What kind of transformer is this ? (EEVBlog #1119) - Page 1
Forum section: electronics / projects

author: BravoV
author_rank: Super Contributor
timestamp: September 07, 2018, 05:11:28 am
content: Just in case you haven't watch (HERE) just jump to 11:30 where Bart started to explain it if you're lazy to watch all of them.Its used in the Cleverscope : CS448 which is an isolated scope. Its the power transformers to power the isolated scope front end, and claimed they are manufactured by them self, and claimed to have low CMRR.Curious as I've never seen one like this before, photo captured & attached below.What kind of winding is that ? I'm puzzled by how are windings go around that whole thing ?As highlighted by arrows, is that a 3 taps windings at both primary & secondary ?Also regarding the magnetic material, the blue toroid is obvious, but at the surrounding black container, is that a magnetic material too ?What are the advantages of this kind of build, say compared to plain ordinary EI core ?Appreciate any other insight or comments.

author: BrianHG
author_rank: Super Contributor
timestamp: September 07, 2018, 05:28:35 am
content: The secondary, output, is on the toroid core.The primary is lifted above the core, in fact, it isn't even in contact with the core. This is needed to give you isolation at the electrostatic level and series capacitance prom primary to secondary < 1 pf. Call this the ultimate galvanic isolation transformer. When it comes to placing a GND of a probe on a 500v H bridge output, switching up and down in 10ns or less, you cannot bridge that swinging GND to anywhere else in the scope's electronics, or, other input channels. Even a capacitance of 1pf from primary to secondary with a 500v swing at the 10ns rise and fall times would feed a disastrous signal right through a conventional coil sitting on-top of coil transformer. You got to keep those primary and secondary conductors as far away from each other as possible.Used to do this for audio DAC, not power, but data where you wanted to super isolate the SPDIF signal.For power, we would use split bobbin transformers to get as far as possible from the mains AC. Obviously, NO Y caps from mains to secondary.The surrounding black material is plastic. Something very low in capacitance, wont build up static electric charge, and is safe for a few Kv.That black transformer case goes below the PCB, since you can see a cutout on the PCB. There are 4 solder points on the PCB, not 3. The center 2 on each side is the main transformer connection. It looks like the outer 2 connections on both sides goes to a support wire holding down the entire apparatus. This short outer conductor seems to go through a single loop in the transformer line. Maybe it is a feedback loop. I'm only guessing here.I believe I once had my similar transformers made at Inductors Inc. I had a 15kv AC hipot. It was a custom job, nothing you can buy off the shelf.

author: BrianHG
author_rank: Super Contributor
timestamp: September 07, 2018, 07:08:44 am
content: With 4 solder contacts on each side of the transformer, and a twisted pair, it is possible the the inner 2 solder spots go to the twisted braid on the beginning of 1 coil while the outer 2 are coming in from the underside of the plastic case to the top side through 2 holes, to make it to the top contacts. This would be my second guess, that it is just the way the outer 2 contacts are braided from the bottom side to the top of the transformer, then send out to the outer 2 pins.Here are some other closeups, different angles, zoomed in with sharpening and a gamma boost to try to get a better look...

author: BrianHG
author_rank: Super Contributor
timestamp: September 07, 2018, 07:14:48 am
content: Labled...Same goes for the right side, except, it is being done through a center hole in the plastic by the center pin, then twisted together, then spread out to the 4 pins on the right side.

author: BrianHG
author_rank: Super Contributor
timestamp: September 07, 2018, 07:20:07 am
content: My guess is on the primary, 1 wire in the twisted pair is power, 1 feedback, or, both conductors are in parallel.On the secondary output, the twisted pair is wired in series at the output for a +/- tracking balanced power supply. All the analog needs the +/- 5v or even +/-12v supply.The turn count is also identical from primary to secondary, 5 loops on each.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 07, 2018, 07:40:50 am
content: Not sure about the clearance distance, but on a first look, it seems to be huge for 1KV. I do recognize some isolators from Texas (SO packages bridging between two boards on the lower right corner).P.S.: 0.5ppm is considered a very accurate clock? I'm off the hook, then! (just because I've designed a 0.025ppm clock gen, and I wasn't sure about the precision on that)

