attempt to copy pearson Wideband CT - Page 1
Forum section: electronics / projects

author: daisizhou
author_rank: Super Contributor
timestamp: October 21, 2023, 01:27:11 pm
content: Hallo every one.I found that many people use Pearson probes to measure high-frequency currents.But this type of probe is very expensive.A Chinese professor analyzed this probe.Here is the video he created1--->https://www.bilibili.com/video/BV1gp4y1M7Xz/?spm_id_from=333.999.0.02--->https://www.bilibili.com/video/BV1wH4y197qT/?spm_id_from=333.999.0.0This is Pearson's official website-->https://www.pearsonelectronics.com/I have drawn simple production schematic diagrams for 411 and 4110, and I hope interested friends can discuss whether my plan is correct togetherhttps://www.pearsonelectronics.com/pdf/411.pdfhttps://www.pearsonelectronics.com/pdf/4100.pdf

author: PartialDischarge
author_rank: Super Contributor
timestamp: October 21, 2023, 02:29:10 pm
content: The video misses (I think) the most important aspects: the core material and properties and the coil cable, is it normal transformer wire or Litz?Also keep in mind that the production of these transformers is not massive so price cannot be that low.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2023, 02:33:20 pm
content: Hah. Yeah, that much is trivial enough; it can be determined with under an hour of probing, you don't even have to take anything apart!The secret sauce is how they get the bandwidth flat up to or beyond the cutoff frequency.What even is a cutoff frequency for a CT? What does it come from, how does one control it?That, and a couple other keen questions, are where the real work is at.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: October 21, 2023, 03:54:15 pm
content: We knew Mr. Pearson, and have several 410, 411 currewnt transformers.For professional use well worth the cost.Mr Pearson's research and patent used special core materilas and a distributed termination, with many taps and term resistors, NOT a simple current transformer as you show.The case is also special for shielding.Pearson 410 BW is 1 Hz..50 MHz.YOU CANNOT COPY IT !!~JonPS: Wrong title of post: suggest to revise title "Cheap knockoff attempt to copy pearson Wideband CT"

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 21, 2023, 03:54:37 pm
content: No knowledge of those particular units, but I know you can play some tricks using two different cores, stacked, to increase the bandwidth.

author: jonpaul
author_rank: Super Contributor
timestamp: October 21, 2023, 05:36:32 pm
content: Pearson patentshttps://patents.justia.com/assignee/pearson-electronics-inchttps://patents.google.com/patent/US3146417A/enHe may have used a very thin amorphous or Si steel grain oriented tapewound coreJon

author: daisizhou
author_rank: Super Contributor
timestamp: October 22, 2023, 04:59:34 am
content: Thanks.The internal structure is like this,It seems difficult to make it under amateur conditions.If you have any better suggestions, please let me know. Thank you

author: jonpaul
author_rank: Super Contributor
timestamp: October 22, 2023, 06:22:47 am
content: daisizhou, you cannot copy or replicate Pearson's years of research and practical magnetics engineering with copies and shortcuts.For years Mr Pearson made prototypes and tested under all worst case conditions, with HP/GenRad/TEK generators and scopes, not cheap junk.Suggest to study the many textbooks and papers on wideband transformers, and the Pearson Patents.See our old friend Steve Smith's classic book, "Magnetic Components design and applications"https://www.amazon.com/Magnetic-Components-Applications-Steve-Smith/dp/0442203977Take into account the DC bias and pulse VT product expected, core saturation and losses vs frequency.DO YOUR OWN RESEARCH to find the best core material, shape, area for your usage.You will learn a lot!There are a few Pearson knockoffs from ex employees (one was a pinter!) that ILLEGALLY STOLE Pearson's designs and production parts to copy and manufacture their own knockoffs.ALL THE COPIES ARE VERY INFERIOR TO A GENUINE PEARSON CT.AmicalmentJonMSEE

author: daisizhou
author_rank: Super Contributor
timestamp: October 22, 2023, 08:39:51 am
content: It seems that the working principle of Pearson probe is very similar to that of AC transformer.Do you mean that Mr. Pearson's design has never been surpassed?Even the final quality of the imitations that emerged lagged behind Mr. Pearson's original design?

author: jonpaul
author_rank: Super Contributor
timestamp: October 22, 2023, 11:19:43 am
content: I have great respect for the engineering and innovation of Mr Pearson.A current transformer is a very special requirement as the primary is a single turn and the minimum voltage drop on the proimary (conductor under test) is required.The secondary drives a specified load eg 50 Ohms.The usual wideband transformer ratio Fh/Fl = BW HF 3db / BW LF 3db = ~ 1000.Some Pearson have 20MHz/1 Hz!Combination of special windings, distrib term, spécial core material, special wire, etc.You may make a transformer or even a CTI think that but you WILL NOT make or copy anywhere near the performance of as a GENUINE Pearson 410/411.Pearson are worth ever penny.Bon chance,Jon

