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In question: What is the power output of a USB port? per Doktoro Reichard's answer:

As stated in Wikipedia

The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices.

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.

So my question is: if I have a USB port rated at 1.5 A, is it restricted to always output 1.5 A? Can it automagically output 0.5 A, when a device, which doesn't need as much power is connected to it? Or does it depend on the USB port? As in, some can adjust but others can't.

andrybak
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1 Answers1

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The posts quoted above by user1686 deal with power supplies, but the answer is the same as in your question : A device will draw only as much power as it needs. If more is available, then it stays unused.

In the case of the USB port rated at 1.5 A, the rating is the total power available. You can for example connect to it a USB dock with three connected 0.5 A devices, to use up the entire 1.5 A. With fewer such devices, only a fraction of the available power will be used.

This is the case with any kind of power supply, including your wall power. A lamp won't be overwhelmed when connected, although much more power is available than it will take.

harrymc
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