I am sorry if this is a bit of a novice question but I am new to electronics and have come to a problem that I can't quite see an obvious answer for.
A while ago I had a bit of a scary encounter when connecting an Arduino powered machine to my pc. The device worked fine with a PC power supply but once I swopped it for an open frame type one I noticed some serious arcing when I brought the USB cable near my pc's port. After some research I determined that this was caused by the fact that the new power supply was not connected to earth internally and once I made the necessary modifications my problem was fixed.
What I am really stumped about is how consumer electronics product manage this floating voltage problem when interfacing with attached devices. What I am wondering is how for example a laptop running from its battery without any possible earth reference can connect to a USB printer without any problems. Is it that the internal regulator in the laptop immediately assumes the USB ground as its ground reference and if so, how would that prevent an initial arc?
Best Answer
I think the problem you experienced was actually because there was a ground connection at both ends. What you saw was the end results of a ground loop.
When you have two systems each with their own power supply, both connected to ground through different routes, the resistance of that ground path can differ for each device. That means that in fact the electrical potential at ground can be different between the two units.
Connect those two different voltages together, with what basically amounts to a short circuit, and you get a spark.
When you have something that has no ground connection there is no voltage difference, since only one side has a potential to real ground. In the laptop the earth connection does not form part of its circuit in any form whatsoever.
Incidentally, the USB specifications actually state that the shield of a USB cable should only be connected to ground at the host end, not the device end - unless that device is purely bus powered in which case the shield forms part of the ground return for the power.