author: BrianHG
author_rank: Super Contributor
timestamp: September 07, 2018, 10:00:11 am
content: Quote from: bloguetronica on September 07, 2018, 07:40:50 amAnything less on the spacing and a 100 MHz signal at 1Kv will bridge right through from secondary to primary. We are not talking about a 60Hz 1Kv, we are talking about over 100MHz at 1Kv. You better believe all that empty air space between primary and secondary is crucial.As for the isolated function generator, it only goes to around 25Mhz, at a low voltage, so the digital isolators are good enough on that side. It is also a low impedance output, not a 1 MegaOhm input reading small milivolts with a sensitive ADC. You can believe that to achieve the posted scope isolation specs with no crosstalk, those special transformers were the sole solution other than making battery powered front ends. Those long optical fiber for the data was also mandatory.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: September 07, 2018, 11:32:28 am
content: Quote from: BrianHG on September 07, 2018, 10:00:11 amI must confess I haven't seen the whole video. Battery power front-ends? So, basically they avoided isolated DC-DC converters and other such solutions. That is new and I was not expecting that. That seems to be extreme!Kind regards, Samuel Lourenço

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2018, 04:56:35 pm
content: It's not. it's a common mode choke.The giveaway is the pairs connect to each side, not across the barrier as a transformer.The custom(!) bobbin is to minimize capacitance; the core may be nanocrystalline, for reasons well covered in this thread,https://www.eevblog.com/forum/blog/eevblog-1104-omicron-labs-bode-100-teardown/but for common mode choke purposes instead.This gives maximum isolation impedance, to the highest frequencies possible.At least, that's just my interpretation of it.The "102" block nearby is probably another CMC, for higher impedance at lower frequencies (it's probably an off-the-shelf like a Bourns SRF0905), and will have a fair bit of capacitance (and maybe transmission line characteristics -- peaks and dips at high frequencies) at modest frequencies (low MHz), where the fancy CMC takes over. Alternately, it could be a transformer -- the component itself could still be a CMC, they're just using it as an isolation transformer proper -- in which case the converter would be a forward converter, and given the low noise, probably a balanced, resonant or quasi-resonant type.Actually, that wouldn't make sense, the isolation capacitance would be utterly awful. Even if it's a power line choke*, the capacitance might not be great. It could very well be a custom part with very low capacitance; this design certainly deserves it.*A data choke is usually twisted pair or bifilar, wound on a toroid or other shape. A power choke is usually two separate windings, on opposite sides of the core, giving much higher leakage which is useful for diff mode filtering.Tim

author: mzzj
author_rank: Super Contributor
timestamp: September 07, 2018, 05:08:47 pm
content: Quote from: BrianHG on September 07, 2018, 10:00:11 amVoltage is usually derated with increasing frequency, also in this case:Maximum Differential Input Voltage±1 kV, derated above 1 MHz.Derated at 20dB/decadeMaximum Common Mode Input Voltage±1 kV, derated above 10 MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2018, 05:10:05 pm
content: Ya know, on closer look, that probably is a CT:CT transformer, and that's why they used twisted pair. I didn't see the multiple connections at first and first-glance-assumed it was all one wire.But again, just a guess until someone desolders one(Gosh, what a sin that would be! Would be great to peek inside those shields too though...)Tim

author: ArthurDent
author_rank: Super Contributor
timestamp: September 07, 2018, 05:29:00 pm
content: It appears to be a CT primary to CT secondary transformer with very high voltage isolation. It is wound with twisted pair like a lot of ham baluns. Here is a file photo of what would be one half of the pictured balun transformer.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 07, 2018, 05:39:17 pm
content: The toroid part very much looks like a transformer, build for capacitance. The 2 twisted wires are likely just for get 2 windings and maybe a split coil for the output. Some of the pictures are quite clear. The only thing slightly odd is that on top it is just some air space with no extra insulation material.The extra ferrite block than might be extra common mode choke, especially for the very high frequencies, where even a few pF could be a significant.

author: BrianHG
author_rank: Super Contributor
timestamp: September 07, 2018, 08:17:56 pm
content: Just to repeat, after analyzing the photo and listening to the interview, this is a power DC-DC converter transformer. The secondary, power output is the twisted pair, on the left side, whose twisted pair is wound directly on the magnetic core at 5 turns.On the outer plastic structure, there is the power input twisted pair going around the core, 5 turns, but spaced off the surface of the core to give that super low pf transmittance from 1 side to the other for the high frequency high voltage steps you see in power supply design work. If a single twisted pair had the primary and secondary as the isolation, that 10ns 500v pulse would migrate through immediately.The primary twisted pair is most likely wired in parallel for lower voltage. The secondary twisted pair is most likely wired in series for double voltage, or, a +/- supply output.All the data is on the 4 fiber-optic cables. There is no data on this transformer.

