[chbot] Earth issues, noise currents, and hair-loss

[Mark Atherton]

Safety comes first, so ground things.

Component protection is important during manufacture, so ground things.

Operation, may be an issue with lots of different grounds, and may rear 
it's ugly head in the form of ground loops, hum etc.

====

In my experience, ground loops are usually caused by earth leakage current.

In a typical building, each power outlet (or maybe a small cluster) has 
it's own earth connection back to a central point (fuse/breaker box).

So, lets connect different loads, each with different ground leakage 
current to each socket.

... and as if by magic, we now can measure (hopefully small) noise 
potentials between sockets in the same room.

Into socket A, let us plug a low level audio signal generator, with 
chassis grounded. Also into this socket, lets plug a laptop.

Into socket B, let us plug a very sensitive oscilloscope, with chassis 
grounded. Also into this socket, lets plug an LED lamp.

Assume that socket A & B have separate ground wires back to the fuse box.

And lets connect the audio between the sig gen and the 'scope.

... and as if by magic, hum, and all manner of other rubbish appears on 
the scope along with the expected wanted signal.

As well as the equipment being connected by the audio cable earth, they 
also share a separate ground, each with their own noise sources.

Noise leakage current from the laptop PSU is making socket A ground 
noisy. Noise leakage current from the laptop LED lamp making socket B 
ground noisy,

For audio gear, one easy fix is to elect a single power socket, and run 
a multi-box from that socket, and only use it for audio.

Hopefully you get the idea.

This is quite a broad subject, and one that can be discussed for hours 
on end.

Have spent many years chasing ground current issues, in many types, 
sizes, and forms and the whole subject is still fascinating.

Try laying out a PCB with digital stuff at one end, RF in the middle 
(next to a switched mode PSU), and analogue stuff, with >100dB dynamic 
range at the other end. It can all be done, but please leave a few 
days/weeks to allow circulating currents be understood by the poor-old 
product designer.

-Mark





On 22/11/2016 2:45 PM, Marshland Engineering wrote:
> Ok not Kaiakora type but electrical ones.
>
>
> I have a digitally controlled soldering iron, PSU and Scope. For the past XX
> years, I never connected the scope earth to the plug earth thinking that the
> probe ground would therefore float with reference to my power supply earth.
>
> Any way, when soldering components on my test PCB, a few LED's started to
> glow. I measured the voltages on the test equipment and I had 90 volts between
> my soldering iron and scope probe earth. A few more measurements and I found
> scope probe earth was the problem.
>
> What is normal? Earth the scope, disconnect all cables to the board before
> soldering, run an isolating transformer?
>
> Often USB powered boards get supply from the PC and some laptops only have 2
> pin plugs so all sorts of earth issues can occur?
>
> Any suggestions ?
>
> Just ordered a battery pocket scope from Aliexpress. That will solve one
> issue.
>
> Cheers Wallace.
>
>   
>
>
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