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I want to power a Pi model B from a powered USB hub. I got surprised that the Pi powers on with only the USB attached. A little search in the forums confirmed me that it is a working solution.
I measured the voltage between TP1 and TP2 and it says 4.25 V. (my experience says it's quite low making the Pi unstable)
If I also connect the microUSB to the hub the voltage jumps to 4.90 V.

Is it safe, from an electronic point of view, to have the Pi powered from two sources?? Can this damage the hardware in some way?

The hub is a Belkin F5U701 with a power adapter that outputs 5V with 2.5A.

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Both connections are actually the same source. It's a know problem of some hub, to backfeed the pi. Looking at raspberrypi.stackexchange.com/a/7308/7863 it doesn't seem to be a problem in newer revisions of pis. So either thrust this info and just connect both the usb and micro-usb. Or take the safe route, and snip the 5v wire in the cable going to the hub. –  Gerben Jun 8 '13 at 20:29

5 Answers 5

The Pi should not be powered from it's own USB ports. It's not a safe way of supplying power. The correct way is to use either the micro USB port or the correct GPIO pins.

The hub you have is at fault here - it shouldn't be supplying power upstream along the data feed cable. You may have to take it apart and cut a wire.

Powering the Pi from two places is not recommended.

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Cutting the wire is a surprisingly good idea =) I didn't think of. –  Alessandro Da Rugna Jun 10 '13 at 7:41
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Powering the Pi from the USB ports and Powering the Pi from the GPIO ports carry the same amount of risk. Either method is bypassing the main protection fuse F3. –  Lawrence Aug 15 '13 at 1:14
    
backfeeding by raspberry usb ports is safe for use but you don't have additional protection for shorts –  RooTer Aug 19 '13 at 10:02
    
So we power the Pi from microUSB, and when we want to plug a hard drive, we plug the second power to the hub and remove the first power from microUSB? and before removing the hard drive we plug the first power and unplug the second power again? Looks very complicated... –  ilius May 18 at 13:36
    
That would be easier to find to high power adapter (like the one that feeds the hub) and just plug it into the microUSB and just leave the power as is. Then we can plug/unplug usb hard drives whenever we want. –  ilius May 18 at 13:38

Never power any device from two sources. The source with the higher voltage will provide the reverse current to the other one and since power supplies have very low internal resistance (by design), eventually one of them will give you a smoke signal.

The correct mode of supplying power to RasPi is through microUSB connector, it has the short protection fuse and other interesting things to protect your power supply and/or schematics.

If you try to explain, what kind of goal you're trying to achieve, maybe there will be a better suggestions available.

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I'm trying to power the Pi plus some energy-hungry devices from a single hub. I didn't expect the hub to backfeed the Pi, it feels so unnatural. It's running fine with the double powering since when I posted the question but I wonder if it's "safe", and if not why. –  Alessandro Da Rugna Jun 10 '13 at 7:39
    
4.25V might be a result of protection diode installed in your hub, usually there's a 0.5V voltage drop over a diode. if it works, touch everything, if there are no hot parts (power supplies, voltage converters?) -- leave as it is. –  lenik Jun 10 '13 at 9:59
up vote 1 down vote accepted

I'm answering my own question because I'm not fully satisfied of other answers. They are correct and I agree with them: do not power any device from two sources, a backfeeding USB hub is doing the wrong thing.
I'm no expert on electronics, and a quick look to Raspberry Pi ModelB schematics didn't tell me much, except that microUSB power input and USB hub have the same +5V line.

My experience is the following: powering the RaspberryPi model B from these two sources isn't affecting the Raspberry functionality and the hardware keeps running fine. On the other side, it also doesn't improve stability or resolve any power-related issue you may encounter with USB devices. I still get some random reboots when I plug or unplug USB devices directly on the Pi.
I implemented on my software a subscription to the hardware Watchdog to reboot the Raspberry when it hangs. Just a couple of articles on Watchdog for reference:
http://harizanov.com/2013/08/putting-raspberry-pis-hardware-watchdog-to-work/
http://pi.gadgetoid.com/post/001-who-watches-the-watcher

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recantha and lenik gave you the correct answers, they just didn't elaborate on their answers to apply to your "performance problem." As recantha pointed out, your USB hub is "back-powering" or "back-feeding" your Raspberry Pi. When you plug a new device into your USB hub, the voltage is likely dropping and your Raspberry Pi is suffering a "brown out" and restarts itself.

The USB-out connections on your hub should be providing the proper voltage (which is why the Pi works when you route a cable from the USB-outs on the hub to the micro-USB power input on the Pi). However the USB data link or input connection isn't supposed to be providing power, so there is not guarantee that it is providing proper power when load spikes when you plug another device into a port.

As to why you shouldn't back-power or back-feed your Pi, I recently wrote an article if anyone needs/wants more information.

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Here is a great project on Kickstarter that allows you to power the Raspberry Pi board http://www.kickstarter.com/projects/1154457775/usb-power-adapter-charger

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Interesting piece of hardware, but it doesn't answer my question. –  Alessandro Da Rugna Aug 19 '13 at 7:21

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