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Oscilloscope probing mains AC DUT questions

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mart1n:
Hello Folks,

I've been wanting to learn more about AC circuits, antique electronics and power supplies. I have a few switch mode power supplies that I've started to study/analyze that are mains earth referenced that I would like to probe with my scope.
I just finished watching the EEVBlog episode on how not to blow up your scope (Thanks Dave!) and have a quick question that I want to verify to maintain my safety.

First off, I don't yet have a differential probe or an isolation transformer.

Question #1:
If I'm extra careful to only attach my ground lead clip to a mains earth referenced rail, I should be safe to then probe the circuit, is that correct? Is there anything else I need to be careful of?

Thanks!

james_s:
No no NO, just don't.

At the bare minimum an isolation transformer is *mandatory*, this is not optional. A differential probe is highly encouraged but don't even think about poking around inside a live mains powered switchmode PSU without isolation. Really you should avoid live testing at all unless absolutely necessary.

You can buy small isolation transformers for under 20 bucks from arcade machine parts suppliers as most of the older CRT monitors required them. Seriously, this is for your own safety and that of your equipment, don't try to take shortcuts, these things are lethal. A switchmode PSU is much more dangerous than the high voltage on a CRT, the 340V rail can pack a hell of a punch.

nigelwright7557:
SMPS often have a bridge rectifier on the front end which means trying to probe anything after that will be just garbage on the scope screen.
You need an isolation transformer..
Then you can connect scope ground to bridge -ve end out and probe circuit and get good results.

You really do have to play safe even with an isolation transformer.
Always turn off and wait for caps to discharge before touching the circuit or attaching/un-attaching scope probes.

On high voltage circuits I always added an LED and high value resistor across main cap to show when it is still charged.
Not fool proof but better than nothing.

tggzzz:
Firstly, an isolation transformer doesn't make the setup safe. It can also interact badly with GFCIs.

Secondly, if you have really understood the failure mechanisms, use an isolation transformer on the DUT. Never ever float the scope.

Thirdly, try and find another way of achieving your ends.

Finally, read and understand the safety references and probe types at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

mart1n:
Thanks for all the tips, warnings and information!
I still feel like my question wasn't answered though...
Assuming I never actually connect my scope probe(s) to an AC mains earth referenced DUT without proper safety equipment (isolation transformer, differential probe), I would
still like to know the answer to my questions for the purpose of technical understanding.

Scenario #1:
With the DUT powered off and any component able to store energy discharged...
I use my DMM to test resistance between test point A and the earth pin on the DUT's power cable to verify that test point A
is indeed a direct connection to mains earth. With the DUT still powered off, I connect my scope probe's ground lead to test point A.
I connect the scope probe to some other point in the circuit and then plug the DUT's power cable directly into a wall outlet.

Assuming mistakes were not made during the setup of Scenario #1 (I understand this is the point of the safety gear as mistakes do happen).
We're also assuming that we power down and discharge the DUT if/when we move the scope probe to different test points, while maintaining
test point A as our ground lead location.

Scenario #1 Questions:

* Is this "safe"?
* Are all probe points "safe"? (Assuming we're not moving the ground lead)
* If some probe points are not "safe", what would those be and why?
I think the answer to these questions are that this is OK. I couldn't think of any reasons why it wouldn't and that's why I'm asking.
Not to take short-cuts with safety gear, but because I want to solidify my understanding.

FYI - I've purchased a bench isolation transformer from Jameco since first posting this question.

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