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Power and ground loops

Posted: Wed Aug 15, 2012 7:02 am
by RaspPP
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

Re: Power and ground loops

Posted: Wed Aug 15, 2012 8:43 am
by RaTTuS
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]

Re: Power and ground loops

Posted: Wed Aug 15, 2012 9:11 am
by bredman
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.

Re: Power and ground loops

Posted: Wed Aug 15, 2012 10:02 am
by Burngate
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

Re: Power and ground loops

Posted: Wed Aug 15, 2012 12:32 pm
by jackokring
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, ...

Re: Power and ground loops

Posted: Wed Aug 15, 2012 1:08 pm
by rurwin
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.

Re: Power and ground loops

Posted: Thu Aug 16, 2012 1:18 pm
by isketerol
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?

Re: Power and ground loops

Posted: Thu Aug 16, 2012 1:26 pm
by RaTTuS
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

Re: Power and ground loops

Posted: Thu Aug 16, 2012 7:55 pm
by isketerol
That makes perfect sense. Thanks!

Re: Power and ground loops

Posted: Thu Aug 16, 2012 10:08 pm
by isketerol
Not even a glimmer. Hub is a Belkin F5U307 - working great for me.

Re: Power and ground loops

Posted: Tue Aug 21, 2012 11:54 am
by RaspPP
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.

Re: Power and ground loops

Posted: Tue Aug 21, 2012 12:20 pm
by rurwin
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.

Re: Power and ground loops

Posted: Thu Aug 30, 2012 12:09 am
by Colly
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. ;)