I was at Frys today and noticed that they had a notebook hard drive external enclosure kit for $9 that has the USB Y cable that fits. They had several brands. Many of them have a different USB connector so you need to see it to be sure that it fits the mbed board. It seems to work OK and provide more power, but still not enough power for an R/C servo with both USB cables plugged directly into the PC. In fact when you plug in the power only cable, the mbed board runs, but does not enumerate so it is getting some extra current. I know you should not tie two different regulated power supplies together at Vcc (connect GNDs only), but isn't it all just one 5VDC supply inside the PC with two fused connections and wires in parallel to provide more current.
In any case, my plan B works. I dug around in the basement for an old 5VDC AC wall wart from something I was no longer using. Clipped off the strange small power connector plug and put color coded banana plugs on the wires after checking the polarity of the wires with a voltmeter. My protoboard has the banana jack power terminals on it, so this works well and is not a big mess. Thinking ahead I got the black and white banana plugs while at Frys.
I am still a bit puzzled as to why some other companies like Phidgets have RC servo control boards that run off USB power only and will run one servo, but I have always thought that was pushing the current limit. I am using the same servo that came with that board.
So I think the lesson is plan ahead and get a protoboard that has the external power connections on it, banana plugs, and a 5V DC out 1-2 Amp AC wall adapter, if you have a complex mbed project with servos, motors, or lots of external devices that draw current. We use the Global Specialties PB 103 ones here for students, they tend to last a bit longer than the inexpensive ones we have tried.
Each USB device can only use .5A at 5VDC. For some mbed projects you could use another .5A.
I seem to recall that some notebook external hard drives use a special USB Y cable. Two USB cables plug into the PC and the second one is used only for more current.
Has anyone tried this?
Anyone know of a source for a cable like that which would plug directly into the mbed module?
If someone like monoprice.com sold it, it would be cheaper and easier than adding an AC wall wart with 5VDC out!