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2018, 08:39:08 pm
content: what kind of power transfer do you get through that kind of construction at that size?

author: BrianHG
author_rank: Super Contributor
timestamp: September 07, 2018, 09:06:18 pm
content: (I cheated here, they are actually using around 2.5 watts of power).....Like I said, here is the ADC they are using, it consumes 1 watt max:ISLA214P50Add another 1.5 watts for the fiber transmitter and analog front end opamps and I get approximately 2.5 watts, maybe even 3 watts.

author: mzzj
author_rank: Super Contributor
timestamp: September 07, 2018, 09:09:58 pm
content: Quote from: coppercone2 on September 07, 2018, 08:39:08 pmfrom couple of watts to maybe 20-30 watts. Not much copper there.1" toroid core is good at least to 100 watts with less radical winding

author: BravoV
author_rank: Super Contributor
timestamp: September 08, 2018, 05:17:41 am
content: BriangHG, thanks a lot for the comments & illustrations.Quote from: BrianHG on September 07, 2018, 08:17:56 pmIts in the video on Dave's interview, Bart Schroder (the designer) said they took almost 1 year to develop the power section to power the scope front end including this transformer if I'm not mistaken, and mentioned too that its a custom in house job.Regarding the outer black plastic surrounding the blue toroid core, I guess that is also a custom made isn't it ?With this construction style, does the parasitic capacitance between the two windings (pri and sec) is relatively low too ?For sure, Bart said it has lowest common mode noise.

author: BrianHG
author_rank: Super Contributor
timestamp: September 08, 2018, 05:56:38 am
content: Here is an example of a mass produced version of a super galvanic isolation transformer, a lot larger than the one I once had made for 10mb data around a decade ago:Note that Cleverscope's transformer case looks like a home made 3D printed, and that plastic case holds the windings at specific locations and angles on both the inner coil and the outer windings to even further isolate the primary and secondary. Remember, they are not trying to separate 60hz sine wave AC, or DC, they need to measure the weak small drive of a gate drive with that probe GND tied to the switching bridge with a 10ns 500v (this is like a spike intermittent radio broadcast) step up and down not to affect the adjacent probe channels which may sit on a different frame ground. If the windings were any closer, that signal would bridge to the chassis & power of the Cleverscope's power system, then to the adjacent input channels. Their only superior choice would be to use a big 25 watt laser diode to beam power to a photovoltaic cell using another fiber link with that huge empty space, for each channel.

author: Hypernova
author_rank: Unknown
timestamp: September 08, 2018, 06:43:18 am
content: Quote from: BrianHG on September 08, 2018, 05:56:38 amDo people actually do that? At that point you may as well go for batteries.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2018, 06:55:22 am
content: Besides a transformer and photovoltaic, there would also be the option to use an acoustic transformer: In the extreme case a glass rod with transducers on both sides. Efficiency is expected to be better than the optical way.The optical way is not that bad anymore: red / NIR lasers are relatively efficient (up to about 50%) and also PV conversion just above the edge is more efficient than with solar light. So I don't think it would need 25 W optical, more like 6-10 W, maybe 25 W of electrical input. I would prefer an acoustic transformer.

author: BrianHG
author_rank: Super Contributor
timestamp: September 08, 2018, 08:25:58 am
content: Quote from: Hypernova on September 08, 2018, 06:43:18 amHere is a 1 watt transmitter/receiver system.https://www.fiberopticlink.com/product/power-over-fiber-system-pof/#tab-id-4

author: Hypernova
author_rank: Unknown
timestamp: September 08, 2018, 09:36:13 am
content: Quote from: BrianHG on September 08, 2018, 08:25:58 amThat is some hardcore gear for just 1W.

author: SeanB
author_rank: Super Contributor
timestamp: September 08, 2018, 09:58:57 am
content: Given that the black former is undamaged by soldering right next to it, I guess it is a custom made injection moulded part, probably made from PTFE with a glass bead fill, to get the right high temperature ability and also not build up a dirt film with time. either injection moulded or powder compressed in a mould, and I will edge to compressed as that is a lot easier on the die. Got to be hand wound as well, there must be a whole room with a few people in there sitting all day with these toroids, the PTFE coated twisted pair wire and carefully winding them, then checking them on some sort of test jig to get the capacitance both low and balanced between the halves of the windings.Looks like the 5 turns wound on the toroid is the secondary, 5 turns arranged so as to be as far apart from the outer primary as possible, and as far as possible crossing at right angles to the windings. Primary is 4.5 turns, probably driven with a sine wave drive to keep the noise low, and probably is regulated on the secondary side by some further switching regulators to give the ADC and buffer stage supplies, after some full wave rectifiers to give 2 symmetrical supplies.

