USB Cycle Charger

There is a simple problem we want to solve with this project. We need to keep my phones and other devices powered or be able to charge them whilst on a bicycle. In particular, an iPhone 3GS needs power when operating as a satellite navigation system. The elements of the design are quite simple:

  • First we have a standard bicycle dynamo. Typically these are 6V ac and put out around 3W of power. You can now get more efficient (and expensive) ones that output 12V. What follows in the design needs to handle both and dynamos that output ac or dc power.
  • To avoid any issues with an ac or dc power source, the next part of the electronics circuit is a bridge rectifier. This will also handle a dc power supply being connected with the wrong polarity.
  • Next we have a large smoothing capacitor, which stores the generated electricity. If the capacitor is large enough, then it can continue to power the devices whilst the bike is stationary for short periods of time. I'm using a 6800uF 50V capacitor which should be a reasonable compromise between physical size and storage capacity.
  • These then feed a voltage regulator, which provides a smooth and clean 5V dc supply to the USB sockets. The chosen voltage regulator can handle input voltages as high as 37V, so will be perfect for this task.

Design

Typically, a USB port can supply 500mA at 5V so the voltage regulator needs to be capable of handling 500mW four each of the four USB ports on my charger, which equates to 2A of current. I've used L7805CV device with a heatsink to handle this current.

Ultimately, I'm wanting to use a 12V dynamo so that I can also provide a cigarette lighter socket and us this for charging other devices I have, which have an in-car power adaptor. My chosen design will easily support this upgrade. I'm also planning a 12V lead acid battery for more powerful lighting and a circuit to safely charge this battery from the dynamo.

My dynamo is a fairly standard 6V 3W ac version, which has a wheel that sits on the sidewall of the tyre.

Note: Although some dynamos have terminals marked as + and - or GND, they generally output an ac voltage and the - or GND pin simply means that this pin is connected to the mounting tab and hence the bicycle frame. This should not be connected or shorted to the -ve supply on the voltage regulator.

I've cheated with the packaging of my protoype as I had a powered 4-port USB hub spare, which perfectly meet my needs. It is a small and light-weight aluminium enclosure with 4 ports and a socket for a 5V dc power supply. This will be fixed to my bicycle using Velcro.

Testing has shown the design to work very well. One thing I don't like is the noise from the dynamo I'm currently using. I'm looking to replace my front wheel with a new hub dynamo built into the wheel. We have also tested the design with a 12V photvoltaic solar cell and it charges my iPhone 3GS perfectlty well.

Updates

Sept 2009 - Dahon announce the BioLogic ReeCharge, a personal power system that mounts to any bike and charges iPhones, iPods, GPS units, or just about any small electronic device, through a USB port.

Aug 2009 - I've come across a company that are selling a product that does something very similar here.

Nov 2010 - There are various design decisions above that mean that this solution will not work with all types of bicycle dynamos. The main reasons for this are the voltage drops introduced by the rectifier circuit and the voltage regulator. These have been addressed in a later project to provide dynamo powered LED lighting.

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