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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.

Alessandro Da Rugna
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4 Answers4

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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.

recantha
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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.

lenik
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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.

Alex Glover
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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

Alessandro Da Rugna
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