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Measure high Voltage 500V DC and Current in a floating system

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David Hess:

--- Quote from: mjoelner on January 27, 2020, 01:35:59 pm ---Using a differential probe actually worked pretty good, but I thought it was a bit to noisy at 50x. One solution then would be to up the resistance a bit, but I cant steal to much of the voltage either. So, I thought about using the a-b differential method using two probes connected together and then measure across the resistor. A calculation in Picoscpoe would then give me the DC voltage across the resistor, and I was hoping to see the same as the handheld oscilloscope. Unfortunately that did not happen. When I connected the probes across the 4 ohm resistor, they both gave me a channel overrange alarm and the numbers made no sense whatsoever. The laptop and the picoscope is not grounded or connected to power anywhere.

Any idea what is causing this, and better, is this what is expected or can I adjust my experiment to give me better values.
--- End quote ---

You exceeded the common mode input voltage of your hacked together differential probe.  The difference is the measurement that you want however each probe and input channel sees the total voltage before subtraction.

The high voltage differential probes have enough attenuation on each input to bring any common mode voltage into range and it is this attenuation which also result in a noisy differential signal at high gains.

floobydust:
Am I right thinking a differential probe can be useless and even dangerous if it has no ground reference? Then there is no common-mode attenuation possible by the input voltage divider, as seen in this commercial HV differential probe schematic I drew, with the CMRR trimpot center-point floating. Differentially, yes there is attenuation. But common-mode the TVS will just clamp and the return path is to the Picoscope/laptop.
I keep thinking the confusion is these DP's have isolation or work when completely floating. They have a need for a CM return path to give them CMRR.

I would try again with OP's setup but earth-ground the Picoscope. Using (the) three-prong laptop power adapter could suffice, but not a two-prong one.
We don't know much about the noise, CM voltage, the load etc. in the "floating system".

David Hess:

--- Quote from: floobydust on January 28, 2020, 03:19:15 am ---Am I right thinking a differential probe can be useless and even dangerous if it has no ground reference? Then there is no common-mode attenuation possible by the input voltage divider, as seen in this commercial HV differential probe schematic I drew, with the CMRR trimpot center-point floating. Differentially, yes there is attenuation. But common-mode the TVS will just clamp and the return path is to the Picoscope/laptop.
--- End quote ---

That is correct; a high voltage differential probe cannot be safely used in an isolated setup.  If it is, then the isolated side will charge up to the common mode voltage.  Older high voltage differential probes included an explicit ground (really common) connection as part of their high voltage differential input.  On newer probes without this, the ground (common) of the output signal to the oscilloscope is the same thing.

Tektronix includes a warning with their differential probes and oscilloscopes with isolated inputs about using them together.

mjoelner:
So, it seems its not that safe to measure the voltage and amps in this setup the way I intended.

Is there any way this could be done safely then, where I and my equipment can safely esacpe without to much harm. Just so it is said, the equipment has very sensitive GFI and trips when its sensing 10mA. So far in my tests it has not tripped at least. I power down and up the gear every time I mess with the cabling.


For a bit more clarity I have attached a file where I draw up both two experiments I have tried this far.
This is the differential probes I have used:
https://www.amazon.com/Micsig-DP10013-Differential-Attenuation-Tektronix/dp/B074K4XPW3
On here they claim: Micsig DP10013 High Voltage Differential Probe is designed for professionals and experts like you to be used with most oscilloscopes in the market.
It allows you to safely measure floating voltages up to 1,300V and has a bandwidth up to 100 MHz.

floobydust:
DP10013 "allows conventional earth-grounded oscilloscopes to be used for floating signal measurements of up to 1300 V of differential voltage and common mode voltage."
The keyword is earth-grounded, so your Picoscope needs to be grounded- not floating along with the laptop. Careful with cheap products with no user manual or safety instructions.

As far as measuring current, the probe attenuates by 50x so a 4R sense resistor 0.8A becomes 64mV plus the probe's 40mV of input-referred noise.  It does not look promising.

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