RaspPP
Posts: 2
Joined: Wed Aug 15, 2012 6:19 am

Power and ground loops

Wed Aug 15, 2012 7:02 am

Hi all,

I was not sure whether to post this here or rather under "feedback and requests", but finally decided to post it here.

I am struggling to come up with a way to cleanly power the Pi and a powered USB hub from a single power source.

1) Powering the Pi with a power source and the hub with a separate, isolated, power source would work. However, this seems ridiculous, since it means adding two power sources with corresponding losses (and costs) to power this otherwise quite energy-efficient system.

2) Powering the Pi from a Hub, as is recommended in some posts, inevitably creates ground loops (there would be two ground connections over the two USB cables), which really should be a no-go from an electronic point of view (BTW, there would also be a loop in the VCC line). I guess this is the reason for many systems becoming unstable.

3) One could fix 2) by interrupting the GND and VCC lines in the USB data cable or the hub (but leave them in the USB power cable). This would avoid the ground loops, but what happens if one unplugs the power but not the data cable? I guess this would break the input circuitry of the D+/D- lines of the Pi's USB port. It further requires messing around with the hub.

What would work better is to power the PI and communicate over *one single* USB port, but it seems this was not foreseen with the low-current polyfuse installed on the regular USB port.

Overall, this leaves me puzzled and I wonder what power configuration the designers of the Pi had in mind.

Any advice is appreciated

User avatar
RaTTuS
Posts: 10415
Joined: Tue Nov 29, 2011 11:12 am
Location: North West UK

Re: Power and ground loops

Wed Aug 15, 2012 8:43 am

power the RPi from the USB hub to the micro USB port
on the port from the RPi to the HUB for data http://www.raspberrypi.org/phpBB3/searc ... e+red+wire
this gets rid of any +ve loop and stops the board being 'back powered'
and means you only have one power plug

you need the ground on the USB to make it work [AIUI]

if all the ground is grounded then there is no issues ? ground is ground is ground [I'm not an electrician]
How To ask Questions :- http://www.catb.org/esr/faqs/smart-questions.html
WARNING - some parts of this post may be erroneous YMMV

1QC43qbL5FySu2Pi51vGqKqxy3UiJgukSX
Covfefe

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

Re: Power and ground loops

Wed Aug 15, 2012 9:11 am

A ground loop only becomes a problem is there is significant resistance in both of the USB cables.

Using at least one good-quality cable should eliminate this problem.

User avatar
Burngate
Posts: 5972
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: Power and ground loops

Wed Aug 15, 2012 10:02 am

bredman wrote:A ground loop only becomes a problem is there is significant resistance in both of the USB cables.

Using at least one good-quality cable should eliminate this problem.
Or sufficient magnetic hum-fields

User avatar
jackokring
Posts: 816
Joined: Tue Jul 31, 2012 8:27 am
Location: London, UK
Contact: ICQ

Re: Power and ground loops

Wed Aug 15, 2012 12:32 pm

It depends how the hub is made, for some 2 would be enough, and for some 3 might work, but it all depends on opto isolation. Like MIDI has.

Bad problems of ground/power loops are
Hum, high current welding transformer effect, lower noise margins and so eventual lower frequency of operation and a higher voltage of operation for the same noise clearance of digital logic gates, and a few more related to EMP loop area.

Good problems of ground/power loops are
Radio, high current welding transformer effect, ...
Pi[NFA]=B256R0USB CL4SD8GB Raspbian Stock.
Pi[Work]=A+256 CL4SD8GB Raspbian Stock.
My favourite constant 1.65056745028

User avatar
rurwin
Forum Moderator
Forum Moderator
Posts: 4258
Joined: Mon Jan 09, 2012 3:16 pm
Contact: Website

Re: Power and ground loops

Wed Aug 15, 2012 1:08 pm

It's a black art, but I would not like to mix the power lines with the data lines since that is just as likely to add noise. So you don't want the data ground to be the same wire as the power ground.

I'd put a ferrite om each of them and then tie them together and even twist them.

isketerol
Posts: 6
Joined: Tue Aug 14, 2012 4:33 pm

Re: Power and ground loops

Thu Aug 16, 2012 1:18 pm

Cutting the red wire ended up with a dead cable - tho I must admit I did not check the cable beforehand to ensure it was good. :-/

Is there a simple way to determine if the RPI is getting backpowered?

User avatar
RaTTuS
Posts: 10415
Joined: Tue Nov 29, 2011 11:12 am
Location: North West UK

Re: Power and ground loops

Thu Aug 16, 2012 1:26 pm

if you have both the micro USB and normal USB plugged in [powered up]
and you remove the micro USB power
and the RPi stays up or the lights stay on then it is being back powered
How To ask Questions :- http://www.catb.org/esr/faqs/smart-questions.html
WARNING - some parts of this post may be erroneous YMMV

1QC43qbL5FySu2Pi51vGqKqxy3UiJgukSX
Covfefe

isketerol
Posts: 6
Joined: Tue Aug 14, 2012 4:33 pm

Re: Power and ground loops

Thu Aug 16, 2012 7:55 pm

That makes perfect sense. Thanks!

isketerol
Posts: 6
Joined: Tue Aug 14, 2012 4:33 pm

Re: Power and ground loops

Thu Aug 16, 2012 10:08 pm

Not even a glimmer. Hub is a Belkin F5U307 - working great for me.

RaspPP
Posts: 2
Joined: Wed Aug 15, 2012 6:19 am

Re: Power and ground loops

Tue Aug 21, 2012 11:54 am

I disagree with the statements that ground loops are not important. Actually, the ground loop formed by the USB cables are large (cross sectional area covered by the loop), and may capture all kind of noise. The same by the way applies for "Vcc loops". This are very basic electronic design considerations one usually follows at a much smaller scale (e.g. centimeters). With long cables, this may very well turn into a problem. Even if only occasionally, it may crash the system. USB uses a differential signals (V+/V-) so that the signal / power ground issue may be less problematic. I'll probably try to remove both the GND/VCC connections in the USB communication cable.

User avatar
rurwin
Forum Moderator
Forum Moderator
Posts: 4258
Joined: Mon Jan 09, 2012 3:16 pm
Contact: Website

Re: Power and ground loops

Tue Aug 21, 2012 12:20 pm

RaspPP wrote: Actually, the ground loop formed by the USB cables are large (cross sectional area covered by the loop), and may capture all kind of noise.
Not if you tie, or better twist, the cables together.

Colly
Posts: 17
Joined: Mon Jul 16, 2012 4:27 am

Re: Power and ground loops

Thu Aug 30, 2012 12:09 am

I want to power my Pi with one of these: http://www.tigerdirect.com/sectors/camp ... gn=WEM3063
but it seems they've been on back-order longer than the Raspberry Pi was. ;)

Return to “Beginners”