Some devices are on the market which dont actually connect to the mains, they draw their power through the usb connector. Some devices seem to be certain digital cameras and newer scanners. I would warn people to be very careful of these as NOT all motherboards seem to be very tolerant of this situation. USB is supposed to give 500ma and many motherboards arent quite up to this and the usb chips 'blow' when power is drawn. I recently installed a 4 port usb pci card for a customer cause his new scanner wanted all the available power but the motherboard wasnt up to it.
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