very low capacitance isolation transformator DIY for mains - Page 1
Forum section: electronics / projects

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 09:55:31 am
content: here is what i came up withhttps://youtu.be/4rNubfeICOEI have done many of them, just two normal Toroids, just two normal E cores, rewound E core, and rewound toroids, one of my previous best is here:http://webx.dk/oz2cpu/230V-trafo-isolation-low-capacity/230V-trafo.htmSTL files :https://www.thingiverse.com/thing:5317001LTSPICE simulation file is also in the thingiverse folderTransformators : TRT300212 (300VA 2 x 12V) talemai am in the making of a follow up videothat should explain a bit deeper the workings of this, and the design challenges of ultra low resistance.

author: jonpaul
author_rank: Super Contributor
timestamp: March 14, 2022, 11:01:16 am
content: We use Topaz medical isolation transformers 0.001 pf readily available as surplus, expect poor regulation and high stand-in lossesDIY windings difficile as Shiel needs safety isolationJon

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2022, 11:18:32 am
content: Quote from: jonpaul on March 14, 2022, 11:01:16 amThe 0.001 pF are obviously not the full capacitance, more like the coupling capacitance after grounding the shield(s).The contruction in the video is a bit odd to use 2 pieces of wire for each loop: one side could easily be directly connected and thus only one connection needed.For the voltage sagging there is not only the resistance of the link, but also the inductance. This may be reduced with the transformers stacked in a different way.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 12:24:36 pm
content: kleinstein, could you please add a drawing of how you suggest to orient the two toroidsto improve this design ?I got two tiny toroids i use for demo of varius suggestionsand i am in the making of a new video, that covers all sorts of possible ways to do it.here are the first 3 experiment pictures attachedA 10 turns, thru them bothB 10 single turns thru them bothC 10 turns thru trafo 1 connected to trafo 2 using 10 turnsso in all 3 examples there are 10 turns to talk about and to compare the output power under load.with one of A/B/C do you think is the best ? and the worst ? equal ?please do not cheat and look thru my YT channel for the answer :-)

author: werediver
author_rank: Contributor
timestamp: March 14, 2022, 12:30:58 pm
content: A relevant video at Marco Reps channel:Extreme isolation (low leakage power in precision electronics).The author explores different approaches, makes capacitance / leakage measurements, picks a balanced solution that suits his needs.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 01:15:12 pm
content: thanks werediveri allready link to that one in my video description :-)however it is not for mains, and it is not for full rated power the toroids capacity

author: jonpaul
author_rank: Super Contributor
timestamp: March 14, 2022, 01:45:22 pm
content: Klienstein:The use of a shield in low cap trsf means Cps is measured with shield as a guard.That results is perhaps 1/10 ...1/200 of unshielded capacitance.All the Topaz and similar ultra isolation transformers use this method.If the shield is unused there is no point to having Faraday shield.Jon

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 01:49:42 pm
content: if you add a shield, your total capacity gets higher !you measure the total capacity by taking all wires from one side together, and measure to the other side,so shield will be connected to one side = higher capacity..

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 05:38:58 pm
content: here is the follow up videohttps://youtu.be/eSZdO_Uca40

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2022, 06:02:56 pm
content: Not useful for your benchtop purpose, but back in the day we needed isolation transformers to power a "dome" terminal floating at high voltage (100 kV DC). We used two toroidal cores from scrapped Variacs (much as shown above) and looped several turns of RG-34/U high-voltage coaxial cable through them, grounding the shield only once, to get a reasonable AC voltage (somewhere around 12 to 24 V) to power the DC supplies in the dome. Of course, we had to strip back something like 30 to 50 cm of shield braid at both ends of the cable, and carefully terminate the shield ends, for HV purposes.Low-capacitance was not in the requirement, but the capacitance from the primary winding to the secondary would have been very low, although the capacitance from the secondary to ground (coax capacitance) was not trivial.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 06:25:55 pm
content: have you see the Austin ring transformer ?https://en.wikipedia.org/wiki/Austin_transformerhow many pFhttps://tower-pro.com/austin-ring-transformer/could be cool if a more bench sized version existed :-)Attached : my own 3D printed sec winding and machine to wind the wire to the rim.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2022, 08:01:45 pm
content: What VA? smaller is better, but it's a compromise. Two transformers back-to-back results in poor regulation. Shaver sockets, are a good source of small isolation transformers.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 08:18:33 pm
content: Shaver sockets ?ROTFL :-)

