, powering the lights with an electrical generator, or dynamo, may be a better option. Dynamo powered lights is self-sufficient and there is no limit to the duration that the lights can be used.But decent dynamo light sets are much more costly than decent battery powered lights Dynamo systems widely-used worldwide and for a long time, although they are uncommon in English speaking countries (having been almost entirely replaced there by battery-powered lights).

There are three main types of dynamos available, each with different attributes:

Bottle Dynamos attach to the seatstay or fork and are rotated by a small wheel in contact with the tire sidewall.Dynamos that rub against the tire rim have a few undesirable properties. They are noisy, they can slip when wet, and they wear the sidewall of the tire. The problem of slippage was solved on the higher end dynamos by the optional use of a material that provides more friction with the tire (which increases the wear even more). On a tire with thick sidewalls the wear is not such a big deal. On a lightweight tire with thin sidewalls, the wear will require more frequent tire changes. Bottle dynamos remain the most popular type.

Bottom Bracket or “Roller” Dynamos bolt between the chainstays behind the bottom bracket and are powered by a roller against the tyre, these are easy to fit and do not wear the tire sidewall, but the location near the ground subjects it to dirt and moisture. Some touring bicycles have wiring through the frame, from the bottom bracket area up to the headset, for bottom bracket dynamos These dynamos are no longer popular, but they are still available from Union and a light set with this dynamo is $70 and includes a 2.4W headlight, and 0.6W tail light.

Hub dynamos are built into the front or rear wheel hub, and are generally the most efficient. Dynamos encased in the hub of the wheel have made resurgence recently. These work in any weather and they do not wear the sides of the tire. They produce some extra drag even when they are not engaged, but the additional drag is minimal. Retrofitting a hub dynamo means building a new front wheel with your existing rim and a new hub and new spokes. A fourth type is a spoke-mounted electromagnetic system, such as that sold by Relight.

Someone can make a dynamo that produces more power. A dynamo behaves as a constant-current device, not constant voltage; this means that the voltage can exceed the capacity of the lamp at speed, causing failure. Historically this was a nuisance, but modern lamps and dynamos often incorporate zener diodes to prevent it, and dynamos can be designed to "saturate" beyond a certain voltage to protect the lamp (saturation is a feature of all permanent magnet generators). Good dynamos can achieve efficiencies of up to 70% (i.e., under 5 W of the rider's output is diverted to produce 3 W of electricity) and provide full output at surprisingly low speeds, often 4 to 6 mph (6 to 10 km/h) is sufficient for full brightnes.

If choosing to use a dynamo powered lighting system then it should be supplemented with additional front and rear "being seen" lights, because dynamo lights are particularly unsuited for “being seen.’’ To compensate for their limited output, dynamo headlights have good optics, which focus the limited amount of light in a narrow beam that lights up the road directly in front of the bicycle; this can be seen in Andreas Oehler's side-by side comparison of beam patterns.

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