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Powered USB Hub

Sun Jan 13, 2013 2:04 pm

I bought a powered USB hub which I have discovered also powers the Pi. If the micro USB power plug is removed Pi still functions from the powered hub with only the hub connected to power up the keyboard and mouse. Is this normal or do i have a snag?

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Re: Powered USB Hub

Sun Jan 13, 2013 2:53 pm

The usb spec's say that a hub shouldn't provide power to it's upstream usb port.
A number of hubs are reported to violate this rule. The first generation of Pi boards had polyfuses to protect the usb connectors against overcurrent. Those couldn't handle the current needed to Power the Pi. As a result, the Pi wasn't getting enough voltage to operate properly like that. The latest Pi rev. have those Polyfuses removed, so they can be powered like that. If you do it, there is no protection fuse anymore as you supply the Pi after it's main polyfuse. So, your situation isn't exceptionel, and if you have a TP1-TP2 voltage of at least 4.85V, your Pi should work fine being powered from the hub. (Unless it's an older rev. 1 board with the usb polyfuses.)

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Re: Powered USB Hub

Sun Jan 13, 2013 7:02 pm

The voltage range for the RasPi is 4.75V to 5.25V. If the test point are between theses figures it will work just fine. In my experience in back-powering, the length of the upstream cable, has a lot to do with the voltage seen at TP1/TP2. I also suspect high loss at the connectors on some cables. If you see voltage under the 4.75V then try a shorter cable.

I have had a Raspi work just fine as low as 4.25V. Usually what will not work at this low voltage is a USB device plugged directly into the RasPi. And though you are powering your RasPi on the wrong side of F3 your hub should have its own protection.

Overall, I have been back powering a modified ver1.0 since August 2011. No problems at all. I have just setup another RasPi to back-power through a hub, a ver 2.0. It works real well also.

You should have few problems as long as the voltage is in the right range. There are quite a few people running their RasPii this way without problems. Give yours a try for a bit. It does reduce the wiring attached to a Ras Pi by a little bit.
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Re: Powered USB Hub

Wed Jan 16, 2013 7:38 pm

Thank you both for your replies. I will keep powering my Pi using the hub unless I hit a problem.


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