Dynohubs are nifty electrical generators built into special bicycle hubs. They were made by Sturmey-Archer in England for several decades, and as of 2010, a new version has been introduced by Sturmey-Archer in Taiwan, though only in combination with a drum brake. (Generator hubs are also currently made by Schmidt, Shimano, Shutter Precision and others. Strictly speaking, those are not "Dynohubs" because "Dynohub" remains a Sturmey-Archer trademark.)
Generator hubs are fairly heavy -- the modern ones less so, thanks to rare-earth magnets and aluminum shells -- but they are totally silent and have no moving parts nor mechanical friction. They work by revolving a ring-shaped multi-pole magnet attached inside an over-sized hub shell around a stationary armature (coil) attached to the axle.
Advances in magnet technology allow modern hub generators to be smaller and lighter than the original Dynohub, yet more powerful. Lights also are more efficient, with halogen bulbs or light-emitting diodes, so that a modern generator system can put out several times as much light as a classic Dynohub system. The original Dynohub's power output was 1.8 watts at 6 volts, while that of most other bicycle generators is 2.4 or 3.0 watts. The headlamp sold with the original Dynohub was carefully designed to make the best use of its low power.
An original Dynohub nonetheless provides ample power for a modern LED lighting system. There are even LED bulbs now which fit the original Raleigh lights. The original headlight bulb is not only much less bright, it is very rare by now. An original Dynohub may still be found from time to time, built into the heavy, steel wheel of a Raleigh Industries three-speed -- usually 32 spokes for the front wheel with the GH6 Dynohub, or 40 for the rear wheel with the AG 3-speed hub. A quick test is to hold a key across the two terminals while spinning the wheel. If the Dynohub is good, it will spark.
Dynohubs are AC devices. Hub generator systems put out useful amounts of light at lower speeds than tire-driven generators, because the alternating-current frequency is low. The bulb lights up on the peak of each pulse of current. At low speeds, a Dynohub-powered light pulses visibly, while a light powered by a tire-driven generator goes dim.
As the speed of any bicycle generator increases, so does the frequency of the AC. This is convenient, because the generator's armature coil passes AC less efficiently as the frequency increases, substantially countering the tendency of the voltage to increase with speed. This inductive effect makes it possible for the generator to be used at a wider range of speeds than would otherwise be possible.
Like other bicycle generators, though, a Dynohub will fry incandescent bulbs if you go too fast. High speed cyclists still have to worry about cooking bulbs, unless using a modern lighting system with a voltage regulator.
A generator hub in a small wheel will produce more power at low speeds and is more prone to blow out bulbs at higher speeds. Some modern generator hubs are designed for use with small wheels, avoiding these problems.
[Sheldon, writing sometime around the year 2000, goes on to describe a primitive voltage regulator which he constructed a number of years before that. If you are going to build one, note that the Dynohub's rated output of 0.3 ampere requires a nicad or nickel metal hydride battery with a capacity of 10 ampere-hours or more (typically, a stack of 5 high-capacity nickel metal hydride D cells -- one example) to avoid overcharging with his setup in the "day" position.
A nickel-cadmium or nickel-metal hydride battery can also over-discharge, suffering permanent damage as the stronger cells drive power backwards through the weaker ones ("polarity reversal"). A"smart" system shuts off charging when the battery is full, and stops discharge at a safe level. Modern generator systems with a stay-on when stopped ("standlight") feature use an LED with a capacitor for storage, to avoid all these issues.
Front- and side-facing reflectors are nearly useless when the bicycle is moving, but they are effective when it is stopped, and are another effective backup to a generator system -- John Allen]
I used to have a Dynohub on a tandem, and the bulb consumption was unacceptable. I solved the problem (and some others) by running the Dyno's output through a full-wave bridge rectifier and then hooking the DC in parallel with a 6 volt (5 x 1.2v cell) nickel cadmium battery. This not only provided light when I was stopped, the Dyno would re-charge the nicads, and, when we went so fast that the voltage rose above 6 volts, the low internal resistance of the nicads sucked up the excess, gaining a bit of extra charge and saving the bulb.
The rectified output of the Dynohub was always connected to the lights. There was no way to turn the light off while in motion. It would have been easy enough to rig a switch for that purpose, but I didn't see the need. The Dynohub has _very_ low drag.
[The drag appears greater when the wheel is off the ground, because of original Dynohub had ow drag. The Schmidt generator has the lowest of all current models, and the new Dynohub, the highest. Still, you are unlikely to notice it while riding. -- John Allen]
I had a three way switch connecting the battery pack to the lights. In the "night" position, the nicads were in parallel with the rectified output of the Dynohub, as described above.
In the "day" position, the nicads were connected to the lights and Dyno through a diode (rectifier). This would allow the Dyno to charge the battery when it was going fast enough, but would not pass electricity the other way, so that the battery would not drain running the lights.
The "park" position completely disconnected the battery, because diodes are not perfect, and there is a slight drain that would discharge the nicads over a period of time.
Dynohubs are suitable for this type of set up because they do not use the bicycle frame as a ground. If you use a full-wave rectifier, you must keep the AC circuit separated from the DC circuit. Since most bicycle generator setups are made to use the frame as one of the wires in the circuit, this is a problem. To use full-wave rectification, either the generator or the lamps must be kept insulated from the frame. [The original] Dynohubs are already insulated from the frame, that is why they have two screw terminals and use twin-lead wiring.
[This is true of some but not all new hub generators.]
No replacement parts are available for GH6 Dynohubs. Please do not write to me asking for Dynohubs or parts; I don't have any, and I don't know where you can find any.
[If you want to rebuild one, have a spare on hand for parts. -- John Allen]