semtex
Posts: 63
Joined: Sat Mar 03, 2012 4:54 am

Where does RasPi get its power from?

Mon Oct 15, 2012 9:20 am

I use a (genuine) Apple iPhone PSU to power my RasPi. This is rated at 1A. I recently got a powered 4 port USB hub which has a 2A PSU. But I was shocked to discover that with the Apple PSU disconnected, the RasPi powered up as soon as I plugged in the USB cable from the hub.

My question is if I have both the USB hub connected and have the Apple PSU plugged in, what is RasPi using as its power source? Can I stop RAsPi from using the hub as a power source? I want the RasPi to just use the Apple PSU and leave the hub PSU to power just the hub peripherals.

Thanks for any advice.

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Burngate
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Re: Where does RasPi get its power from?

Mon Oct 15, 2012 10:07 am

semtex wrote:My question is if I have both the USB hub connected and have the Apple PSU plugged in, what is RasPi using as its power source?
It depends on which PSU is providing the higher volts.
semtex wrote:Can I stop RAsPi from using the hub as a power source? I want the RasPi to just use the Apple PSU and leave the hub PSU to power just the hub peripherals.
If your Pi is rev 1 then unsoldering F1 and / or F2 (or the 0 ohm links) will do what you want.
Alternatively, cutting the red wire in the USB lead to the hub, or opening the hub and cutting a track, will do it.

Many people find that using the hub to back-power the Pi is advantageous - only one PSU to switch on/off - but I can see situations where that might overload the hub PSU.

As others have said, hubs shouldn't do this, and many don't, but many do

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johnbeetem
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Re: Where does RasPi get its power from?

Mon Oct 15, 2012 6:47 pm

Burngate wrote:... Alternatively, cutting the red wire in the USB lead to the hub, or opening the hub and cutting a track, will do it.

Many people find that using the hub to back-power the Pi is advantageous - only one PSU to switch on/off - but I can see situations where that might overload the hub PSU.

As others have said, hubs shouldn't do this, and many don't, but many do
I have heard of hubs that have an internal switch to control whether power goes upstream or not. My own hub has a Schottky diode to make sure power only goes downstream, so it's compliant with USB standards. OTOH, now I'm mostly using my RasPi with an Atrix Lapdock, which deliberately provides upstream USB power.

The original rev 1 boards with polyfuses at F1 and F2 had problems with hubs that send power upstream. Newer rev 1 boards replaced F1 and F2 with 0 Ohm jumpers to avoid these problems.

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