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tedhale
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Joined: Thu Sep 20, 2012 4:52 pm
Location: Williamsburg, VA, USA
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Forcing a USB hub to back feed power

Sat Mar 09, 2013 5:05 am

There are varying opinions on whether back feeding power through the USB port is a good idea or not. However, for one project this is what I wanted.
The following post on my blog describes how to modify a USB hub to force it to backfeed power.
http://raspberrypihobbyist.blogspot.com ... power.html

Of course, you could just get a really cheap powered hub and hope that it viloates the standard and already back feeds power. :D Some of them do.
- Ted B. Hale
http://raspberrypihobbyist.blogspot.com

klricks
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Location: Grants Pass, OR, USA
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Re: Forcing a USB hub to back feed power

Sat Mar 09, 2013 2:53 pm

Rev 1 boards have the 'poly' fuses on the USB ports. Each port is limited to 140mA... so that is not enough to run the Rev 1 RPi. (300-500mA)
So to backfeed power via USB on a Rev 1 board you would have to bypass fuse F1 & F2 by soldering wires across the fuses.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

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Lob0426
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Location: Susanville CA.
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Re: Forcing a USB hub to back feed power

Sat Mar 09, 2013 8:24 pm

I have been using a hub to back-feed a RasPi. I had to rewire the hub to back-feed. Soldered jumpers between the polyfuse and the uplink port. i also installed a ground wire straight to the connector to help with low voltages. My WordPress server is running on a modified hub and a modified ver1.0 RasPi. This one is hardwired as I had some issues with low TP1/TP2 voltages. Turned out to be loss at the connectors and wire lengths. I recommend a short USB cables for best performance. I had no problems with a ver 2.0 board.

See this post for some more details. It does not show the version 2.0 board version, which is also mounted in a VESA mount with the hub.
http://www.raspberrypi.org/phpBB3/viewt ... 41&t=16473

P.S. this setup powers a HDD and a WiFi with no problem. I also have no problems hot plugging USB devices with either version board. Even the HDD can be hot plugged without causing a reset. Overall stability is better than a RasPi under normal power.
512MB version 2.0 as WordPress Server
Motorola Lapdock with Pi2B
Modded Rev 1.0 with pin headers at USB

http://rich1.dyndns.tv/
(RS)Allied ships old stock to reward its Customers for long wait!

totoharibo
Posts: 4236
Joined: Thu Jan 24, 2013 8:43 am

Re: Forcing a USB hub to back feed power

Sun Mar 10, 2013 8:14 am

it is possible to power up the RPi and the 2 USB ports.

Simply put the power on the +5V and GND GPIO pins.

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