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With the rev. 2 of the Rasberry, the USB protective fuses are removed.

Does this change increase the max current limitation of 100mA per USB?

If not, what does imply the elimination of the fuses? Can the Raspberry be powered through the USB port if the external USB device is powered (i.e., a HDD).

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2 Answers

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There is no limitation on USB currrent in rev 2 board other than:

  • how much traces would be able to handle. 2.5A should be safe.
  • how much current you can get from input. There is a 1.1A polyfuse (with 700mA "hold current") on microUSB port. Since RPi itself is using somewhere between 300mA to 550mA, not much is left for USB devices. If you are using other way to power your RPi up, this limitation does not apply.

Keep in mind, however, that hotpluging any USB device that takes much current may reboot your RaspberryPi because of voltage drop. So its suggested not to use high current USB devices or plug them before booting.

And yes, you can now (almost) safely power your RPi from USB if your device back feeds power. Keep in mind, however, that USB specification says that devices should not do that (but some USB hubs and probably other devices do not have protection about back feeding power despite what specification says). Since there is no current limit, you should watch out for devices that could give more than 2.5A of power at faulty conditions as this may burn traces on your PCB.

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Wait, it still goes through the first fuse, doesn't it? – Alex Chamberlain Sep 27 '12 at 11:08
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You mean input fuse? If so, then yes, you can't get more than 1.1A (700mA without big voltage drop). But only if you are powering your RPi from microUSB. I will mention that in my answer. – Krzysztof Adamski Sep 27 '12 at 11:21
@KrzysztofAdamski so, with rev 2 board, I should be able to power directly from pi's usb a 250mA Wifi adaptor without any worry if I have a decent power supply (lets say 1.2A)? – Sergio Lopez Sep 27 '12 at 14:31
@SergioLopez: Yes, you should, at least if you don't plan to hot plug it. Keep in mind however, that a lot of power adapters will provide much smaller voltage when their load is close to max so be sure to check that voltage when your wifi is working. – Krzysztof Adamski Sep 28 '12 at 7:58
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Yes, there is no fuse (or any other protection) on 5V GPIO pin. I don't know what are the current limitations of this pin, however (for example, its traces may be smaller than USB power pins as it's designed to output maximum couple mA). So it may or may not be safe to input as much as 2.5A there. – Krzysztof Adamski Oct 1 '12 at 10:14
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I bought a rev2 RPi and a Wi-Pi adapter from Farnell / element14.

It works without a hub when you just do a simple console session over SSH.

But if you transfer more data than that (in my case a bit of multicast traffic, audio 48000 @ 16bits, 2ch), it draws too much power and you get the classic CPU stutter from your RPi..

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