Homebrew USB Charger

There are plenty of examples on the net on how to build your own usb charger, but since we didn’t have any on our site I figured I’d share it with you.  I found an old wall wort in my junk drawer that I couldn’t remember what it went to, so I figured it would be great to put it to a good use as a USB device charger. Because I didn’t have a clue what the maximum current draw requirements were for a USB device, I found the requirements on USB.org’s website.

Difficulty : MEDIUM - SAFETY WARNING, Shock Hazard
Time: 1 hour

The victim

To start off with I cut off the existing dc output wire of the wall wort and then cracked open the case (If you’re more patient you can probably get the wall wort open without destroying the plastic case, but I ended up putting it in a vice to crack it open). From there I de-soldered the AC wires that are attached to the AC outlet prongs and removed all the electrical innards from the plastic case. After that, I de-soldered the transformer output wires from the circuit board so I could re-locate the board after I installed it into its new box.  Since the voltage regulator is only rated for .5 Amps decided to remove and replace it with a 1 Amp LM7805 regulator. I ended up attaching a heat sink to it to keep the regulator cool, as shown below.  Btw, the transformer is rather over sized so I’m not worried about pulling too much power through it.

Next, I found a used project box to put the transformer and circuit board into. I didn’t re-use the original enclosure because I destroyed it while opening it up. I found an old beat up ac cord and an extra panel mount USB connector, then I drilled two holes into the project box to make room.  I soldered the new ac cord in onto the transformer and then glued the transformer into the project box with hot glue.  To get the stepped down voltage from the transformer to the circuit board, I ran another set of wires from the other end of the transformer to the circuit board.

AC to DC converter

Everything mounted in the box

After the circuit board was placed inside of the box I got to work on soldering the USB connector to a small proto board, shown above.  I decided to add the proto board to make mounting and connecting a couple of components much easier (plus when i tried to solder wires and components on to the USB connector pins directly I was running out of room and started creating shorts).  On the proto board I added a 1 Amp PTC (thermal resettable fuse) in series with the positive wire from the output of the AC to DC converter circuit and the output 5 V of the USB connector.  The PTC will provide device protection without the need to ever open the box and replace a fuse. I also decided that I wanted add a power indicator on the circuit, so a green led and a resistor in series with the 5V output was installed.  I installed the green led by drilling a hole in the top of the enclosure and used an clear-ish epoxy to mount it (I have to admit that the led is a bit dimer than I would have liked through the epoxy).   In order to charge the iPod correctly, the USB specification states that the data puts must be shorted together. As a precaution, I soldered a 51 Ohm resistor between the data pins on the usb connector. This way the pins are pulled together with low resistance rather than a short.  To finish things off, I attached the ground wire from the circuit to the ground on the usb connector and was ready to test.

Ipod Charging

Above, my 5th gen i pod video attached, it drew about 700 mA which was well within the limits of the supply hardware.

With the project complete I’ve got a functioning USB charger to keep at work which cost me nothing to put together (much better than paying $30 for one).

USB Charger Complete

Side view of charger

It is pretty rough looking, so I just hide it behind my computer monitor =).

Update (8/19/09):  Here is the circuit diagram for the charger.  Enjoy!

Circuit Schematic