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I am not very good in electronics and I am a bit confused. I've read some articles which say that maximum TOTAL current for the 4 USB ports is 1.2A.

But how much can a SINGLE USB port provide?

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It depends. 8-)

The USB specifications I think, say up to 500 mA normally. I think I have heard that some (all?) Pis have extra polyfuses (besides the one in main 5V supply rail) that limit the USB ports to 140mA (each?).

There is also the fact that there is a total limit (curtasy of the aforesaid main supply rail polyfuse) on the total current that the RPi, anything powered by the Power pins on the GPIO connector, any attached camera module as well as any other USB device directly connected to the other Ports or via an unpowered Hub. For a Model B this is around 1.2A (IIRC) and the Pi itself can take up to 700mA - so you can see there may not be too much to play with. This also indicates why a powered hub is recommended if it is feasible.

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    I think they stopped using those USB polyfuses a few years ago on everything. You can certainly get at least 500 mA on a +/2 model w/ max_usb_current=1 set in config.txt. – goldilocks Dec 31 '15 at 19:44
  • Yes, I think that happened at the same time they put an extra mounting hole on the PWB in the space where those Polyfuses were... – SlySven Dec 31 '15 at 19:48

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