>> Yes, they are powered from the USB bus. It's four wires - two are the bus
 >> and then there's +5 and ground.
 >>
 >> There are companies that sell USB-powered lights for illuminating your
 >> laptop keyboard :-)
 >
 >
 >Also USB phone rechargers :o) which makes me think we can pull a fair amount
 >of power out of the USB port :o)

Universal Serial Bus System Architecure by Don Anderson states the following:

Current Budget

A fully rated port must be able to provide five units of current (500ma) to
the attached device.  Self-powered hubs (including the root hub) having
their own local power supply can provide the maximum rated power to each
port.  However, bus-powered hubs have only the bus power that they receive
from the upstream cable to distribute to all of their USB ports.  This can
severely limit the amount of current that is available for USB devices that
attach to bus-powered hub ports.  The minimum current available at a port
is 100ma.

Regards
Jean-Michel.

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