author: Wolfram
author_rank: Frequent Contributor
timestamp: October 22, 2023, 02:07:12 pm
content: It's a combination of distributed termination giving flat HF behavior and very high permeability core materials giving a low LF cutoff frequency. It's not trivial to reproduce the over 7 decade range of the originals, but it's also not impossible, it just requires an understanding of the fundamentals and some engineering effort.A two-winding ferrite core CT with midpoint termination per half-winding will give you five decades of bandwidth, see "Inductors and Transformers for Power Electronics" by Van den Bossche and Valchev, section 11.5.2.2 for an example. Replacing the core with Vitroperm 500F one from Vacuumschmelze will get you at least another decade on the low end, maybe more. To fully match a Pearson likely requires a more spread out termination scheme, and some optimization of the winding geometry to control the parasitics tighly. Then there's the effort of adequately electrostatically shielding the winding.Further discussion and some interesting teardown photos can be found here:https://4hv.org/e107_plugins/forum/forum_viewtopic.php?id=175696

author: HighVoltage
author_rank: Super Contributor
timestamp: October 22, 2023, 03:12:50 pm
content: I have used many many Pearson CT over the years and sometimes looked for alternatives with no success.The models I have at hand are 110, 410, 411, 6595, 2879, 2100, 4997At one time one of my Model 110 broke down from a wrong hookup and I was happy about that, because I could take it apart.It was much more complicated than I had anticipated. Too bad, in those days I did not take pictures.The price is OK for what you get.Once in a while you can get lucky to find a good used on on ebay for a low cost.But usually they even sell for lots of $$, even used and in unknown condition.Another specialty for Pearson CT are the shielded version with rise times in the low nano seconds.I was able to measure the current of high voltage spark discharges of more than 1000A and less than 10 nsTry this with any other manufacturer. I don't think so.I was lucky enough to visit them in California once.It is a very impressive company.And I have my doubts that a DIY project will give you good results.One can never have enough of these current transformers ....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 22, 2023, 03:25:49 pm
content: I mean, it's still not so hard to see what's going on. We can invoke transmission lines to analyze things in a bit more detail still.Probably the simplest model to see what's going on, is to say: suppose we have a slitted cylindrical primary winding, and we wind a solenoid (single layer helix) secondary over it. The secondary is round wire, elevated above a plane -- well, a curved plane, a cylinder, but it is locally flat so we can see it is a transmission line cross-section (round microstrip), and it's just bent around in a helix. Except at the slit, where the plane is discontinuous. Thus we model the transformer as N transmission line segments in series, each port "stacked on top of" the last, with an EMF connected in series at the gap.This would be a cylindrical core geometry, like a UR core with a single-turn primary, but it also works if we bend the core around into a torus, so we have a helico-toroidal secondary winding, and a slitted-toroidal-shell primary. Also, notice it doesn't matter whether the primary is inside or outside the secondary, as long as the secondary-to-primary distance is shorter than the distance between turns (or even if comparable, it doesn't really matter, we just get a little dispersion, and maybe that's damped out by the next step anyway). So we can wind the toroid core with the secondary, and wrap a shell around it, and with that slotted shell, we don't care what the current distribution is outside of it, so we have a nearly ideal current transformer -- pass some current through the center, any orientation, and we'll read it. All that's left are the internal dynamics.Suppose we short the secondary at one end, to the primary (shell/shield), so we get a 1:N autoformer made of N series transmission line segments. The voltage imposed between all pairs happens ~simultaneously, and propagates in both directions away from the slot (of equal and opposite polarity). After one propagation delay, each wavefront is incident on its immediate neighbor, except at the start where it's shorted to ground so will reflect out-of-phase in the next delay, and the end where the output transmission line is connected. Eventually, the last (N-1, adjacent to the end) slot will have propagated to the start, then back out the end into the load (assume a termination resistance for simplicity), and no further change occurs. Thus for a step input, we read one step output, then another, and so on until N steps -- the step response is a staircase, which in practice will smoosh out to a ramp, and probably some other ringing or lumpiness, in any case limiting full (-3dB) bandwidth to of the order of the total winding  length.Which is basically what we expect for any transmission line transformer, anyway. This is a Ruthroff TLT, where the TL segments are wired in series, and the bandwidth is given by the total electrical length.There is also the Guanella TLT, where TLs are fed in parallel at one side, and wired in series at the other end; by matching lengths, and using magnetic cores to maximize CMRR between the two ports of each TL, we can make any arbitrary ratio autoformer. (More specifically, a general series-parallel connection can be used at one end, andits parallel-series inverseat the other end, thus affording any rational ratio, with factors given by the pattern of series and parallel connections. More than a small ratio is... rather tedious, though.) The propagation delay is thus the simple electrical length of the TLs (all identical, so all waves are series-parallel transformed exactly in sync), and bandwidth is "infinite" -- in practice, limited by CMRR of the pairs, and how "lumped equivalent" all the connection points turn out to be. (When is a series-parallel network no longer series-parallel? When its dimensions, compared to the speed of light, and frequency of