Electronics > Beginners

Proper Scope Probing

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t1d:
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?

tggzzz:
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 in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

IanMacdonald:
I think it's better to use the term chassis for a return connection, because 'ground' is ambiguous. It may mean chassis, or it may mean supply earth.

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.

t1d:
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.


--- 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.

t1d:
Thanks, Ian, for your reply!

--- Quote from: IanMacdonald on November 01, 2018, 09:25:04 am ---I think it's better to use the term chassis for a return connection, because 'ground' is ambiguous. It may mean chassis, or it may mean supply earth.
--- End quote ---
See Above.


--- Quote from: IanMacdonald on November 01, 2018, 09:25:04 am ---If the center rail is connected to supply earth, then connecting a probe outer to -9v will cause a short.
--- End quote ---
It 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 am ---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.

--- End quote ---
The 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.

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