iDevice charging voltage divider

Other Hardware General Electronics
1

Apologies if this is a repost but I couldn't find the right answer.

I am working on a clock with dual usb ports that I would like to be able to charge any usb devices. As you probably know, iPhones look at D+ and D- for voltage to determine what it is plugged into, either a wall port or usb port, and how much current to draw, either 500mA or 1A. The iPad wants to draw 2A. I ripped apart a dual usb car adapter, cut out the 12V adapter part, soldered on some jumpers and it works perfect for all my devices. I am using a 5V 4A power supply that outputs a steady 5.12v

I copied the resistance of the circuit on back of the usb port and got the following schematic with the voltages. (luckily resistance is written on them, I assumed the 513 refers to 51.3k)

I recreated it on a breadboard, it charges my iPhone 5S and my girlfriends iPhone 4 and iPad 2 perfectly fine but will not charge my iPad 3.

I cant figure why it behaves differently under identical conditions. My best guess is that the resistance is in the right ratio but is too high and there is not enough current to charge up a capacitor in the iPad's battery charger circuit. Any suggestions?

2

The voltages you indicated on your drawing, are they no load voltages or with the unit connected?
As regards the resistance values, 513 would indicate 51 followed by 3 zeros making it 51K ohm.

3

Could they be sensing resistance rather than voltage? Or use the resistor network to detect a voltage drop that is the result of overloading (a sort of power sense)? In those cases the values of the resistors matter, not the voltages they produce.

4

I'm pretty sure it's a resistance sense, not voltage sense. You don't want spurious voltages on the data pins of a USB plug. That's a good way to burn up your iP(hone | ad).

5

The voltages you indicated on your drawing, are they no load voltages or with the unit connected?
As regards the resistance values, 513 would indicate 51 followed by 3 zeros making it 51K ohm.

thanks for the tip, this is the no load voltage.

6

rclymer:
this is the no load voltage.

Change the resistor values, load it and re-measure as the iThingies are rather picky about what they measure on the two lines.

7

If I'm not mistaken (please double check), you will need:
2.75V on D-
2.065V on D+

8

This may help:

9

rclymer:

The voltages you indicated on your drawing, are they no load voltages or with the unit connected?
As regards the resistance values, 513 would indicate 51 followed by 3 zeros making it 51K ohm.

thanks for the tip, this is the no load voltage.

Measure the voltage with the device connected.

You might need to lower the value of your resistors to match the impedance of the iDevice.