author: max_torque
author_rank: Super Contributor
timestamp: September 08, 2018, 12:46:19 pm
content: silly question, but, given you only care about CMRR when actually making a measurement, and we know the high speed sampling means the device cannot actually stream continuously, then perhaps a (super) capacitor could be charged during the non measurement time by a simple relay connected DC/DC ?? Given the low power requirement for the front end, you wouldn't need that much "on time" to charge it?(ie in effect, during your measurement period your front end IS battery powered!)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2018, 02:58:37 pm
content: The normal relays have problems providing the required insulation. So it would need special high voltage relays. For a full insulation with a floating bucket cap it would need 4 contacts and capacitance could be comparable, maybe even high.With something like 2-3 W the power requirements are not that low.

author: grythumn
author_rank: Contributor
timestamp: September 08, 2018, 03:16:32 pm
content: Quote from: max_torque on September 08, 2018, 12:46:19 pmDidn't he say in the video that they were streaming continuously, but decimating the data stream internally before putting it out over USB3?-R C

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2018, 06:49:36 pm
content: Most likely a molded part, glass filled liquid crystal resin or the like -- same thing bobbins and connectors are usually made of. Failing that, GF nylon would be cheaper and probably okay as far as not falling apart during soldering.FYI, PTFE can't be molded, at least not easily as far as I know. The next best thing is PFA or something related.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 12:32:28 am
content: how come it does not have a electrostatic shield?can you do 2 slotted shield each to a separate ground plane? isolation? Cant you put teflon plate?

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2018, 12:40:04 am
content: Quote from: coppercone2 on September 09, 2018, 12:32:28 amDoesn't need one.The front ends are in cans where the waveforms are digitized and then passed on via fiber.After a year of development they'd have used shields if they were needed which arguably they should've even just to help protect their IP.

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 12:58:10 am
content: i mean on the transformer that looks kinda crazy

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2018, 01:34:15 am
content: Quote from: coppercone2 on September 09, 2018, 12:58:10 amSo do I. Watch the vid, just the PSU took a year of development......that includes the toroids for the PSU's.

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 02:16:55 am
content: Quote from: tautech on September 09, 2018, 01:34:15 amI watched the video. He just says it took a year to develop. Did I miss him talking about electrostatic transformer shields?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2018, 02:29:43 am
content: Shields would only increase capacitance without affecting EMI (which is minimized thanks to other filtering and presumably a nearly-sinusoidal resonant converter).Tim

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 02:41:12 am
content: how many DB can you get with conventional shielding techniques vs these advanced novel methods?is his design a theoretical feat? So if we want to approach these performance levels that is the only way thats reasonable?

author: BrianHG
author_rank: Super Contributor
timestamp: September 09, 2018, 02:44:34 am
content: Quote from: coppercone2 on September 09, 2018, 02:41:12 amIf I remember from the video, it was quoted as over 100db isolation at 50Mhz. You get more crosstalk, less isolation, at 50Mhz on 2 parallel 1 inch long traces, 20 mil spaced apart on a normal PCB.If you study that black plastic case, you will notice it has guides for the inner core's windings so they are in fixed positions. The outer windings never run parallel over the inner core windings. It is beautifully designed to make sure no RF crosses from side A to side B without any shield which would add a capacitive link between the inner winding and the frame-ground where that shield would be tied to. You cannot do that in this application where the GND connection of the probe may be an active modulating 500v signal.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2018, 02:45:00 am
content: Quote from: coppercone2 on September 09, 2018, 02:16:55 amNo you didn't miss anything except arising to the obvious conclusion (for me) that no shields are needed. Why could you think that guys that design stuff like thisandtake a year to perfect it could possibly overlook shielding if it was needed. They wouldn't, period.

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 02:47:29 am
content: Quote from: tautech on September 09, 2018, 02:45:00 ami wanna know engineering details on its behavior and when this technique should be used and why the other one does not work I am not questioning that it works, and I want to know for sure that its not done that way for cost reasons.

