Quick and dirty dynamo powered bike USB charger

One day a biker friend of mine asked me to give him a solution for charging his phone from the bike's dynamo while on longer trips. Sounded pretty simple and a nice challenge for a low cost solution to be found.

His bike has a Shimano HB-NX70 dynamo that's nothing more than a alternating current generator rated for 6V and 3W. That voltage rating is an estimative figure though as it all depends on the wheel's speed so you can have much less when cycling slow or much more when speeding.

That being said, the solution? Whatever I had laying around as left overs or in the junkbox. A small plastic case, small piece of prototype PCB, a PC bracket with two USB connectors, a DC/DC converter (a.k.a. BEC in RC hobby world), and some discrete components: a rectifier diode bridge, a big fat filtering capacitor and a little protection with a zenner diode.
 

On the bike's original electrical instalation, the dynamo powers a head light together with a tail light. I had to open the head light anyway because it wasn't working so I pulled the AC from there.

 The BEC starts operating at 6Vdc and goes all the way up to 23Vdc. I put it all together in a hurry and didn't give the time to pull up a schematic or do some multimeter work on the dynamo's output and how all things work together.

All I can say is that when the time came and we gave the charger a test spin, it worked, charging a phone with 600mA which is decent enough.