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Pi running "without power"

Sun Sep 08, 2013 5:33 pm

I wanted to reboot my pi today, so I pulled out the power Micro USB plug ... and nothing happened. The power LED stays on. Checking on the monitor confirmed that it was happily running along!

Apparently, the Pi can also be powered via the USB out, that's connected to a powered USB hub (although I think this is also a design fault in the USB hub). Or is this "intended behaviour"?

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Re: Pi running "without power"

Sun Sep 08, 2013 5:43 pm

This is expected behaviour on current models of Pi which can be back-powered from a USB hub. A typical configuration is to attach one of the USB ports on the hub to the input micro USB port of the Pi, and an output USB port on the Pi to the hub input port (many hubs require that before they will power the output ports).

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Re: Pi running "without power"

Sun Sep 08, 2013 6:06 pm

Yes, it is a design fault on the hub. Unfortunately many hubs have this issue and the manufacturers aren't interested in fixing it as it would increase the cost of the hub.

If you are really worried about it, you could carefully cut open the cable linking the hub to the Pi and cut the red wire and isolate it with some tape so it can't touch anything else.

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Re: Pi running "without power"

Sun Sep 08, 2013 6:19 pm

I am not at all worried about it; in fact it comes in handy as I intend to power both the hub and the Pi from the same battery pack; this means one voltage regulator less.

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Re: Pi running "without power"

Sun Sep 08, 2013 6:27 pm

It's possible (on the majority of hubs) but not recommended.

You bypass the F3 polyfuse if you power the Pi from its own downstream USB ports. This removes overcurrent protection on the main power input. If you want to power the Pi from a hub, then you can use a type A to micro AB cable and plug it into one of the hub ports, then "cut the red wire" on the type A to type B/micro B cable between Pi's type A downstream USB port and the hub.
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Re: Pi running "without power"

Mon Sep 09, 2013 12:20 am

Or you can see if your hub has a fuse (polyfuse most likely) to protect it, then just keep back-powering the RasPi. My RasPi server has been running this way for over a year without an issue.

Powering a RasPi this way is no more dangerous than using the GPIO! (still no protection as this also bypasses F3) Actually it is probably less dangerous if you read all the posts about Raspberry Pi's that have been killed by mistakes with the GPIO. Yet you will see many posts recommending power by GPIO.

You cannot easily plug a USB cable into the wrong pin!
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