Rogowski current probe

[flyglas]

Hello,
does somebody have access to a (perhaps broken one) rogowski current probe for use with an oscilloscope?
I cannot find any pictures from the inside of such a probe (integrator part of the probe) anywhere in the internet.
I would be very happy if someone can open a probe and take some pictures.

[Hydron]

I second this, but I'm also interested in the construction of commercial probes, especially the skinny high-frequency ones (you can get them thin enough to thread through a TO-247 leg!

It's one thing winding a skinny coil, another to do it such that it doesn't pick up all sorts of garbage from E-fields etc yet still is flexible and high-bandwidth (the obvious ideas like a shield tend to work against these).

[johnf]

tiduby4a.pdf

[davekni]

Sometimes a very cheap simple solution works fine, especially if there's a way to calibrate.  I once needed to probe output current of a large H-Bridge.  Wrapped 10 turns around the end of a felt-tip marker and wedged that between the two wires of the output cable.  Coil ends connected to a scope.  Captured waveforms and integrated with software.  Used a known 800A waveform for calibration.  It was then very useful to capture high-frequency details of the current waveforms that weren't visible previously.

Also tried a similar test on my DRSSTC H-Bridge output.  This time I added a non-shorted turn of copper foil around the coil for shielding.  Worked fine, but I didn't keep it.  My CT already has a scope tap, and digital integration isn't convenient for real-time viewing.

[johnf]

Hydron
I built a tiny current transformer to go over the source lead of a TO220 fet (actually a 5 pin PowerIintegrations SMPS device) to actually see what was going on
I had already tried a 0.01 ohm resister to scope over but introducing the resistor caused other effects and masked my problem.
I used the core out of a ISDN modem that had common mode chokes on the data ports i pulled the core out and wound with 38awg wire 200 turns on it and high voltage varnished it up along with a 1.0 ohm burden resistor. Net effect was this causes 0.000025 ohms of impedance on the source lead which in my case was low enough to go back to Power Integrations for them to ponder over. I ended up making a second transformer and sending it to them for their use in their lab

[johnf]

Kludgesmith asked me for some pics of the common mode choke i stole the cores out of so here they are

[johnf]

another pic

[johnf]

available from
https://www.digikey.com/product-detail/en/pulse-electronics-network/PE-65855/PE-65855-ND/3533394

[flyglas]

Here is a patent of a commerical unit:

https://patentimages.storage.googleapis.com/85/26/79/fd153583c08c2a/US6614218.pdf
https://patentimages.storage.googleapis.com/b4/12/0a/f6a3234c6cc7e7/EP1073908B1.pdf

[Hydron]

Sometimes a very cheap simple solution works fine, especially if there's a way to calibrate.  I once needed to probe output current of a large H-Bridge.  Wrapped 10 turns around the end of a felt-tip marker and wedged that between the two wires of the output cable.  Coil ends connected to a scope.  Captured waveforms and integrated with software.  Used a known 800A waveform for calibration.  It was then very useful to capture high-frequency details of the current waveforms that weren't visible previously.

Also tried a similar test on my DRSSTC H-Bridge output.  This time I added a non-shorted turn of copper foil around the coil for shielding.  Worked fine, but I didn't keep it.  My CT already has a scope tap, and digital integration isn't convenient for real-time viewing.

Hmm, will need to give the similar simple setup I tried and wasn't happy with another go given I have a better scope these days.

As for small CTs, I spied these guys in a power supply teardown once:

(from https://www.tomshardware.com/uk/reviews/corsair-ax1600i-psu,5406-3.html)
Can also be seen here: http://www.jonnyguru.com/wp-reviews/CorsairAX1600i/DSC_2384.jpg

I never had any luck finding a way to order these unfortunately - they look super handy! Any lead on them would be most welcomed.

[flyglas]

@Hydron: I have asked the manufacturer of the GaN switches in the Corsair PSU about the tiny current sensor via mail. Perhaps they will answer my question.

Yesterday i bought a rogowski coil PEM CWT from ebay. Perhaps the unit will satisfy my thirst for knowledge.

[Hydron]

May I ask how much you paid? I haven't really looked seriously, but what I did see was silly money (i.e. business purchase $$$, not hobby). I am certainly interested in what you find!

[flyglas]

Hi Hydron,

I paid 174 EUR including shipping from Italy to Germany. That is a fairly good price.
--> PEM CWT 6R

[Hydron]

Thanks,

I did see this internal picture, may be of interest as you wait for your unit:
https://gmw.com/wp-content/uploads/2019/03/CWT_Fuse_Replace_Instructions_GMW.pdf

Is not showing the cable entry from the coil (goes in the hole at the top), but there isn't a whole lot going on in there! Would be good to see and compare to the inside of the one you have coming.

I am definitely interested in how the coil is made, but unfortunately this is difficult to do non-destructively unless you have an x-ray machine handy.
From the info I can gather from PEM publications etc, some of their probes look to be shielded, some not, so there may be quite a range of performance in the presence of high dV/dt.

[flyglas]

Thank you very much for the fuse replacement PDF.
Half of the tiny PCB is a power-supply and the other half is the integrator and output driver.
Nothing much in there.

My probe arrived during the week, but the probe is in a not working condition.
As long as the situation is not resolved with the seller, i will not dismantle the probe.

[klugesmith]

Eager to hear your results.  Sorry to hear the instrument does not work.

I have taken some steps toward winding a flexible Rogowski coil for the purpose outlined in "current transformer for narrow spaces" thread.  Commercial ones for AC power metering come close, but all seem to have small end diameters bigger than 7 mm (0.28 inches).  If the current to be measured is a sinusoid of known frequency, we can skip the voltage integration.  I expect to get about 10 mV out, from 200 amperes at 60 Hz. 

What dimensions are easy to see on little commercial probes like the PEM?
For any Rogowski coil, the sensitivity (in units of inductance) is simply u0 A N/L,  where A is the small loop area, and N/L is the winding density in turns per unit length.   Commercial ones with different size big loops, made from the same tightly wound flexible rod material, all have the same sensitivity.

[flyglas]

What dimensions are easy to see on little commercial probes like the PEM?

The smalles probe is the PEM CWT Ultra Mini with only 1.7 mm in diameter.
I have worked with this tiny probe a few years ago. In my application I have mounted the probe between a minibrick and a PCB on top of the minibrick.

[Hydron]

That's a shame about the probe - the pics and description looked good.

Did you notice much influence from nearby dV/dt transients when using the CWT Ultra Mini? That one isn't shielded (it looks like the only shielded probe they do is the CWT miniHF) which concerned me about it's use near fast switching transistor legs, but if interference from nearby voltages isn't such an issue then DIY versions become much more practical.

[flyglas]

A solution with the seller was found. The repair of the probe is in progress.

I did not noticed the influence of nearby dV/dt transients with the CWT Ultra Mini.
We also performed double-pulse tests with unshielded probes.

[Hydron]

Excellent, very interested in how the repair goes and to see what's inside.

Do you know whether the coil is damaged, or if the fault is in the integrator box?