Dtm King
Posts: 4
Joined: Mon Jun 17, 2013 7:38 pm

USB current protection and limits?

Wed Jun 26, 2013 7:18 pm

Hello,

I am a beginner to the Raspberry Pi and the workings of it. What is the limit the USB ports on the Raspberry Pi can take in terms of current? I am trying to connect a Web Camera, AND a WiFi dongle coupled with a USB hub for the mouse and keyboard. It works for a few minutes, but then shuts down or freezes. I was just trying to calculate up all the current I need in order to power all of this under the 5V power supply. Also, what is the max current the Pi can intake into the USB power supply Port. Could I simply get a 5V, 2A power supply to provide enough power to the USB ports for them to work? or would I blow the fuse?

Thanks!

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redhawk
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Re: USB current protection and limits?

Wed Jun 26, 2013 7:27 pm

The Raspberry Pi has polyfuse protection F3 which is rated at 750mA or 0.75A this is the maximum current drain permitted on the +5v supply rail powering the USB ports and RG2 regulator.
If you exceed this amount then the polyfuse would increase it's internal resistance causing the +5v output voltage to drop thus USB devices fail and sometimes the SoC could trip or hang.
It's also worth noting that some power supplies have been implicated in causing the Pi to malfunction, poor designs could result in premature drop in output voltage even before maximum current rating is reached.
Sometimes changing the power supply for a different model can solve most power problem, however a better solution would be to use a USB hub with external power supply instead.

Richard S.

Dtm King
Posts: 4
Joined: Mon Jun 17, 2013 7:38 pm

Re: USB current protection and limits?

Wed Jun 26, 2013 7:41 pm

redhawk wrote: a better solution would be to use a USB hub with external power supply instead.
Thanks, I am looking into that! :)
Would it also be possible to power the Pi through the powered USB hub, meaning having only one voltage source? or would I be forced to take two voltage sources to power both the Pi and the Hub?

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piglet
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Joined: Sat Aug 27, 2011 1:16 pm

Re: USB current protection and limits?

Wed Jun 26, 2013 9:28 pm

That's what I do. However I did take the short-cut of wiring a micro-usb lead directly to the usb hub's power input, rather than using a usb port...

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redhawk
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Re: USB current protection and limits?

Wed Jun 26, 2013 9:35 pm

Backfeeding power using the USB is generally not advised because it bypasses the F3 polyfuse protection.
Without protection your Pi could be damaged if the input voltage goes too high and out of spec.
If you do find your externally powered hub is backfeeding the Pi then you'll need to open it up and cut the red wire.

Richard S.

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