One for the electronics experts here...
I recently bought a charge/sync cradle for my PDA. It was a cheap one from Hong Kong, bought on eBay.
http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=120322181676
At first, I thought it was fine. Connects ok, and charges. But then I noticed that it only charges when the computer is on. So I investigated further.
With just the charger plugged in, it charges. With just the PC on and the USB plugged in, it charges. Both plugged in, PC off it doesn't, and the LED is dim.
It appears that the 5V from the charger is connected directly to the USB plug, and thus feeds back into the computer when it's off. I noticed this last night when I turned it off, a couple of the computer's LED's were dimly lit until I disconnected the cradle.
So, I'm thinking of a solution, and I have three options so far.
#1. Insert a Schottky diode in the positive line from the USB port, to block the reverse current but still allow charging in the absence of the 5V adaptor.
#2. Disconnect the 5V from the USB completely, relying solely on the charger for charging.
#3. Perform #2 with a relay, so that it will still work just with the USB connected, but plugging in the charger will switch over 5V from the USB to the charger. There is plenty of room inside, and maybe even another LED to indicate which source it's using.
My main question is, will the data connection be affected if the 5V isn't connected to it?
The 0V will always be there.