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 02:50:14 am
content: Quote from: BrianHG on September 09, 2018, 02:44:34 amI wonder if you can extrapolate some kind of theoretical model/equation for shielded torroidal transformer and their capacitance/coupling so you know when your grinding your mind doing something impossible. What the maximum shield limit is.

author: BrianHG
author_rank: Super Contributor
timestamp: September 09, 2018, 02:55:29 am
content: Yes, the outer winding crosses the inner winding only at 2 points at 2 sides on the transformer, and at 90 degrees, not in parallel, traversing at the wrong angle. For everything else, you need to speak with a specialist in the field & build and test measure with believe it or not, high voltage signal tracers/network analyzers up to 100 Mhz. Which was also divulged in the video that they did such tests and their test hardware only went up to around 65Mhz where a plot was shown on paper.

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 03:54:07 am
content: Well I did 2 estimated size donut shaped shields layers surface area with 2mm spacing and the result is like 40pF for dual shields coupling ground planes to each other if anyone is curious

author: BrianHG
author_rank: Super Contributor
timestamp: September 09, 2018, 04:11:17 am
content: Quote from: coppercone2 on September 09, 2018, 03:54:07 amThat's like a 0.07 ohm short at 50Mhz.You got nothing on this scope's transformer's somewhere less than 0.04pf capacitance from primary to anything else. That's more than 1000x less capacitance.

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 04:15:08 am
content: I guess you just can't use shields at higher frequencies for isolation in a transformer

author: BartSchroder
author_rank: Newbie
timestamp: September 09, 2018, 05:31:39 am
content: Hi,It took us a while to design this transformer, so we will keep it as a puzzle for you to delve into.The transformer is just for power isolation. We use the optical fibers for communication.However, the key requirements for the transformer are:1. It has to have as low capacitance as possible. As several have pointed out, a 500V 8ns edge will go through a capacitor like butter if the capacitance is too high.2. That it be as balanced as possible - this is so that the common mode output voltage, which could get into the circuit being measured, is minimized.3. That it be reasonably efficient. We have to put 3W across the gap.The former is 3D printed, and yes the shape is important, as is the material. The toroid is a fairly standard one.The driving system is also pretty important, and you can use your imaginations on thatcheers, Bart

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2018, 05:49:44 am
content: Thanks Bart. I had a look at your wares at the Auckland EMMEX show earlier this year, very impressive capabilities.We had a chat too but of course I wasn't waving any flags to reveal any allegiances.

author: BrianHG
author_rank: Super Contributor
timestamp: September 09, 2018, 06:13:33 am
content: Quote from: BartSchroder on September 09, 2018, 05:31:39 amI was off by 0.5watts. The only thing, which I'm sure is a secret, is that coupling capacitance which I'm sure you will keep a secret. Though, I did mention that it was in house 3D printed...And Bart, I am thoroughly impressed the extent you went through on your scope to achieve the degree of performance you achieved. I am nutty in the audio world and know about going to engineering extremes to achieve a particular spec, or, many times for DAC units with absurdly good enough isolated outputs.

author: BravoV
author_rank: Super Contributor
timestamp: September 09, 2018, 09:44:11 am
content: Quote from: BartSchroder on September 09, 2018, 05:31:39 amThank you Bart for sharing few hints, really appreciate it.

author: mzzj
author_rank: Super Contributor
timestamp: September 09, 2018, 05:31:42 pm
content: Quote from: Kleinstein on September 08, 2018, 06:55:22 amAnd honorable mention for the the old brute solution of motor-generator combination. Actually really good option if you need say 100kW with 4000kV isolation and low capacitance

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 06:24:03 pm
content: can you parallel a buncha other transformers to make a non leaky one so long you don't care its big?

author: coppercone2
author_rank: Super Contributor
timestamp: September 09, 2018, 06:37:54 pm
content: lol

author: Cerebus
author_rank: Super Contributor
timestamp: September 10, 2018, 01:19:02 am
content: Let me quote Bart's article on the design of the Cleverscope CS448, available herehttp://www.eevblog.com/files/CleverscopeSep2018.pdfQuoteOn the subject of power supplies with low common mode noise, Keithley have a US letters patent, No. 7,868,692on a somewhat similar design. There the secondary has been constructed out of coax cable, using the cable's screenas an inter-winding electrostatic shield.You can see the physical construction of the Keithley SMPS in Dave's teardown of the Keithley DMM 7510.Here it is on YouTube, cued up for the first mention of the SMPS layout:http://youtu.be/uvgJ2zAxgAY?t=978He gets into it properly much later in the video:http://youtu.be/uvgJ2zAxgAY?t=2825

