Converting a DSO to a battery powered device to measure differential high voltage

[rxpu]

I need to make a differential high voltage measurement for a power inverter. ( up to 4 channels)

The classic way is to use a high voltage active differential probe capable of measureing +-1000V cat3

But they are very pricey. Each one cost more than the osci and 4 of them is a huge investment.

I came to an idea to construct a battery powered 230Vac generator to supply the osci.

I plan to attach high Voltage passive probes to measure high voltage differential voltages (ex. gate of the IGBT of the upper leg.)

Will it work.

I also plan an isolated enclousure to operate the osci and plan not to use the panel buttons but control the osci over WIFI ()or remote probram) to avoid high voltage link between the body of the osci and my own body through ground.

What do you think abaout it?

 

[wwfeldman]

i'm not s fan of amazon, but this scope (or something comparable) may be be easier and more practical
see page number 184 / pdf page 209/237
no need to reinvent the wheel

Siglent SDS1104X-E 100Mhz digital oscilloscope 4 channels standard decoder: Amazon.com: Industrial & Scientific

Siglent SDS1104X-E 100Mhz digital oscilloscope 4 channels standard decoder: Amazon.com: Industrial & Scientific

www.amazon.com

 

[dick_freebird]

Running the 'scope off a battery (or just a plain
old isolation transformer) would free you from
common-mode limits but the differential voltage
comes down to the vertical channels' front end.

For a power inverter you might assume you do
not care about milliamps, so maybe just make
a resistor divider per channel all returned to the
same ground, maybe 990/10 ohms and now
you have a 1/100 scaled voltage that ought to
play with regular probes just fine, and with a
7pF tip the time constant is still sub-100ps, way
faster than any inverter's edge rates and pulse
widths. Might need to take some care with the
power ratings and tempcos, creepage and so on.
Maybe make the "990" out of (10) 100-ohm
segments and one of those, a parallel 1K. Or just
take the real scale factors and forget round
numbers, which would take up the tolerances
too.

 

[rxpu]

i'm not s fan of amazon, but this scope (or something comparable) may be be easier and more practical
see page number 184 / pdf page 209/237
no need to reinvent the wheel

Siglent SDS1104X-E 100Mhz digital oscilloscope 4 channels standard decoder: Amazon.com: Industrial & Scientific

Siglent SDS1104X-E 100Mhz digital oscilloscope 4 channels standard decoder: Amazon.com: Industrial & Scientific

www.amazon.com

Thanks for the link. The scopes that have integrated isolation have very low capture memory.
So i try to adapt the isolation to a 14Mpt memory.
My application is an LLC converter. (@400kHZ-1000kHZ).

--- Updated Jan 30, 2021 ---

Running the 'scope off a battery (or just a plain
old isolation transformer) would free you from
common-mode limits but the differential voltage
comes down to the vertical channels' front end.

For a power inverter you might assume you do
not care about milliamps, so maybe just make
a resistor divider per channel all returned to the
same ground, maybe 990/10 ohms and now
you have a 1/100 scaled voltage that ought to
play with regular probes just fine, and with a
7pF tip the time constant is still sub-100ps, way
faster than any inverter's edge rates and pulse
widths. Might need to take some care with the
power ratings and tempcos, creepage and so on.
Maybe make the "990" out of (10) 100-ohm
segments and one of those, a parallel 1K. Or just
take the real scale factors and forget round
numbers, which would take up the tolerances
too.

Thanks for the useful info. I already have a 1:100 HV probe. But as you said I should find better ones with lower capacitance to conmpansate for the High resistance.

 

[danadakk]

https://www.paulvdiyblogs.net/2017/10/differential-amplifier-probe-make-buy.html

One could think of doing this optically, with an isolated processor that G corrects
the emitters used to translate V across the "barrier". You would have fiber isolation
results. Or same with transformer approach. Alternative is HV R's in dividers, those
can be pricey.

What are accuracy and resolution and environmental targets ?

Then there are actual probe mechanical design to meet the user isolation. Distances, isolation....


Regards, Dana.

 

[FvM]

I presume the OP wants to solve a practical measurement problem rather than designing a new probe.

The sketched isolated operation of a battery supplied scope can be considered as a pratical solution. Drawback is the required room for an insulated safety box. Also, if the circuit involves fast common mode transients at the probe ground, the scope will still pick up a lot of noise. Common mode choke torroid cores and static screen for the oscilloscope box may be required. But still a usable solution.

High speed optical isolated probes are available from major manufacturers, but very expensive.

 

[danadakk]

https://www.analog.com/en/about-adi/landing-pages/001/safety-and-regulatory-compliance-information.html

Above isolators for HV work.

Here is a diy, V not CAT 3 but principles could be used.

High-Voltage Differential Probe - Circuit Cellar

Amplifiers in Action A high-voltage differential probe is a critical piece of test equipment for looking at high-voltage signals. In his article, Andrew describes his design of a high-voltage differential probe with features similar to commercial devices, but at a considerably lower cost. It...

circuitcellar.com

Regards, Dana.