Electronics > Beginners

Proper Scope Probing

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tggzzz:

--- Quote from: t1d on November 01, 2018, 02:58:09 pm ---Thanks, tggzzz, for your reply!

--- Quote from: tggzzz on November 01, 2018, 09:19:31 am ---Your first test should be with a handheld meter, to check sanity before connecting expensive equipment.
--- End quote ---
Yes, I did that, as step #1. No problems.


--- Quote from: tggzzz on November 01, 2018, 09:19:31 am ---One critical point is how you derive the 12Vac. If from an isolated transformer, it is OK. If not then you have to understand the internals of whatever is supplying the 12Vac.
--- End quote ---
The PSU is a 12vac, two-pin, wall wart, in the USA. So, its floating.


--- Quote from: tggzzz on November 01, 2018, 09:19:31 am ---EDIT: Another critical point is that your GND line is not well defined. Its voltage will be set by the ratio of capacitances and loads, all of which are poorly controlled. Silly me. Too little coffee. I missed the implicit wire between one "gnd" and another. But the point in the next message is correct: do you mean 0V, chassis, protective mains earth, or ground? In a power supply it would be more normal to refer to it as 0V, and then to explicitly connect it to chassis/PME/ground as appropriate.
--- End quote ---
Let me feed this back to you, to make sure that I have it correctly. On the schematic, I have the center of the positive and negative supplies named as ground and I use the circuit ground symbol. This is not tied to the chassis, nor earth. Are you saying that it would be more clear to name this as "0v?"

The only place that I used the term "ground," in my text description was for the scope probe return. So, in relation to the terminology discussion, above, is this what needs to be renamed? I think you mean the former... Actually, both.

--- End quote ---

Scope probes' screen are, with important rare exceptions, tied directly to the protective mains earth. What that means relative to any groundsoil in your vicinity will depend on the electrical installation!

"0V" is what you are using as the reference voltage for your circuit; usually that's the midway between positive and negative supplies. You have marked that as "GND" = ground, which it probably isn't.

For an extreme example, consider a high precision voltmeter. Often they have two "0V". The "earthy 0V" is used for the display, switches, logic and anything a user might touch. This is similar to that found in most equipment. The other "floating 0V" is completely floating w.r.t. earth and ground, and is used for the analogue front end. This allows, for example, the +ve input to be at 100V relative to earth and the -ve input at 110V relative to earth, but the voltmeter input only "sees" the -10V differential because the "floating 0V" is pulled to 105V relative to earth. Communication between the two parts is via optocouplers or similar.

And then there are the important cases where your equipment's "0V" is not at the same potential relative to earth as another person's equipment's "0V". Connect the two and currents can flow where you don't want them.

And in industrial applications you never presume that "your ground" is the same as "someone else's ground"; consider operating from different phases etc etc.

Ground potential is a convenient fiction. Consider a lightning strike to the ground. It can be that there is a large potential difference between the ground under your left and right foot - in which case current can flow through your legs :(

Having said that, people often refer to the local reference 0V as "ground", which isn't correct.

As for wall-warts, who knows what's inside them, especially with very low cost warts where corners have been cut. Hence my specification for a transformer.


--- Quote ---
--- Quote from: tggzzz on November 01, 2018, 09:19:31 am ---Your should read learn and inwardly digest the safety references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
--- End quote ---
I will give it a look. I have watched Dave's video, on how to not blow up your scope. That's how I knew I should verify how to do this test.

--- End quote ---

t1d:

--- Quote from: tggzzz on November 01, 2018, 03:37:57 pm ---As for wall-warts, who knows what's inside them, especially with very low cost warts where corners have been cut. Hence my specification for a transformer.
--- End quote ---
Yes, I could tell that you were being specific, about it being an isolation transformer. But, as my wall wart does not have an earth pin, I doubt that there is any cross over...

MiDi:
To summarize tggzzz:

Earth is connected via power socket to central earth point of building, the neutral at some point is connected to earth.

GND in general is just a reference point for the circuit and could sit at any voltage referred to earth.

tggzzz:

--- Quote from: MiDi on November 01, 2018, 07:26:21 pm ---GND in general is just a reference point for the circuit and could sit at any voltage referred to earth.

--- End quote ---

Not necessarily. That reference is 0V.

Overall the nomenclature is complex and contradictory, with terms varying across countries.

Sometimes GND is ground. Sometimes the reference voltage is the earth. Sometimes there are more than one reference voltages in a device.

In simple self-contained equipment it is often possible to incorrectly elide 0V, earth and ground. In larger and more complex equipment, such elision can lead to subtle and not so subtle behaviour.

Fundamentally there are electric fields, magnetic fields, electrostatic potentials and currents. Everything else is a simplification that is valid in some circumstances and invalid in others. As an example, consider a digital signal going from an output to an input in a wire on top of a ground[1] plane. The return current will travel in the ground plane, but not everywhere in the ground plane and certainly not the shortest route from receiver to transmitter. It will actually be concentrated underneath the signal wire, no matter how much that wire meanders across the PCB.

[1] both Vcc and 0V planes are "ground planes" in this context - and they act as a very effective distributed capacitor :)

MiDi:
I do not see any disagreement in your post, but want to add one point:


--- Quote from: tggzzz on November 01, 2018, 10:11:06 pm ---It will actually be concentrated underneath the signal wire, no matter how much that wire meanders across the PCB.

--- End quote ---
Mainly true for transition of voltage levels aka high frequency AC (RF), but in steady state aka DC the current will spread over the plane.
At frequencies with wavelength in the order of trace-length or plane-dimensions you enter a different world, but that is way off topic...

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