This page provides an overview of Bicycle Generators, including the common types and their relative advantages and disadvantages.
Common Features of Bicycle Generators
All bike generators that I have ever seen have had several common traits. They all put out AC current, and all can produce 3 - 6 watts of power. Most bike generators are designed to produce a 6V output voltage, and a few are designed for 12V. All bike generators 'saturate' at a certain output current, in order to avoid frying whatever they are powering. This is done by limiting the amount of ferrous metal surrounding the windings in the generator. (Don't ask for any more details, please). Most bike generators, when used on an average sized bicycle wheel, will reach their full output power when going around 12 - 15 miles per hour. The saturation trait gives them the slightly odd feature that you can (theoretically) make them put out any output voltage you want, at up to whatever the saturation current is. The down side to this is that you have to spin them really fast get higher voltages, when you have any kind of load hooked up. (Faster than you could easily get going on a bicycle). For a hypothetical 3 watt, 6 volt generator and a load that somehow limits the output voltage to 3 volts (say, a battery that is being charged) then the maximum power that you can get out of the generator, due to the saturation current, is (3 volts) x (1/2 amp) = 1.5 watts. If your battery is 6V, then the generator is limited to 6 volts, but the saturation current is the same, so you can get 3 watts out of the generator. With higher voltage batteries, you can get even more power from the generator. (Charging a battery will not provide any load to the generator until the generator voltage surpasses the battery voltage, and after that the load increases quickly for small increases in generator voltage) If the same hypothetical generator has a resistive load (say, a light bulb with a fixed resistance of 12 ohms) Then when the light bulb has 1/2 amp flowing through it, it will have 6 volts across it, and so the generator, which cannot supply more than 1/2 amp, will be limited to 6 volts.
The Unregulated Battery Charging Circuits I describe take advantage of the fact that you can get some power out of bike generator when it is going slowly, but its maximum power output goes up if you increase the limiting voltage. That is why the third circuit (the one on my bike) is designed to switch between a 3.6V limited circuit to a 7.2V limited circuit as soon as the generator is going fast enough to put out more than 7.2V. The maximum power output of the generator is higher with the higher limiting voltage, so the overall circuit efficiency goes up. There is no point in trying to switch to a yet higher limiting voltage, because I could not sustain the speed on my bike to make it useful.
There are three main kinds of bicycle generators. I will describe pros and cons (as I see them) of each below. The three kinds are: Hub Generators, Sidewall Generators, and Drum Generators.
This is, in my opinion, the best kind of bicycle generator. This is the kind that I have on my bicycle. This type of generator is built into the hub of a bicycle wheel (usually the front). These are by far the most elegant, and the most efficient (no friction to overcome between a roller and tire, like the other two kinds of generators). These are also impervious to dirt, unlike the other generator types, since they are away from all the muck the tire goes through. This kind is by far the most reliable. This type also will not slip when the tire is wet, and cannot damage the tire. The disadvantages of hub generators are that they are heavy, harder to install (you must build a custom wheel) and they tend to be a little less powerful than the other types. Hub generators are by far the most expensive. (A used dynohub will cost around $40 - $50 and you still need to pay to build the wheel.) I do not know if Sturmey-Archer still builds dynohubs. I have seen dynohubs with manufacture dates from the early '50s up to the middle '70s. I would not recommend heavy use of a dynohub built before the '60's due to poor metallurgy. My dynohub was in new condition when I got it, and now has tens of thousands of miles on it. It was built in 1965. Today, modern hub generators are made by Shimano and Sachs, but these are quite expensive and still relatively hard to find. There are other hub generators, (bendix made one back in the 50's) but I have never seen a serviceable example.
This is by far the most common type. They are shaped like a little barrel with a small roller that runs against the side of your bike tire. These are small, but powerful for their size. They are reasonably reliable, but you have to work a bit to keep them clean and oiled, especially if you ride when it is wet. They will wear out after a few hundereds or a thousand miles of use, but they are easy to find and replace. The problems with this kind of generator are that they can damage your tire, and they will only work if your tire has a surface that the wheel can run against, and they can slip, and they are noisy. They are also kind of ugly. They do have another small advantage over Hub generators, in that they can be disengaged from the wheel. (Though the extra drag from a hub generator is totally unnoticeable.)
These are basically like sidewall generators, except that their roller is designed to contact the top of the tire instead of the side. They are less likely to damage the tire. They are also a little less noisy, since the roller is larger diameter. They are a little more discreet than sidewall generators are. Unfortunately, they will get completely clogged with dirt and die, if run under anything other than perfect weather and paved streets. I have never had much luck with drum generators for that reason. These are a lot less common than sidewall generators, but still easier to find than a hub generator.
Most bike lights these days run off of batteries. Fancy battery systems can be good for up to 40 watts light output, but they will run down. (And the batteries for these systems are usually big and heavy, too.) Many 'fancy' battery systems have a couple of hours of run time, or less. In my experience, a well-focused 6 watt headlight is sufficient for most circumstances,and with the proper generator, you can run after dark indefinitely (albeit with a noticeable drag penalty). With a 1.5 amp-hour, (the capacity of average Ni-Cad C cells) 6V battery, you can run the same headlight for 1.5 hours. (multiply the amp-hour capacity of the battery by the voltage, and then divide by the headlight wattage to figure how long it should run)
Finding a Generator
There are new bike generators available, but most are fairly poor quality (that statement does not apply to new hub generators). It is not hard to find older, but better generators in good condition by looking around secondhand bike shops (any big urban area will have some) or thrift stores. Usually they have barely been used, and just need oil and a good cleaning. You can tell by how worn the roller is, or just by how much dirt is on them. For a hub generator, if the axle turns freely, but is not sloppy, and it does not make any bad noises, it should be OK. Make sure to thorougly clean and repack its bearings before using it, if it is used.