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Two DealExtreme USB power adapters fail to charge MP3 player

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nzo:
Hi all...

I was going to build a small 5v USB power adapter to charge my generic MP3 player when I'm away from my desktop PC.

Someone here suggested it was not worth building, since adapters are so cheap to buy via DealExtreme. So I thought stuff it and bought two of them: this one and this one.

I was surprised to find that neither of them charged the player.

The PC USB feed works perfectly and the player charge icon shows it is being charged.

When I use either of the wall adapters, the player LCD screen lights up briefly, bypasses the charge icon, goes straight to the play interface and "nothing" happens.

I prised open both adapters and there is definitely +5v between pin 1 and 4 of each USB socket. Thinking the problem may be the micro-mini resistors on pins 2 and 3, I removed them but charging still does not happen.

What is it about the PC USB feed that causes the player to charge but not the adapters?

Got me beat. Any suggestions much appreciated.

IanB:
I think the MP3 player has to believe it can draw current from the USB cable. Either it can negotiate with a real computer on the other end, or it checks for particular voltages on pins 2 and/or 3 (that's what the resistors are for, they divide the 5 V down to a precise intermediate signal voltage). Most likely in your case the MP3 player does not find the voltages it is expecting to find. I know iPhones are very fussy about this, but it is news that a generic MP3 player may also be fussy.

westfw:
What brand of MP3 player?  Some (apple) are infamous for playing a bit loose with USB standards so that they'll only recognize "real" OEM charger units.  Adafruit has a lot of experience getting their "mintyboost" charger to work with various devices, and they have posted a bunch of info about the hows and whys: http://www.ladyada.net/make/mintyboost/icharge.html (and other mintyboost documents.)

alm:
Not sure if I'd call it playing loose, does the USB standard require devices to do anything if they get supplied with 5V without a proper USB host? I think USB would require negotiation with the host first. Just connecting a dumb 5V regulator to the USB power pins is outside of the USB spec, as far as I know. There is a USB battery charging spec, but I doubt that a cheap DX charger will conform to it. There is a slight chance that Apple does, though ;).

nzo:
Sorry westfw, I haven't the foggiest idea of the brand. The player came with a sparse leaflet with no brand ID. It is 2GB, with mini USB socket. Looks like a typical Chinese Apple knock-off.

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