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2018, 03:01:11 am
content: Quote from: BrianHG on September 09, 2018, 06:13:33 amNot hard to guess from photographs. It'll be in the single digit pF.Tim

author: BrianHG
author_rank: Super Contributor
timestamp: September 10, 2018, 03:37:59 am
content: Quote from: T3sl4co1l on September 10, 2018, 03:01:11 amIt will bee much less than that. Even 1pf has a seriously low enough resistance at 50Mhz. I would say below 0.1pf.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2018, 03:56:06 am
content: Well no, if there's 0.3m of wire on there (a SWAG) and the windings are spaced at approximately twin-lead transmission line distances (Zo ~ 377 ohms, say), then c_p ~ 8.8pF/m and C ~= 3pF.Mind the probe cables themselves have much more capacitance to free space (or each other, when routed roughly together as in the video). As long as the cables and acquisition modules are well shielded, this only loads the switching node, without affecting the measurement. The displacement current can affect the measurement through any unshielded path, which is traditionally the ground clip, but that has been done away with in this probe design (at least, as I understand it).This has very little to do with shielding in conventional terms, like channel-to-channel isolation.It may well be as simple as making the power supply behave, regardless of what's going on across it!I don't understand the extreme low noise requirement; that could be because of lack of filtering (which takes up valuable real estate) or poor PSRR within the module. It wouldn't make sense for it to be due to interference at the probe end (i.e., conducted emissions from the DC-DC converter travelling up the probe and adding to the measurement at the ground clip), since the CMRR of the probe must already be so good to begin with.Whatever the cause, it was clearly necessary, and it's hard to do much better without getting much more esoteric.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: September 10, 2018, 04:27:21 am
content: i read the topez transformers have something like 0.005 pF capacitance between secondary and primary..jim williams used it in his ultra low noise 10hz thing and its used by people on audio forumshttp://www.surplussales.com/item/_tp/91095-11.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2018, 05:15:31 am
content: Mind the definition used for shielding: there are three capacitors, one from each terminal to ground, then the coupling capacitance between the two terminals.0.005pF is probably a measurement limit; similar limits showed up on vacuum tubes (grid 1 to plate capacitance).The limit here is total capacitance, so shielding only makes things worse.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2018, 04:47:16 pm
content: There are 2 properties for the transformer that do matter. One is a low capacitance from the ground to the probe side. This is to keep common mode loading effects small. This is mainly to avoid loading the circuit with a capacitance to ground: there is still the probe cable and case - so no need to go super low here and a few pF total are unavoidable.The second point is injected common mode signal from the transformer drive. This is from capacitive coupling from different point's in the windings. If this is not symmetric some of the drive voltage can appear as common mode voltage. For this second part shielding can help, but shielding also adds to the capacitance to ground. In a higher frequency instrument the capacitance to ground is more important than with a DMM.

author: Daixiwen
author_rank: Frequent Contributor
timestamp: September 11, 2018, 12:09:22 pm
content: Quote from: Kleinstein on September 08, 2018, 06:55:22 amAt last! A practical use case for uBeam!

author: ogden
author_rank: Super Contributor
timestamp: September 11, 2018, 06:38:15 pm
content: Quote from: Daixiwen on September 11, 2018, 12:09:22 pmIt could actually work, just transformer oil instead of air between TX/RX transducers shall be used.

author: mk_
author_rank: Regular Contributor
timestamp: September 11, 2018, 08:44:28 pm
content: btw: Tek did a similar transfomerdesign 20 years ago...http://w140.com/tekwiki/images/6/61/Ths720std-scopeanalog.JPG

author: thm_w
author_rank: Super Contributor
timestamp: September 11, 2018, 09:45:46 pm
content: Quote from: coppercone2 on September 09, 2018, 06:24:03 pmYeah series/parallel would work, but think about the board space difference (as you said), and efficiency losses every step of the way.Not for power, but see fig 4 and 5 here:http://www.analog.com/en/analog-dialogue/articles/transformer-coupled-front-end-a-d-converters.html

author: BrianHG
author_rank: Super Contributor
timestamp: September 11, 2018, 11:30:47 pm
content: Quote from: mk_ on September 11, 2018, 08:44:28 pmThose look like the custom super-isolated SPDIF isolation high voltage coils, except my primary and secondary were in transparent tubes like the blue coils, around 15 years ago, made by Inductors Inc, I think, or, one of their associated predecessors.

