Author Topic: Pearson current monitor model 1330 teardown (100kA)  (Read 378 times)

Offline Mads Barnkob

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Pearson current monitor model 1330 teardown (100kA)
« on: June 01, 2024, 08:22:28 PM »
Repost: Was requested from a reader, as the large pictures are no longer available on the original 4hv thread.

January 2015.
So, there I was, happy about my latest purchase, a huge and extremely noise immune 100kA current monitor.



Testing it on a small cap bank did however reveal that something was wrong with it.



I tried to write Pearson tech support for help with the above image attached.
Quote
I have a second hand Pearson current monitor model 1330 that is very ill behaving compared to my model 101.

I made some different measurements with a little description on the bitmaps from my oscilloscope. The measured currents are around 12-16kA.

The 50 Ohm attenuator mentioned in the image is a 50 Ohm -10dB attenuator.

I also tried looping the coax cable through large ferrites for common mode noise suppression without any luck. Neither did it help with a 50 Ohm termination at the oscilloscope input.

I put the different measurements together in a imgur album.

The reply from Pearson
Quote
Thank you for the Imgur data. First off, I would not recommend using our current monitors with external terminations other than 50 Ω and 1 MΩ. Our current monitors have a 50 Ω output impedance and the sensitivity is designed to work into an oscilloscope at 1 MΩ input impedance. Using the 50 Ω setting on the oscilloscope with cut the sensitivity in half. Also, the 1330 has an internal series resistor to make the output impedance 50 Ω so putting different loads in parallel with the unit may not have the intended result.

What is the input setting on your oscilloscope? Are you using an external 50 Ω termination with the 50 Ω attenuator? From #1, at ~14 kA I would expect the 1330 channel to be ~10-20 V with a -10 dB attenuator, depending on termination. Since the sensitivity appears off it could be a damaged series resistor or termination resistance. From #2, it looks like the internal termination of the 1330 could have been damaged. Has this unit been used for measuring damped ringing waveforms in the past?

Have you measured the terminal resistance of just the 1330 with the DMM? The 1330 should measure ~46 Ω. If it is much higher this usually indicates a damaged series resistor which can affect the sensitivity when using a 50 Ω load.

I requested help on repairing the unit and all communication stopped after that.

As I had nothing to loose, except money already lost, I decided to take it apart to find a faulty component and maybe repair it on my own.

The green paint is thick, hard and sticks very good to the copper surface, it was a messy and long job just to remove enough of it to find out how it was assembled.




Prying the first copper shield off was a whole evening job. Notice that the two shields are not connected in the through-hole, there is only a tiny narrow slot for the magnetic field to enter the CT.




Carefully chipping the epoxy off was a job that took some evenings week upon week, it was over a long period.




After I had removed the inner copper shielding I lost all hope and interest when there was just another layer of epoxy and this time chipping it off was not an option, now there is components and a ?brittle? core involved. I left it on a shelf for a long time.

Last week I found a large pan on the scrap yard and thought, I will try to cook it and see what that does to the epoxy. Just add salt and pepper for a tasty transformer soup.




The last remaining copper part with connector in it came loose in first boiling run and a cable revealed where the series resistor was located. Carefully removing the softened epoxy, the series resistor came apart in tiny pieces and from the look of it, it was once warm, maybe too warm. This was however also the only visible damage I found.




Connector got out in the free




On the inside of the through-hole, which by the way was only accessible for electromagnetic waves through a thin 2-3 mm wide slot between the shieldings from each side of it, there was another weird coil of 5-6 hair thin wires, wave shaped, in parallel on a wide piece of tape, tapped for each 30mm with a gold wire into the distributed load resistors on the regular large coil. See schematic.




The thin wave-shaped wires should be visible from these two different angles



Schematic made from my best guess on how its connected, number of distributed load resistors it not accurate.




As you can see, I tried replacing the 39R series resistor, identified by the rings it left in the epoxy, and not with a great result, DC resistance is still far from specification.

Tearing it down, the money was not all a waste, but still a pretty expensive teardown :)

End of story, so far. February 2016.
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Offline klugesmith

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Re: Pearson current monitor model 1330 teardown (100kA)
« Reply #1 on: June 05, 2024, 01:42:04 AM »
Glad to see the re-post, Mads.  Nice work.
Am reminded that I have a couple of Pearson high frequency CT's that turned up at flea market 10 or 15 years ago.
I had to walk 1 km to get $50 or $100 cash for the purchase.
So far untested, but they beckon me to retire from day job and clean up the home workshop.

As you requested, I will re-post my story and pictures from 4hv report (in about 2010) about rigid, reproducible Rogowski coils.
My calibration pulse generator, like yours, has RLC circuit with similar dynamics as pulser of interest (a can crusher).
With 1/100 of the voltage and current. 1/10000 of the energy.  Nice thing about ironless instrumentation is linearity over decades of signal strength.
 

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Re: Pearson current monitor model 1330 teardown (100kA)
« Reply #1 on: June 05, 2024, 01:42:04 AM »

 


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