A
hub dynamo is a small
electrical generator built into the hub of a
bicycle wheel that is usually used to power
lights. Note that the hub dynamo is (usually) not actually a
dynamo, which creates
DC, but a low-power
magneto that creates
AC. Most modern hub dynamos are regulated to 3
watts at 6
volts, although some will drive up to 6 watts at 12 volts.