This may seem tirade-ish, and if so I’m sorry. This was spurred by the story posted here at TUAW. Basically, there’s a little USB pass through dongle that you can buy to eliminate your iPad “not charging” woes. TUAW kind of misses the boat on bothering to explain or understand it, so here’s how it works, in comment-on-the-article form (so read the article first):
Technically, in order to obey the USB spec, if it didn’t negotiate for > 100mA, it can’t have >100mA If it tries to draw more than 100mA and it’s attached to a USB port with a power supply line that legitimately can’t provide it, the result is a drop in voltage on the line, and bad things happen. Maybe it’s connected to a 5V supply that can handle 40930485 amps, it doesn’t matter. It’s just not allowed.
Of course, most things can supply at least 500mA, and to be sure, lots do regardless of negotiation. If I plug a dumb little USB fan or whatever else into my computer, it may use more than 100mA. Technically it’s not within the USB spec in this case, but whatever. So basically, you’re safe drawing up to 500mA from a USB port without asking, although you probably won’t get USB certification.
But of course, with a dumb charger, if you want to be within spec, you can’t just *go* and draw what you like. There has to be some other methodology. iDevices for a long time have employed resistors on the data(+) and data(-) lines of the USB port to handle this. LadyAda tore apart a few chargers to figure this out, and she found something like this: if you want the iDevice to take 500mA, hold data+ at ~2.8v and data- at ~2.0v . edit: sorry, I didn’t read the LadyAda page well enough, 2.8/2.0 is 1A, and 2.0/2.0 is 500mA. If you want it to charge at 1A, do something similar but with different values.
Finally, there’s the more recent USB Charging spec. The pertinant part here is that on modern devices, you’re within the letter of the USB spec if you short data+ and data- together as a message to the device, “you can draw any current you like, from 500mA to 1.5A.”
And something like that is going on here. It could be one of two different things. It’s definitely safe to say that on the male USB plug, only V+ and GND are attached. That way there’s no data negotiation at all. On the female side, it’s possible that resistors are in place as in the paragraph above to tell the iDevice “I’m a dumb charger, draw 500mA.” Although I lean away from that conclusion, since I BET that in such a case, the iDevice would still warn about too little current to charge. The other option is that, as per the Charging spec, the lines are shorted together. That’d technically tell the device “draw whatever,” but I THINK that iDevices only draw 500mA in this case, which like I said is generally safe. And it could be that the iPad draws an amp, and the dongle just hopes that most users’ computers will be able to handle it.
The bottom line is this:
The dongle is not magic! It doesn’t make power out of nowhere. It doesn’t matter that it’s small and light, or that the iPad is big and not. The takeaway here is that the dongle skirts the letter of USB law to say “computer, give me all you’ve got, whether you like it or not” and to tell the iPad “here’s all the power you desire!” Maybe it works across the board, maybe not, but I’ll definitely tell you I won’t buy one. I’m 100% positive that if I stick that thing in my late 2008 MBP (with its 500mA max of usb current, yes, I tested the hard way), I’m DEFINITELY getting an overcurrent USB warning. And if I don’t, the thing’s not helping anyway.
EDIT: I forgot to mention, but it’s worth doing so, you can’t simply tell the device “you have an amp” if you don’t, in fact, have an amp to back it up. It won’t just fall back to 500mA, that’s not how Ohm’s Law (v=ir) works. If the device tries to draw an amp, one of two things NECESSARILY happens. Either the computer CAN support the draw, but doesn’t SAY so (in which case, there’s certainly good reason it doesn’t say so, and good reason you shouldn’t be drawing an amp), or the computer CAN’T support the draw, and the USB bus fires an overcurrent warning and the power rail is shut down. And no charging at all takes place. In no case is it a good plan.
UPDATE: For shiggles, I did some quick testing with my USB breakout cable. As per LadyAda’s findings, my iPhone 4 with a nearly-full battery draws around 575mA from it’s in-package wall charger, stock configuration. Stock configuration has data+ at 2.8 and data- at 2.0, which is what you’d expect from a charger UL rated for 1A supply. Similarly, when I disconnect the charger’s data lines entirely, and instead show the phone 2.0v/2.0v, I measure a draw of around 510mA. Again, that’s what I’d expect. I’d also expect to see that, charging full swing, it draws more connected to 2.8/2.0, but I’ll have to wait until I have a low battery to test.