Thursday, June 6, 2013

Mobile Power: Homemade USB Charger

Before heading out for a tour of the Pacific Coast, I hobbled together this USB charger to use with my SONdelux hub on a 700c wheel:

Please excuse the mess; I threw everything together the night before I left. The next version will be much sleeker. It weighs almost nothing and cost me only about $5. The circuit I used is a [very] slightly modified version of this one (which includes detailed instructions):

 I got most of my parts from Jameco with a couple additions from Radio Shack:

Diode bridge rectifier 100V 1.5 amp [turns the AC current coming from your hub into DC, far left on the schematic]
2 Zener diodes 15V 5W [placed end-to-end at the beginning of the circuit for overvoltage protection; apparently you can hit 100V with these hubs which would fry your regulator]
* 2200 uF capacitor [for power smoothing, I might add a bigger on to future iterations]
* .47 uF capacitor [tantalum should be used for this and the next one]
* LM2940 regulator [this is a low dropout regulator, which means you will hit 5V at lower speeds. You could also use an LM7805, but they waste more voltage]
* paper clips and a binder clip [the heat sink for the regulator wouldn't fit into the box, so I just clipped some stuff onto it as a very rudimentary heat sink]
* 22 uF capacitor [tantalum]
* Female USB A [there are four pins on it, the outside ones are + and - and the middle ones should be soldered to each other for some devices to charge]
*Prototyping board [soldered everything on this, got it at Radio Shack]
*Project box [also from Radio Shack, although, with better planning, this could all be a lot smaller]
*9 volt battery leads

I added two new wires to my hub and wired them to my front rack, where I added the battery leads to be able to snap the charger on and off (or other things I make in the future):

I decided to use the the charger to charge an intermediary battery since I didn't want to risk frying my devices by giving them variable voltages when I slowed down or stopped. I also liked the idea of being able to charge anything I needed once I got to camp. I chose the inexplicably, but perhaps appropriately named Dynamo-On-the-Go battery from RAVPower which was able to charge my phone and mp3 player several times between charges [note: the first one I ordered didn't work, but the replacement seems fine so far]. If I wanted to charge my stuff directly while riding, I would still use the battery and charge my devices while charging the battery (they have separate inputs and outputs). There are some really small ones that would probably work well, like this one.

And here's the thing all hooked up:
So far, so good, although I really only had to charge the battery once on my twelve day trip. I cheated and stayed inside a couple times where I charged my phone. But I was able to charge the battery to full with the charger and fill up my mp3 player and phone. More road testing to follow...

Here are a couple shots from the trip:

Mark's first braze job is still holding together after some loaded trail testing. More photos on my photo blog.


  1. Looks like a nice ride. Did you go south or north?

    I had a cobbled together USB charger a few years ago, built with plenty of assistance from a friend who had successfully been using one for several years. Eventually, the capacitor burst and I fried the secondhand Droid phone I was using. Since then I haven't had the chance to build another, until a shelf full of commercial USB chargers was staring me in the face in Amsterdam. Using the B&M USB-Werk for now, direct to the device, although I am thinking a battery is essential.

    1. I went south. It was, as always, a great ride despite the rain and unexpected headwinds.

      I'm hoping the zener diodes in my circuit will prevent things from exploding, but I'm not an expert on such things. Most of the circuits I've seen online don't have overvoltage protection.

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