theunit
Posts: 31
Joined: Fri May 11, 2012 7:52 am

USB power isolation

Tue May 15, 2012 9:20 am

USB/ethernet seems to be a huge issue with the Pi at the moment.

Many people have said that the problems are likely due to power issues. There have been many suggestions by people to isolate the usb power form the Pi so there is definitely no current draw over USB.

Has anyone actually tried this? And if so, does it fix the usb/ethernet problems?

When i can get hold of a cheap USB lead i don't mind hacking, I will try this myself. I envision the best set up to be a powered USB hub connected to the RP by a cable with no V+ connection. Then all other devices connect via the hub.

User avatar
__----__----__
Posts: 16
Joined: Tue May 15, 2012 9:33 am

Re: USB power isolation

Tue May 15, 2012 9:57 am

Several people have tried that with success.

bredman
Posts: 1415
Joined: Tue Jan 17, 2012 2:38 pm

Re: USB power isolation

Tue May 15, 2012 10:35 am

Be careful not to expose too much of the cores within the cable and replace the shielding if possible. There are high-speed signals on the D+ and D- lines, and they don't like being left swinging in the wind.

User avatar
fulburncams
Posts: 70
Joined: Wed Nov 30, 2011 8:29 pm

Re: USB power isolation

Wed May 16, 2012 1:07 pm

Hi

I got one working with a 5cm long breakout cable . Just need to break the 5v line ( normally red), and feed the adapter with 5v ensuring no connection back into the Raspi. You will need to join your supply Ov to the Ov ( usually Black ) of the Raspi. I am working on a more robust , shorter and screened version now I have proven the theory. Good luck,

Regards
Steve

theunit
Posts: 31
Joined: Fri May 11, 2012 7:52 am

Re: USB power isolation

Wed May 16, 2012 7:37 pm

So i got me a cheap 6-inch USB extension lead today and proceeded to disconnect the V+ wire.

I then tested the hub connected via this lead, the hub did not even get detected. So it seems for my hub, even though it has it's own power supply, it needs the 5V from the USB to power up. This is clearly the reason why I was having USB problems, the Pi probably couldn't supply the juice the hub required.

I think I may take a look inside my hub and try to connect a wire from the 5V in from the power supply to the 5V usb wire which is now not connected. Anyone see a problem with that? I might as well power the hub from the hub!

Only worry I have is that if i go to plug the hub in to another computer not via my modded lead then both 5V supplies will be competing.

PS: My modded lead definitely works as i tested it with my external hard-drive. (It has a Y-connector so I plugged the power lead in to a USB power socket and the data lead in to the Pi via my modded extension lead.)

User avatar
jbeale
Posts: 3498
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: USB power isolation

Wed May 16, 2012 7:45 pm

theunit wrote:Only worry I have is that if i go to plug the hub in to another computer not via my modded lead then both 5V supplies will be competing.
Sounds like a good hack. As long as you are modding things, if you want to use the hub elsewhere with normal cables, you can always disconnect the +5V pin on its input USB port. Of course it will always need its external power supply to be connected after that. Depending how much current it draws, maybe you could get away with a moderately low-value resistor instead of an open to the +5V USB line, so you could perhaps still use it either way, but that mod is more "iffy" and might cause trouble, I don't know.

theunit
Posts: 31
Joined: Fri May 11, 2012 7:52 am

Re: USB power isolation

Wed May 16, 2012 9:02 pm

So i got the hub apart. Mostly tiny surface mount components which I am sure i am never going to be able to connect to with my soldering skills.

Luckily for me, there is huge diode connecting the USB in V+ and the DC in V+. Nice big connections which are easy to solder on to. I did a quick test bypassing the diode with a wire and it worked! (Very tricky business using 1 hand to hold a wire on the 2 connections and the other to type lsusb and press enter - it took a few attempts)

I should easily be able to permanently bypass that diode.

However, desoldering the usb v+ from the mini USB connector looks like a hard and fiddly task. Do you think i could just scrape away the track on the pcb?

Return to “Troubleshooting”