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Rpi USB power up

tabarus12

Hi guys,

 

I bought an externaly powered usb hub that can handle a lot of power for the things connected to it and so I decided to try it on my Rpi.

I connected the hub to my Rpi and connected the external power addapter to the hub and the Rpi sprung to life, It seems the hub ca supply the power needs for the Rpi without the Rpi having an external power supply.

 

Does anyone know if its safe ro run the pi on the input power of the usb connections on the board or can I run it with only the hub power?

  • Hi Omar - you can do this, although there are some cautionary tales about it not being a good idea because the power circuit isnt protected by a fuse (I think). What I do all the time is to take a feed from my powered USB hub, but with a 'USB to mini USB' lead which I then plug into the standard power input supply on my Pi. If you check out my post at http://www.element14.com/community/docs/DOC-53239/l/raspberry-pi-as-a-media-centre theres a photo of how its hooked up - in this instance Im taking the power from the USB socket on the TV (which is in effect a powered USB hub).

     

    It certainly cuts down the clutter and saves having yet another plug socket tied up !

     

    Peter

  • in reply to PEarle

    If you have a rev1 board there are polyfuses between the usb sockets and 5v. These polyfuses have a relatively low rating which means it's not necessarily possible to power the Pi back through it's usb ports reliably.

     

    Later rev1 boards had the polyfuses replaced by zero ohm resistors, so these can be powered from the usb ports.  If you have rev2 board (with the two mounting holes) then the polyfuses and resistors have been completely removed from the circuit and these should be ok to be powered this way as well.

     

    Strictly speaking, the USB hub should not supply power to it's upstream port, only downstream ports should get power.  This theoretically means that the only way to do it is to plug a usb cable into a downstream port on the hub and to the micro-usb port on the Pi.  However, exactly what works and what doesn't is complicated by cheap hubs that don't follow the specs properly, some will feed power back to their upstream ports as well.

     

    On the downstream side, the hub shouldn't supply more than 100mA until it has negotiated with the device for higher power at which point up to 500mA can be supplied. When connecting to the Pi's micro-usb port all that's in use is the power connections, so the communication to ask for extra power cannot happen. In any event, 500mA may not be enough for the Pi and any peripherals.

    Again this is complicated by both cheap hubs that don't follow the spec and by 'USB Charging' ports that have different behaviours again.

  • Thanks foryout response but I have been running my pi from the usb using the hub I bought and its working fine, actualy I even ran some prototypes and they run great, but thanks for the info anyways.