ferrymanr
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Isolating USB device from RPi USB power

Sat Jul 21, 2012 10:20 am

I am having problems with powering of external USB devices.
1. My externally powered USB hub tries to feed power back to the RPi when normal power is removed from the RPI power connector but not the hub. This does not seem to be a good idea. Do all brands of hub do this?
2. My audio device does not like working through a hub but does work when directly connected to the RPi but draws too much current and has no external power connector.

I thought of getting around these problems by constructing a male to female USB adapter with the 5V wire disconnected at the RPi end. In this way no power could be taken from or fed to the RPi. To enable powering the external device the +5V wire from the external device USB connector could then be fed direct from my 5V power supply. RF engineers might call this a bias tee. Is this a technically acceptable move?

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AndrewS
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Re: Isolating USB device from RPi USB power

Sun Jul 22, 2012 10:10 am

Yeah, as long as the GND, D- and D+ wires are still connected to the Pi, I don't see any reason why it wouldn't work.

From what I've read on these forums, it tends to be the cheaper (i.e. not 100% standards complaint) USB hubs that 'back-power' the RPi, and as you note this is a bad thing - can cause excess current to flow through the Pi. Again the recommendation is to cut the 5V between the RPi and the hub (or cut the 5V connection inside the hub, if you want a neater solution).

ferrymanr
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Joined: Fri Mar 16, 2012 11:09 pm

Re: Isolating USB device from RPi USB power

Sun Jul 22, 2012 3:43 pm

I did try stopping the USB hub feeding the RPI whilst retaining the reverse capability by fitting a 1A schottky diode in series with the +5. Stops the hub feeding the RPi but introduces almost 0.2V drop when requred for use without external power.
As a separate issue I have now upped my 5.0V supply to 5.2V to allow for cable/connector losses so I get 5V when tested at the capacitor on the RPi. Seems more stable with the correct voltage on the board. I have one micro-USB cable which has so much resistance the RPi only gets 4.5V. There is some real rubbish on eBay!
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jbeale
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Re: Isolating USB device from RPi USB power

Sun Jul 22, 2012 4:28 pm

I made a USB power isolator with a short USB extension cable, separating out the +5V line, and tapping into the GND return line. It works fine to power a nominally-unpowered hub separately, without back-feeding the RPi. Does require having a separate +5V power supply of course for the hub. But if your hub already has its own power, then you wouldn't need anything more.

here is a photo:
https://picasaweb.google.com/1099282360 ... 2578510498

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williamhbell
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Re: Isolating USB device from RPi USB power

Sun Jul 22, 2012 8:35 pm

Nice hack. By the way, can you hear the HDMI imaging when the audio is turned up high and the PI is not isolated from the USB-hub connection?
http://www.raspberrypi.org/phpBB3/viewt ... 66&t=11971

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Montekuri
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Re: Isolating USB device from RPi USB power

Mon Aug 27, 2012 3:24 pm

Is there any harm to R-Pi if it is powered just from the 'frontal' USB?
I say, do not connect the micro USB power connection and just feed the R-Pi by the 'frontal' USB.

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Lob0426
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Re: Isolating USB device from RPi USB power

Mon Aug 27, 2012 4:42 pm

The RasPi will actually run back-powered from the hub. The 140ma USB fuse will cause it to reset time after time. The newer RasPii coming out, have a 0ohm resistor where the fuses used to be. These RasPi will back-power without resetting. The series of resets are what will crash your SD card, not the back-power! Unplug the hub first then the RasPi, or cut the power wire in your hub cable. Mark the cable so you know it was changed.

I know this because I am deliberately back-powering a RasPi, and have been for about a month, with no ill effects as of yet. Runs XBMC with a USB HDD and WiFi dongle. Only one cable to the RasPi, for power and Data. One PSU to the USB hub!
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