Electronics > Beginners

Floating the DUT

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james_s:
What exactly are you trying to do?

I use a differential probe occasionally when repairing a tricky switchmode power supply or CRT monitors, other than that I've only ever used my scope on low voltage stuff. Outside of a few niche areas you shouldn't really need to be looking at stuff where this is even an issue, the vast majority of electronic circuits operate on low voltage DC.

fcb:
It's certainly classic to 'lift' the ground (disconnect) of a scope in the good old days. It was also frowned upon by management, but widely ignored.  When PAT testing became a thing, you would reconnect the ground, wait for the PAT test to be done and then disconnect again. Times have moved on and scopes have switching supplies in them that leak to ground so wouldn't do it; and we have diff. probes, low cost battery scopes etc.

tggzzz:

--- Quote from: fcb on January 28, 2020, 04:45:45 pm ---It's certainly classic to 'lift' the ground (disconnect) of a scope in the good old days. It was also frowned upon by management, but widely ignored.  When PAT testing became a thing, you would reconnect the ground, wait for the PAT test to be done and then disconnect again. Times have moved on and scopes have switching supplies in them that leak to ground so wouldn't do it; and we have diff. probes, low cost battery scopes etc.

--- End quote ---

It may be classic, but newbies should be aware of the info in https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 especially the "example fatality".

magic:
Nope.

If the DUT has no power transformer or you ground anything before the transformer (like an SMPS regulator), :-BROKE is guaranteed regardless of whether the device is connected with a 2 or 3 prong plug. You may disconnect safety earth and float the chassis of the device, but the mains voltage which goes into it doesn't float, it is what it is.

fcb:

--- Quote from: tggzzz on January 28, 2020, 05:16:21 pm ---
--- Quote from: fcb on January 28, 2020, 04:45:45 pm ---It's certainly classic to 'lift' the ground (disconnect) of a scope in the good old days. It was also frowned upon by management, but widely ignored.  When PAT testing became a thing, you would reconnect the ground, wait for the PAT test to be done and then disconnect again. Times have moved on and scopes have switching supplies in them that leak to ground so wouldn't do it; and we have diff. probes, low cost battery scopes etc.

--- End quote ---

It may be classic, but newbies should be aware of the info in https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 especially the "example fatality".

--- End quote ---
I should also have said DON'T do it. Ensure your equipment is grounded correctly - including 'scopes.

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