Quasi-high-output USB iPad charger from 12V with extension cable

I was disappointed in the performance of the “high output” dual USB car chargers I found that claimed a total of 4.2 Amps to charge two iPads but in fact could not provide that much current to even one device (failing to keep the iPad charge level from dropping while in use, despite getting an indication showing iPad charging).  Also, I wanted the works on longer cable to get things out of the way, while providing an additional 12v cigarette lighter socket for other uses.

A box with two high-output USB charging ports and a 12V socket was planned. The Murata 78SR-5/2-C seemed encouraging, promising 10 Watts at 5V output.  I set up a voltage divider as described by Ladyada to supply voltage to the USB data lines and encourage the iPad to draw more current,  but never managed to get more than about 0.9 A charging current:

nine hundred milliamps

In place of the iPad I tried an appropriate resistor rated for high power and found that the Murata could indeed put out two amps.  I tried various combinations of resistors to get different voltages on the data lines but never managed to get the iPad to draw even a full amp.  Oh well…  I moved the project from breadboard to protoboard and soldered it up, and cased it up with a Hammond extruded aluminum enclosure I had around.  Here’s a photo in a mirror showing the back with the 12V receptacle and power input line.


Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *