Originally Posted by
Leisesturm
How does a generator, dependent on the wildly varying speeds of a bicycle wheel maintain a constant alternating current?
It doesn't! There is a Busch & Müller tail light (
Busch & Müller) that monitors the frequency of the AC from the dynohub. When it detects a sudden decrease in the frequency, it increases the brightness of the light... a brake light without a switch or any other connection to the brakes themselves(!).
Originally Posted by
Leisesturm
What I do know, however, is that LED bulbs (Light Emitting Diodes) must have DC in order to work. And the polarity of the DC matters, as an earlier poster discovered, since all diodes block current flowing in the 'wrong' direction, it's just that most diodes don't also emit light.
LEDs, being diodes themselves, can rectify an alternating current. They just don't produce light when they are "reverse-biased". Unless the frequency of the AC is very low, the intermittent light emission is not detectable by the human eye.
But that's not the reason most lights designed to operate off of a dynohub won't work properly on DC. If the light has different levels of brightness, there is a more complicated circuit that provides pulses of DC to the LED array. See "Pulse Width Modulation". That circuitry is designed to work off of AC in most lights intended for use with dynohubs. Many of these lights are also made in a slightly different format to operate on DC (for e-bikes) as noted earlier.