Oscilloscope measurements on grounded USB device - Page 1
Forum section: electronics / beginners

author: NickEdison
author_rank: Contributor
timestamp: December 26, 2021, 12:03:43 pm
content: Dear forum users,If this topic has already been discussed, my apologies, but I cannot find it.I would like to perform a measurement referenced to an arbitrary point in a Device Under Test (DUT) which is connected to earth via the USB port of my PC, such that I can debug it in real time. Furthermore it is connected to an Isolated PSU supping the power electronics <65 DC. I am aware that if the reference point would have the same potential as ground, that I can connect the ground clips to that point to perform the measurement.However, I would like to connect my reference somewhere with a different potential (that is continuously changing) as ground and would like to use all 4 channels on my oscilloscope. A solution would be to buy 4 differential probes, but that is quite expensive.Would it in this case be safe to connect the scope via a power lead that has no earth terminal? I am aware that this is generally considered a no go because the BNC connector is also at this potential. But I am not sure why it would be in this case, if so please explain why?

author: madires
author_rank: Super Contributor
timestamp: December 26, 2021, 01:04:22 pm
content: Have you considered getting an USB isolator to break the ground connection via the PC? Modern scopes have usually an SMPSU with filter caps connected between live and earth, and neutral and earth. Without mains earth they would create a capacitive voltage divider and the scope's ground would be around half the mains voltage. They pass only a tiny current of several µA, but it's enough for a tingle. And it can also screw up your measurements.

author: NickEdison
author_rank: Contributor
timestamp: December 26, 2021, 01:32:53 pm
content: No I did not, but its definitely a good solution. I will get one ASAP, thanks a lot!The information about the caps is also a good remark. I don't think the small current would be a problem for my current measurement though.I would like to perform the measurement today though if possible, but If I understand correctly, it can be done safely although it's not preferred?

author: madires
author_rank: Super Contributor
timestamp: December 26, 2021, 01:59:11 pm
content: It's not the recommended way. BTW, you can set up two channels as a quick n' dirty differential probe. Remove the ground clips from the probes, invert one channel and add the other one (both channels must have the same settings).

author: NickEdison
author_rank: Contributor
timestamp: December 26, 2021, 11:06:43 pm
content: That's also a nice tip, however it wouldn't have solved my problem because I had to use all 4 channels.As an alternative I used my laptop, I realized it has floating USB ports.Thanks again though for the information, I am sure it will be useful in the future!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 27, 2021, 11:10:22 pm
content: I like to keep it simple.First I have made sure that my PC is properly grounded.Without a proper ground there is half the mains voltage (although at low current) on the box of the PC because of thy Y capacitor in the filter in the power supply. This is plenty voltage to kill most electronics.Second precaution is that I always use an USB hub in between my PC and my electronics tinkering. If it blows, I open a drawer and get a spare one. These things cost almost nothing.Third precaution:Also make sure your oscilloscope is properly grounded. I also like to power both the PC and the oscilloscope from the same extension cord, so there are no weird things going on with ghost voltages.This should be plenty for most electronics tinkering. Next upgrade would be an ESD mat.I do have an USB isolator, but I have not used it yet. It's probably in some drawer somewhere... I also have an Isolation transformer, but I only use it when I need it, and that is not very often.