Proper Scope Probing - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: November 01, 2018, 09:02:12 am
content: I am newer to oscilloscopes. I am aware that some caution is needed, when probing power supplies. So, I want to make certain that there is no problem looking at the output, of this power supply.I propose to connect the ground, of the probe, to the negative output, of the 7909, and the tip, of the probe, to the positive output, of the 7809. Will anything go boom?

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2018, 09:19:31 am
content: Your first test should be with a handheld meter, to check sanity before connecting expensive equipment.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.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.Your should read learn and inwardly digest the safety references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: November 01, 2018, 09:25:04 am
content: I think it's better to use the term chassis for a return connection, because 'ground' is ambiguous. It may meanchassis, or it may mean supplyearth.If the center rail is connected to supply earth, then connecting a probe outer to -9v will cause a short.Other point, do not route AC connections via the chassis (as this diagram suggests may be the case) because that will induce 50Hz hum into everything. AC2 should be hardwired to the junction of C10/C11.

author: t1d
author_rank: Super Contributor
timestamp: November 01, 2018, 02:58:09 pm
content: Thanks, tggzzz, for your reply!Quote from: tggzzz on November 01, 2018, 09:19:31 amYes, I did that, as step #1. No problems.Quote from: tggzzz on November 01, 2018, 09:19:31 amThe PSU is a 12vac, two-pin, wall wart, in the USA. So, its floating.Quote from: tggzzz on November 01, 2018, 09:19:31 amLet 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.Quote from: tggzzz on November 01, 2018, 09:19:31 amI 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.

author: t1d
author_rank: Super Contributor
timestamp: November 01, 2018, 03:11:28 pm
content: Thanks, Ian, for your reply!Quote from: IanMacdonald on November 01, 2018, 09:25:04 amSee Above.Quote from: IanMacdonald on November 01, 2018, 09:25:04 amIt is not, so I should be safe probing the full span of the positive and negative rails, simultaneously; correct? Probe pin on positive and probe outer/return (alligator clip) on negative.Quote from: IanMacdonald on November 01, 2018, 09:25:04 amThe PSU is on the same PCB as the circuit. But, it is isolated. There are only small ties between the PSU 0v plane and the circuit 0v plane. Additionally, there are some individual traces (isolated, by their margins), as needed, between the two sections.Thank you, both, for your excellent help.

author: t1d
author_rank: Super Contributor
timestamp: November 01, 2018, 03:19:08 pm
content: Thanks, Ice Tea,That video is how I knew I should double check what I am doing.P.S. I grew up on Lipton, but, as an adult, discovered that I like Luzianne, much better. ;-)

author: MiDi
author_rank: Frequent Contributor
timestamp: November 01, 2018, 03:22:47 pm
content: Quote from: t1d on November 01, 2018, 02:58:09 pmGND in schematics is fine if not connected to earth.Input of Oscilloscopes are earth referred, the GND of probe is tight to earth of wallsocket.

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 01, 2018, 03:26:14 pm
content: You already know enough to be concerned. You're at least halfway there

author: t1d
author_rank: Super Contributor
timestamp: November 01, 2018, 03:34:18 pm
content: Thanks, MiDi... If I had thought about the wall wart being a float, I would have worried less... That, probably, would have led to me risking the probe, without verifying what I was doing. So, better to ask and to learn...

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2018, 03:37:57 pm
content: Quote from: t1d on November 01, 2018, 02:58:09 pmScope 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 havetwo"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 legsHaving 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

author: t1d
author_rank: Super Contributor
timestamp: November 01, 2018, 03:53:19 pm
content: Quote from: tggzzz on November 01, 2018, 03:37:57 pmYes, 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...

author: MiDi
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:26:21 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2018, 10:11:06 pm
content: Quote from: MiDi on November 01, 2018, 07:26:21 pmNot 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 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. both Vcc and 0V planes are "ground planes" in this context - and they act as a very effective distributed capacitor

author: MiDi
author_rank: Frequent Contributor
timestamp: November 01, 2018, 11:10:47 pm
content: 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 pmMainly 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...

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2018, 11:40:31 pm
content: Quote from: MiDi on November 01, 2018, 11:10:47 pmAgreed.I included it solely as an extreme example of why it is unwise to elide the concepts of 0V, earth, ground. There are many other examples, of course.