Output and optics of Bicycle Power supply
Dynamos are generally limited to 0.5 A output, nominally 3 W at 6 V, although 12 V dynamos are available and the best nominal 6 V hub dynamos can produce 6 W at speed, as detailed here (a bottle dynamo is likely to slip if run at twice the nominal power, a hub dynamo does not have this problem). A dynamo behaves as a constant-current device, not constant voltage, so additional lamps added in series will draw the same current. However, this is only approximate, and the voltage can exceed the capacity of a single 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 brightness.
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.
To produce light when the bike is stationary, some dynamo lights have a stand light facility, usually a single blue-white LED powered by a capacitor, which runs for around five minutes. It is now common to use all the dynamo’s output to power the front light; the alternative is a 2.4 W headlight and a 0.6 W tail light. Most good dynamo rear lights now use LEDs instead of incandescent lamps. Increasingly, dynamo headlights use LED light sources for the main beam as well as the standlight; some manufacturers include B&M, Supernova (both German) and Solidlights (British). In these newer lamps, the dynamo output requires substantial cleaning up and rectification by electronics before it can be used to drive an LED, whereas a conventional filament lamp will run happily on the “raw” AC from the dynamo.