Originally Posted by
CliffordK
LEDs require DC input.
It is easy to convert from AC to DC with a bridge rectifier, and perhaps some capacitors.
Going from DC to AC can be done in a number of ways. I believe high frequency is also used to regulate power.
Many of the more advanced headlights should already have some provision for stopping, and low voltage, thus using either batteries or capacitors.
Dynohubs are AC. The hubs are alternators with permanent magnets, nothing more, no rectifiers, no voltage regulators, etc.
The rectifier is built into the light if an LED. And the light usually has some ability to protect itself from too much power, such as when going down a hill.
The old incandescent sidewall lights usually lacked over voltage protection, but those now are mostly in the scrap heap although I have a few in in a box somewhere. (I added zener diodes to mine to trim off the peaks on downhills.)
Exception - some taillights lack the rectifier, but since you are supposed to wire your taillight to the headlight, the manufacturer(s?) that makes taillights without rectifiers provides rectified current to the taillight.