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Can someone explain clearly what is ground loop and when we should worry about it?

Shall I worry about it when I connect two USB wires in parallel to deliver power to my 3G modem?

I have something like this:

enter image description here

I was asking about this earlier, here is my question:

Raspberry Pi and USB modem - can I connect it like this?

I did it, it works, but in comments under another discussion (here) one guy (ppumkin) pointed, that this will be ground loop. I know what ground loop is, I know how that can ruin audio quality in hi-fi system, but... can ground loop like this affect 3G modem or WiFi sensitivity or cause other issues?

I have ground loop with 2 USB wires 1m (~3ft) length, wires are very close...

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  • \$\begingroup\$ I think the problem might be in some current coming from the adapter (leftmost red wire) but returning through the ground of the usbs sockets. I don't think this should happen or be a problem at all, but I'm not sure... \$\endgroup\$ – Vladimir Cravero Aug 4 '14 at 23:12
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Small difference in ground potential not really a big issue in 'digital' electronic.

For audio, it the 'ground loop' produce 10mV noise, it will be treated as analogue signal and can be heard and reduce SN ratio. Same for precision/small signal instrumentation.

Digital signal is treated as hi or lo, once passed a threshold value and hence tens of mV difference, as above, would have no effect.

One specific about cellular, it use TDM and have highly pulsating current drawn profile. Jump from 2A peak then down to says, 50 to 100mA. Use thick positive and negative wire for the phone module. Check if your USB style power supply work well under this. If not, use bigger power supply to be safe. Always use thicker wire. Some USB cable inside wire is really thin and one cannot tell easily without cutting it. My portable hard disk has issue (as it draws 2.5 watts) with some USB cable from shop. Original 'good cable' come with the disk are fine.

If I do this, I feel better with bigger size power supply and self wire cable (screw terminal, etc). More peace of mind as I see what cable I use. (although your diagram may work well, if the cable is good quality, etc.)

Hope it helps

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