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2018, 11:26:22 pm
content: Somewhere I have a couple of high isolation off-line power supply modules with similar spaced construction of the power transformer but they operate on 60Hz. The nameplates specify both isolation voltage and capacitance in picofarads but in a practical application, external common mode capacitance would dominate.For isolated oscilloscope inputs, there is an assumption that the probe ground is attached to a low impedance source which is not a problem for their intended applications. They achieve better common mode rejection than a high voltage differential probe assuming that their lower common mode impedance is acceptable. If you were desperate, you might use two together for a differential measurement.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 15, 2018, 09:13:31 pm
content: David Hess - "Somewhere I have a couple of high isolation off-line power supply modules with similar spaced construction of the power transformer but they operate on 60Hz."Here are some photos of a 60Hz transformer that was designed for extremely low coupling for a metrology application 70-80 years ago. The secondary has a copper shield around 3 sides and is held in position by lucite plates. The shield doesn't complete on the 4th side because that would be a shorted turn. You can see the approximately 1/8" minimum spacing between the secondary coil form and any metallic part of the transformer. Breakdown voltage was not a consideration, just capacitive coupling. One metal and one lucite plate have been removed so you can see the construction.

author: BrianHG
author_rank: Super Contributor
timestamp: September 15, 2018, 09:48:36 pm
content: Quote from: ArthurDent on September 15, 2018, 09:13:31 pmThat would be a great transformer design for my preamp/DAC modules. I wouldn't need any separate line isolation transformers or any Y caps. It would almost be like the device was battery powered with no connection to the wall.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 15, 2018, 10:21:45 pm
content: "It would almost be like the device was battery powered with no connection to the wall."That was basically what the supply was used for. It replaced a dry cell that powered a piece of equipment and they wanted the A.C. supply to be as noise free as the original battery supply and eliminate the hassle of replacing batteries.

author: David Hess
author_rank: Super Contributor
timestamp: September 16, 2018, 01:45:59 am
content: The one I have looks very much like that. The enclosure is Lucite plastic or similar.

author: coppercone2
author_rank: Super Contributor
timestamp: September 16, 2018, 02:29:09 am
content: I think that is commonly known as the R-core

author: TerraHertz
author_rank: Super Contributor
timestamp: September 16, 2018, 01:10:03 pm
content: Quote from: mzzj on September 09, 2018, 05:31:42 pmI'm glad _someone_ mentioned this. Yeah it's great for huge voltage differentials. I saw one used to run electronics up in the top Faraday sphere of a giant VanDeGraph generator used to power a particle accelerator. Several MeV, used a big rubber belt to carry charge for the HV, and also used to turn a generator up in the top.But even for this small instrument I don't understand why this isn't an obvious solution. Seems like a good one to me. One small brushless motor, one fiberglass shaft, four small brushless permanent magnet generators. Capacitive distances in centimeters rather than millimeters, much higher power throughput, and sine wave outputs. So what if it has one moving part?One thing I don't understand with this scope. The probes are standard form, with the 'ground' clip connecting through the probe coax braid, via the isolated BNC connector, directly to the shield box on the ADC blocks, right?Doesn't this result in quite a lot of parasitic capacitive coupling between the channel ground/reference lines? Which could upset operation of the system being probed?Hmmm... but I guess the only way to eliminate that would be to have the ADC box up at the probing point, so you'd have fiberoptics from there back to the common digital signal processing box. But that gets you back to the problem of how to supply power to the ADC box, though a no-copper cable along with the optic fibers.Ha ha... compressed air turbine? Great, a scope that makes a dentist drill whine... (But I expect it could be silenced.)

author: coppercone2
author_rank: Super Contributor
timestamp: September 16, 2018, 02:12:03 pm
content: what is the cuttoff for differential mode noise for a motor mechanism? I imagine the shaft loses alot of energy to torque, the coils have some kind of impedance too

author: David Hess
author_rank: Super Contributor
timestamp: September 17, 2018, 10:41:19 am
content: Quote from: TerraHertz on September 16, 2018, 01:10:03 pmThat is a consideration; the probe ground needs to be attached to a low impedance point for proper operation. Unlike a differential probe, the common mode impedance is relatively low.

