If you’ve ever tried to charge your iPhone using “odd” USB dongles, you’ll find there is some secret sauce preventing the device from even accepting a charge. The folks at Adafruit Industries and the lovely ladyada have reverse engineered the magic and found that Apple uses:
The four resistors create a voltage at each of the data lines thats not 3.3V but rather 2.8 and 2.0 (or so) volts. The problem is that when you do this, the iPhone starts to draw as much as 1Amp!
Essentially, they are able to pull 1Amp out of the charger, which the iPhone registers and begins to charge. There are also special data line resistances and some other crazy jazz that I failed out of college trying to learn.
It’s a fascinating look at the deep science behind these chargers and a testament to the efforts of a few dedicated hackers who still care about resistors and voltage.