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HV differential probe versus isolation transformer
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scoper007:
Looks like Micsig has new series HV diff probe! (Chinese web) :popcorn: https://www.micsig.com.cn/MDP/
wasedadoc:

--- Quote from: james_s on June 25, 2023, 07:02:07 pm ---
--- Quote from: nctnico on June 22, 2023, 09:35:13 pm ---
--- Quote from: schmitt trigger on June 22, 2023, 05:48:33 pm ---Can you post a link to the Micsig video?
I have used diff probes extensively, and as long as you know the limitations, they work fine. Specially for 50/60 Hz signals.

But to your question, I always use an isolation transformer when working with power line circuits. When you are developing a circuit, sometimes I connect other equipment in addition to the scope or DMM. Thus better safe than sorry.

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That is how you create a death trap because the GFI will not kick in when you touch the live wires. This has been discussed to death already. A differential probe +GFI is the way to go. If you are worried about excessive currents then put a light bulb in series but even then there are much better ways. When I repair a switching mains power supply, I don't connect it to mains at all before I have tested the power converter stage is actually working. I use a bench PSU to apply power (typically around 12V) to the switcher controller and to the DC bus (several tens of volts). That gives enough information on whether the circuit is OK or not.

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That's assuming you have a GFCI. I know they're used for the whole house in some areas, but in North America they're typically only used for wet locations. Nothing stopping someone from installing one in their lab but I would never assume it to be there.

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Don't know about elsewhere but in the UK plug-in RCDs (our name for GFI) are readily available for about £10.  Just interpose between wall socket and plug. https://www.toolstation.com/rcd-adaptor/p12514
schmitt trigger:

--- Quote from: Luxor92 on June 22, 2023, 08:42:00 pm ---
--- Quote from: schmitt trigger on June 22, 2023, 05:48:33 pm ---Can you post a link to the Micsig video?

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Sure, here you are.


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As a friend of mine used to say: there is never a problem with spending money, when you have enough of it.
The video shows that the optical coupled probe has superior performance characteristics, but also with a superior price tag.
I routinely use the exact same model Tektronix diff probe as in the video, and can tell you that as you are aware of its limitations, it is an excellent probe.

One of the limitations is common mode rejection. For some reason, when I probe an off-line SMPS, I have found that employing a good isolation transformer increases the CM rejection. I use a medical grade 171 series transformer from Hammond, which has an electrostatic shield between primary and secondary.

And I am perfectly aware of the electrical risks.
AVGresponding:
When I used to work on UPS's, especially bigger ones, isolation transformers were never a possibility. We used Fluke Scopemeters (I now prefer Tek THS700 series), and a great deal of care.
PwrElectronics:
At my employer, we've been working on motor drive inverters and DC-DC converters running off 700VDC for a number of years.

We use differential probes for nearly all HV measurements and for looking at gate signals, etc.  Scopes are grounded to earth via the normal AC power plugs.

In the early days of working on the 700V stuff, we didn't have much in the way for diff probes and I recall trying to get a look at the high side gate signals using a TEK THS720A scope.  With isolated inputs, it was not rated for that sort of voltage but since it is battery powered, I figured just run off battery with only that 1 probe connected and use a wood stick to press the control buttons, etc.  Basically, the scope was floated.

For reasons unclear, as the DC voltage increased, the gate waveform I was looking at became more and more distorted.  Later was able go get a diff probe and that issue was not present.

Nearly 30yrs ago I worked at a plant that made power supplies (mostly off-line types).  The line repair techs (repaired test failures) used to chop off the earth pin on their AC powered bench scopes and float the scope to whatever the internal DC link voltage on the power supply was when troubleshooting things.  That could often be 200V.  They got away with it but for sure was not a safe practice.  That place at that time didn't really have any sort of safely rules or training....
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