author: mzzj
author_rank: Super Contributor
timestamp: September 17, 2018, 06:35:19 pm
content: Quote from: TerraHertz on September 16, 2018, 01:10:03 pmBearings are the weak point of motor-generator pair. And it's relatively expensive compared to simple transformer, typical price range industrial thumb-sized brushless motors what I have seen is around 100-200 eur per pc!

author: TerraHertz
author_rank: Super Contributor
timestamp: September 18, 2018, 07:52:24 am
content: Quote from: mzzj on September 17, 2018, 06:35:19 pmBearings - PSZ ceramic sleeve bearings. As used in pond pump motors, so they are pond-dirt cheap.Small brushless motors - You're looking in the wrong place then. Under $5 is more realistic. For eg:https://www.aliexpress.com/store/product/Fast-Shipping-11-1V-10430rpm-Brushless-Hall-Sensor-DC-Motor-Three-phrase-External-Rotor-for-DIY/2397059_32872311023.htmlOr just make it easily replaceable, and use an under-$2 cheapo standard brushed DC motor.With the right mass-produced cheap brushless motor frame you could probably use the same 'motor' units for the four generators too, just leave off the hall sensors and driver IC. Replace with a 3-ph diode bridge and regulator.

author: BrianHG
author_rank: Super Contributor
timestamp: September 18, 2018, 08:37:10 am
content: Quote from: TerraHertz on September 18, 2018, 07:52:24 amOnly need 1 motor, and 4 generators, on a belt drive.The belt and pulleys cost a fraction than the drive electronics, it's EMI shielding, overall size and weight compared to having 3 additional motors.Or, skip the motors and make a huge spring wind-up generator. No motor side drive electronics, or, load on the scope's logic side's power supply....

author: coppercone2
author_rank: Super Contributor
timestamp: September 18, 2018, 06:26:17 pm
content: if you do this for gods sake include spare bearings and belt inside the chassis.and no brushed DC motor. That's garbage.

author: rf-design
author_rank: Regular Contributor
timestamp: September 23, 2018, 11:18:15 am
content: Quote from: BartSchroder on September 09, 2018, 05:31:39 amLet me contribute a little bit to the now common puzzle:1. Common-Mode-ImpedanceAs somewhere below indicated the isolated scope have a much lower isolated-common to chassis-common impedance. So the DUT iso-common connection should be able to drive the iso-common mode impedance without disturbing this voltage in relation to the iso-probe tip voltage. For medium to high power bridge application the effect could be minor but lower power application could be also affected by the stated butnotnumbered iso-common mode impedance.From the local probe-tip common to iso-gnd on the board it is the common-mode inductance of the cable. Typical the probe cables have only resistive center but metalic outer shell. The resistive lead allow a flat higher frequency response with typical 1M scope loads. The common-mode inductance resonate with iso-capacitance. This is in part the stray from the built and the coupling capacitance of the iso-power transformer. So resonate frequency could be within the measurement frequency band and the currents could be still significant if you meet the frequency. One solution is to make also the coaxial-shell resistive to damp the common-mode impedance Q.2. Symmetric Isolation Power TransformerFiguring out the equivalent circuit diagram there are 4 stray inductance. 4 coupling capacitance and one coupling inductance. The driving voltage should be as symmetric as possible to minimize dynamic charge injection. This a very important property for low voltage measurements where dynamic charge could disturb uV measurement which are still DC but get shifted by nonlinear high impedances. In Bench-DMM this property is well understood. For iso-scope application not only the PWM should be 50% but also the wave-shape, or rise/fall, of the driving should by balanced. In effect the balance should be on the charge level. So adjusting modulation amplitude to counteract four-coupling capacitance mismatches in the actual iso-transformer is a common procedure in special HV measurement builtups.3. Iso-Power Transformer IntermodulationDepending on the stray-cap ratio of the transformer to the board part of the iso-common-mode current will flow through the alternating bridge parts on the driving and rectifying side. This create an intermodulation voltage which is seen also in perfect charge-balance. This could be disturbing of this power-drive signal is seen in the scope-signal an will be more problematic for higher iso-common source impedance from the DUT.Overall the product is a very reasonable approach for 200MHz measurements with low impedance common-mode drive. At higher bandwidth it is difficult to get the GS/s ADC's to the isolation side. Tek and R&S play the analog-signal isolation game. Fluke is doing analog opto/magnetic. Also if the common-mode terminal gets high impedance the impact could be higher than the measurement value.There is up to know no golden and cheap solution but we are a step further. The Remaining is my personal puzzle.Thanks Bart for yours and your group contribution.BR Reiner