This instructable will show how to build a USB charging port for an electric bicycle. This project is basically a DC-to-DC converter which takes power from the main battery of an electric bicycle (usually 24V, 36V or 48V) and converts it down to 5V needed to power a USB charging port.
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Step 1: Schematic and Parts List
Here is the schematic and the parts list with Digi-Key part numbers.
The resistor network is needed for some phones (e.g. the iPhone) that need to see 2V on USB pins 2 and 3.
Step 2: Circuit Board Layout
Here is the printed circuit board layout. There are parts on both sides of the board in order to create a more compact layout.
Step 3: Bare Board (back From the Fab)
I had a prototype board fabricated .. and here are some shots of the bare boards.
Step 4: Completed Prototype Board
Here are some photos of the prototype board with the parts installed and soldered.
Step 5: Testing
Testing just consisted of supplying a 48V input voltage and verifying the output voltages at the USB connector.
Pin 1 should measure 5V and pins 2 and 3 should both measure 2V
After verifying these voltages, the real acid test is being brave enough to plug in your smart phone.
I have tested this charger with a Samsung Galaxy Android phone and an iPhone 5 and both charged just fine.
Step 6: Packaging
I used shrink-wrap tubing to enclose the design.
Optionally inject a little hot glue just before shrink-wrapping to make a waterproof casing.