Bicycle Power adaptor for Freerunner

Al Johnson openmoko at
Tue Feb 17 22:48:28 CET 2009

On Tuesday 17 February 2009, Alexander Mueller wrote:
> Hey all,
> is anybody out there who is using the FR on longer trips for navigation
> and/or geocaching? How did you solve the power problem?
> I would love to have an adapter from my hub generator for the
> Freerunner. Should not be too difficult. Something to convert AC to DC
> and then maybe some switching thing to getthe right voltage (DC/DC
> converter or what).
> Does anybody have a solution or some hints?

I have run a Psion 5 and Garmin GPS from my lightspin generator. I used a 
bridge of schottky diodes and a capacitor to get DC, and the car PSU to feed 
them their nominal 6V and 3.3V. The car PSU used ordinary fixed voltage linear 
regulators. The Psion only starts using external power at 5.5V and this was 
available above ~12mph with this setup. The lightspin needs another tweak, but 
that's because it's rather an unusual generator.

The voltage characteristics of different dynamos vary somewhat under load, but 
if you use schottky diodes for the bridge and a low dropout 5V regulator you 
will probably be ok from fairly low speeds. The schottky diodes have a lower 
voltage drop than normal rectifier diodes which is the reason to use them in 
this case.

Charging a battery would be a better solution, as someone else suggested, but 
would be more complicated. to implement well.

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