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I have a KB Gear JamP3 MP3 player which connects to my PC with a USB cable. (No drivers needed in Windows 2000, Windows XP and recent MacOS!) The problem is that it always runs off the two AA cells, even when plugged into a powered USB port. Filling up the built in 16meg plus the 32meg MMC memory card can easily suck half the charge from a pair of cells.<p>I want to build a tiny circuit that will detect when power is available at the USB then cutover from the AA cells to the USB power. Since full USB power is 5 volts and 500 milliamps, the circuit will also have to regulate down to 3 volts and 320 milliamps. That's the specs on the player.<p>I've had the player open and there is plenty of room inside. Looks like I could fit it in between the buttons on the front where there are a couple of spaces about 3/8" square, possibly a little bigger. If I have to do point to point wiring, I don't care. I'll just pot the parts in liquid electrical "tape" or hot glue. I also need to know which two pins on the Type B USB connector are for power.<p>Here's a link to a scan of the inside front of the case and the front of the circuit board. The yellow dots are where the buttons are. http://home.rmci.net/gregg1/jamp3case.jpg At my screen resolution of 1152x864, the image is actual size.<p>Thanks.

So nobody has any ideas?

http://www.maxim-ic.com/appnotes.cfm/appnote_number/190<p>I found this by searching Google for "battery switchover".<p>Ron

That looks like the thing! I wonder how small it can be made?<p>I'm also in Idaho.

I would just use a little double diode ckt.<p>
|\ | | /|
to usb ----------| >|-----|< |------ to batt.
|/ | | | \|
|
|
To ckt power/regulator<p>I use this ckt for battery back-up on a
motherboard. (my ascii drawing sucks)<p>it uses a surface mount component like about,
part # bat54cct. it measures a whapping 3mm x 1mm<p>edit: argh, it pulls out the white space! To
explain, the power sources from usb and batt
go through their seperate diodes and the output
is where the two cathodes meet.<p>[ December 04, 2002: Message edited by: billdar ]</p>

bildar, can you do up a drawing of that and e-mail it to me at [email protected] ? Remove the "spiced meat product" to send e-mail.