A hub dynamo is a small electrical generator built into the hub of a bicycle wheel that is usually used to power lights. Most modern hub dynamos are regulated to 3 watts at 6 volts, although some will drive up to 6 watts at 12 volts. The market was largely pioneered by Sturmey-Archer with their Dynohub product of the 1930s-1970s, which competed effectively with contemporaneous sidewall (bottle) and bottom-bracket units. The Sturmey product was heavy, having a steel case, and like all bicycle dynamos of the time tended to overdrive the lamps at high speeds. Sturmey-Archer have recently reëntered the hub dynamo with their X-FDD product, which combines a hub dynamo with a 70mm drum brake in an aluminium alloy housing[1] The Schmidt Original Nabendynamo (SON) can power two 6-volt lamps in series at speeds above about 12mph, and Schmidt manufactures lamps designed to facilitate this. These lamps have optics based on the Bisy FL road lights. The efficiency of the SON is quoted by the manufacturers at 65% (so just over 5W of the rider's output is diverted to produce 3W of electrical power) but this applies at only 15 km/h (10mph). At higher speeds the efficiency falls. Automotive alternators, in contrast, achieve efficiencies over 90% by using electromagnets to adjust the field current. Bicycle dynamos instead use permanent magnets to elminate the need for a battery to excite the field and initiate electrical generation. Shimano offer a variety of hub dynamos under the Nexus brand, such as the DH-3N70 (also called DH-3N71), which is advertised as having significantly less drag than the Nexus NX-30. However, the Shimano hub dynamos generate more drag than the SON . Some of the Shimano units come with an automatic light-sensitive switch.
