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Message: <blockquote data-quote="gudaya" data-source="post: 11146153" data-attributes="member: 160604">dc 5 volts ,ma count vary on versionThe USB 1.x and 2.0 specifications provide a 5 V supply on a single  wire from which connected USB devices may draw power. The specification  provides for no more than 5.25 V and no less than 4.75 V (5 V±5%)  between the positive and negative bus power lines. For USB 3.0, the  voltage supplied by low-powered hub ports is 4.45–5.25 V.<a href="http://en.wikipedia.org/wiki/USB#cite_note-35" target="_blank">[36]</a> A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A  device may draw a maximum of 5 unit loads (500 mA) from a port in  USB 2.0; 6 (900 mA) in USB 3.0. There are two types of devices:  low-power and high-power. A low-power device draws at most 1 unit load,  with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A  high-power device draws the maximum number of unit loads permitted by  the standard. Every device functions initially as low-power but the  device may request high-power and will get it if the power is available  on the providing bus.<a href="http://en.wikipedia.org/wiki/USB#cite_note-usb.org-36" target="_blank">[37]</a> Some devices, such as high-speed external disk drives, require more than 500 mA of current<a href="http://en.wikipedia.org/wiki/USB#cite_note-37" target="_blank">[38]</a>  and therefore cannot be powered from one USB 2.0 port. Such devices  usually come with Y-shaped cable that has two USB connectors to be  plugged into a computer. With such a cable a device can draw power from  two USB ports simultaneously.<a href="http://en.wikipedia.org/wiki/USB#cite_note-38" target="_blank">[39]</a> A bus-powered hub initializes itself at 1 unit load and transitions  to maximum unit loads after it completes hub configuration. Any device  connected to the hub will draw 1 unit load regardless of the current  draw of devices connected to other ports of the hub (i.e. one device  connected on a four-port hub will draw only 1 unit load despite the fact  that more unit loads are being supplied to the hub).<a href="http://en.wikipedia.org/wiki/USB#cite_note-usb.org-36" target="_blank">[37]</a> A self-powered hub will supply maximum supported unit loads to any device connected to it. In addition, the VBUS will present 1 unit load upstream for communication if parts of the Hub are powered down.<a href="http://en.wikipedia.org/wiki/USB#cite_note-usb.org-36" target="_blank">[37]</a> In Battery Charging Specification,<a href="http://en.wikipedia.org/wiki/USB#cite_note-battchargespec-39" target="_blank">[40]</a>  new powering modes are added to the USB specification. A host or hub  Charging Downstream Port can supply a maximum of 1.5 A when  communicating at low-bandwidth or full-bandwidth, a maximum of 900 mA  when communicating at high-bandwidth, and as much current as the  connector will safely handle when no communication is taking place; USB  2.0 standard-A connectors are rated at 1.5 A by default. A Dedicated  Charging Port can supply a maximum of 1.8 A of current at 5.25 V. A  portable device can draw up to 1.8 A from a Dedicated Charging Port. The  Dedicated Charging Port <a href="http://en.wiktionary.org/wiki/short_circuit#Noun" target="_blank">shorts</a>  the D+ and D- pins with a resistance of at most 200 Ω. The short  disables data transfer, but allows devices to detect the Dedicated  Charging Port and allows very simple, high current chargers to be  manufactured. The increased current (faster, 9 W charging) happens if  both the host/hub and devices implement the new charging specification.source :wiki</blockquote>

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