Paul Pickering -August 29, 2015
Traditionally, the amount of power available from a USB port has been limited – USB 2.0 provides only a single 5V power wire with a maximum of 500mA.
That isn't enough for laptops and Hard Disk Drives (HDDs), but it's sufficient for many smaller devices; USB has become the sole source of external power for cell phones, MP3 players, and similar products, which charge their batteries from other USB ports on laptops, vehicles, or even wall sockets. There are also peripherals such as solid state drives, and "dumb" devices such as LED lamps, that run solely from USB power with no internal batteries.
The latest USB 3.1 specification increases the current capability to 900mA if legacy Type-A connectors are used. If the new Type-C connector - which has four power/ground pairs - is employed with USB 3.x, the current rating can be as high as 3A, but still at only 5V.
Impressive, but no threat to the supremacy of the wall wart. With the arrival of the USB Power Delivery Specification (USB PD), though, USB is evolving from a data interface capable of supplying limited power to a primary provider of power with a data interface.
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