A couple of students from my school asked me to help them make phone chargers that worked from the dynamo of a bicycle.
(There’s still a regulator missing in the last picture, this hadn’t been soldered on yet)
The PCBs rectify the AC voltage from the dynamo using low voltage drop schottky diodes, buffer and filter it with a bunch of capacitors, and then stabilize it with a low dropout 5V regulator. The average bicycle dynamo outputs around 1A at 6V, a voltage just high enough to overcome the voltage drops of the diodes and regulator. The current in this case will be 500mA max, as the USB standard is only specified for that current and devices will not draw more current than that.
For anyone interested in the circuit, it’s much like this one.