interest, are relatively large.)Now, I'm not familiar with the exact model shown in the diagram (shunt resistors across the length of each TL), but it seems like the intent is, at least in part, to partially terminate waves that would otherwise be propagating along the stack, which introduces considerable insertion loss (it's no longer a pure transformer), but I mean the point is to be a current transformer, so we're fine putting a burden resistor inside the thing one way or another; but that should only about halve the risetime (not quite eliminating, but perhaps reducing below significance, the wave reflected from the start short-circuit).We could also simply replace the start short with a Zo terminator, which if our windings within the shield happen to be 50 ohm Zo, we can have 50 ohms at both ends and thus match to a cable quite easily. So we don't need a distributed termination to do this, we can use a single resistor.There may also be something said for high Zo. Notice in the Guanella case, Zo = sqrt(Z1*Z2), it must be intermediate (the geometric average). We have Z2 = 50Ω (or, give or take a modest factor depending on how we terminate one or both ends, or use a min-loss attenuator / L-pad), and Z1 = 50Ω / N^2, so need Zo ~ 1/N. Even for N=10, that's a terrifically low impedance, difficult to reach even with planar construction (PCBs with wide traces and narrow spacing). What we're really after, is some way to connect these transmission lines all in series, and over a short enough distance that we don't mind the speed of light along that distance; we don't get much CMRR for routing TL pairs over ground plane (and we have no choice but to keep ground at most a modest distance away, with everything packed into a shielded enclosure), but if that TL impedance can be made high relative to the signal impedance, and maybe we can do that well enough / in such a way that, we don't mind the resulting leakage inductance from the mismatch; then perhaps we can get something faster after all. Not much faster, it must be a compromise between leakage and capacitance; but perhaps instead of straight halving the risetime, we can 1/sqrt(N) it or something.Which for a decade of bandwidth gain, and for N in the ballpark of 100 (maybe as low as a couple 10s, but surely not many 100s), that seems reasonable enough.So I guess the conclusion goes something like: we're making a TLT, hybrid between the two named cases, where each turn is locally terminated, Zo is relatively high so there's normal mismatch and stray inductance, but this is balanced by the stray capacitance (of any given turn with respect to its neighbors) on the nodes connecting them in series; and they're not really nodes but distributed by way of being turns in a coil, and all these parameters happen to work out well enough.Well, if that's what's going on, then the variables in play are pretty clear; we could wind a toroid, wrap it with tape of given thickness, then of copper tape (soldering seams if necessary), slitting around the outside to break the shorted turn, and also exposing the turns within. We can vary height of the tape and thus Zo of the round microstrip, we can connect resistance R between adjacent turns, and we can connect a load at either end of R_L. Adjust all variables until bandwidth is maximized for given N, and R_L = 50Ω. If R_L = 50Ω isn't available directly (say, the tape thickness would be prohibitively small or large), make a compromise between an L-pad and reduced turns count. Which works upwards or downwards, so we can cover all cases.Incidentally, note that the "ideal" case also exists. In particle accelerators, the beam timing must be known precisely. Particles are bunched into, erm,bunches, so that as they zip down the beamline, a packet of charge goes by; indeed the peak current can be quite extraordinary, despite the charge being so small, simply by way of the bunches containing say some 10^9 particles, but in the space of some mm, or µm even. This current is induced in the beamline (a metal tube), and reflected back as an EM wake, which in part slows the particles, but in straight motion, doesn't really do much (when the particles' path is bent in a magnetic field, it is however ejected as bremsstrahlung, or synchrotron radiation as the case may be). Well, simply slit the tube -- instead of joining sections at a mated metal-to-metal flange, insert a dielectric between flanges, and connect myriad transmission lines across the gap. Bingo bango, 1:1 CT, hope you like microscopic picosecond pulses! Obviously, this is rather hard to deal with, so they do have some filtering and sharpening afterwards to make practical use of it, but, if you like, that's one extraordinarily elaborate way to generate very short electrical impulses.Bandwidth can of course be extended by filling some of the gap with a core; we then have the TLs connect, through a via in one flange, to the opposite flange, the connections being made inside the toroidal core's ID, so we have a one-turn primary and secondary as expected. The outer flanges (the flange flanges, as it were..?) can then be a short circuit, maintaining overall shielding (and vacuum and cryo sealing too, probably).Tim

author: kelvinsz
author_rank: Newbie
timestamp: January 09, 2024, 07:20:49 am
content: Here is tread in the forum 4HV which mentioned the teardown of a Pearson Transformer model 1330, the measurement range of which is up to 1000kA.Big thank to Mads Barnkob for his sharing of teardown report! We can image that teardown such a product filled with potting material is a tough works!https://4hv.org/e107_plugins/forum/forum_viewtopic.php?id=175696Please be careful, some descriptions in this teardown report are not correct. For example, the "tiny narrow slot on the shielding layer" is NOT designed for the magnetic field, just to avoid a full covered metal shielding layer will form a secondary winding turn.The key to ensure the high frequency performance of the current transformer is the magnetic core performance and how to eliminated of use the distributed capacitance between winding turns and layers.The video in Bilibili just shown that the professor of Yangzi University doesn't under Pearson Transformer well.