Tuesday, May 25, 2010

Power From USB Port

USB port is the only port in the PC which could deliver sufficient power for the working of low power and also high power devices. devices. The 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%).



A USB port is capable of delivering 100mA(max)per port and supports upto 5 ports thus delivering a maximum current of 500mA. A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0. A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0.

Low-power devices draw at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. High-power devices draw the maximum number of unit loads supported by the standard.

Thus it is always highly safe to draw a current of 100mA. A possible circuit for drawing 5V power after filtering is shown below. This circuit is  fully tested and proved circuit. Personally I tested this on my PC. However do all things at your own risk

You might have noticed that the circuit is nothing more than a filter with a power indicater

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