SV_huMMer
100+ Head-Fier
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- Jul 11, 2005
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Anything that won't need anywhere close to the 5v USB power (that even some motherboards don't send out on all USB ports)
Small correction here, if I may.
The voltage always remains (at least, should remain) 5V, or within very narrow margins from this (USB specs are +-5%, with some exceptions going down to 4.4v). Otherwise the device will not initialise or will not function properly.
What does vary and matters in this situation is current (measured in A(mperes) or mA). Devices that comply with various USB specs, shall provide (or consume) up to a certain current.
In USB specs there's a thing called "unit load", and it's 100mA for USB 2.0 and 150mA for USB 3.0. A single device may consume no more than 5 Unit loads (500mA) for USB 2.0, and no more than 6 UL (900mA) for USB 3.0. However, you're absolutely right - even in PC motherboards not all USB ports will provide the maximum current (not voltage) of 500mA.
USB OTG specs often provide for a much lower current limits, which render some of the attached devices inoperable due to power (current) limitations. Galaxy S3 is one of the beefiest phones on the market, as it easily allows full-current (500mA) devices to be powered from its OTG connector.