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2022, 08:29:37 pm
content: Note that small (lightweight) transformers usually have a lower-than-nominal step-down ratio, in order to deliver the rated voltage at rated current despite losses in the iron and leakage inductance.(Weight and cost are more important for the usual application of these devices than efficiency.)Therefore, when run backwards as step-up transformers (second transformer in an isolation pair), the output voltage will be lower than expected.

author: Hydron
author_rank: Super Contributor
timestamp: March 14, 2022, 08:30:41 pm
content: Quote from: oz2cpu on March 14, 2022, 08:18:33 pmThey're even better when you realise that the small, inefficient transformer inside CANNOT be turned off (because that would require a switch, which will obviously instantly electrocute you despite decades of advancements making them effectively sealed to minor water splashes), constantly sucking a few watts due to magnetising current.And then you get to the pull-cord lights (because again, people cannot be trusted with a modern switch in a bathroom unless it's meters away from water, impossible in the UK where there won't be room to swing a cat), which have the bonus of instantly attracting dirt and becoming gross

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 08:34:04 pm
content: Exactly Tim, a transformator is turn compensated for its loss,this means under load, X volts in gives Y volts out, as written on its label,but if you reverse the direction,give it Y volts in, and expect to get X volts out, you get the loss factor less,That is of course expected. and a good point to mention.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 14, 2022, 08:39:49 pm
content: and about shaver sockets :please take one, and measure its capacity like it did in the first video,tell us the pF value,and then please try to light up a 60W bulb, and tell ud the loaded output voltage.You need a little bit of load capeability too, for your lab experiments,lets say you need to handle your scope, or signal generator, or power a switchmode supply where you need to measure in the primary part of it,you surely need it to be well isolated.

author: Gyro
author_rank: Super Contributor
timestamp: March 14, 2022, 09:04:09 pm
content: Quote from: oz2cpu on March 14, 2022, 08:39:49 pmOut of interest, I just went and did that very thing (I keep a salvaged shaver socket transformer for low power isolation purposes). Primary - Secondary: approx 40pF Secondary to chassis: approx 75pF. I shorted the primary to avoid errors due to any inductance contribution.QuoteYou'll be lucky. They're rated for about 20VA max.Edit: With 75pF from each winding to chassis / EI laminated core, that must be the main contribution to the Primary - Secondary capacitance on a side-by-side split bobbin isolating transfomer. TheactualPrimary-Secondary coupling capacitance is probably very low.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2022, 09:36:47 pm
content: For most uses one does not need super low capacitance between the primary and secondary.Often the more anoying part is that the capacitance is a mix of both sides of the turns and there is thus some effective voltage between the 2 sides. This gets better with shields, especially with 2 shields for the primary and seconday. Than the effective voltage gets smaller (a fraction of a turns voltage) and with case in the symmetry this can be a small fraction.A main use for an isolation transformer is for protection / safety and there a few 100 pF are not such a problem. A simple torroidial transformer may still not be that good, as the isolation is relatively thin and may fail when over heating. So it would be important to have reliable protection from overload.Quote from: oz2cpu on March 14, 2022, 12:24:36 pmIt is not necessary an improvement, more an alternative configuration with somewhat more coupling capacity and less inductance in the coupling link. The idea is to have the cores stacked along the axis. So like in the much smaller Metron low noise DCDC converter.This way the couping link goes directly from one hole to the other and than can go around the cores/ primaries on all sides. Seprating the return wires to all sides decouples the parts and should reduce the unwanted inductance. I don't know for sure of the inductance is really relevant. Chances are it is on a large scale - it just depends what is large. In transformers in the >100 kVA range some exra spacing that causes extra inductance is used to limit the short circuit current. So at that scale the inductance is quite significant.

author: JohnG
author_rank: Frequent Contributor
timestamp: March 15, 2022, 12:10:33 am
content: Quote from: Kleinstein on March 14, 2022, 09:36:47 pmYes, one effective coupling turn is all that is needed, as long as it can carry the current. You can extend this concept if you wish by using a small pipe as the center conduction and a large diameter pipe as the outer conductor, with disc-shaped conductors on each end.Note that whether you use a wire loop or a full coaxial loop, if you tie a single point on the coupling loop to an AC ground, you get a "poor man's" interwinding electrostatic shield that is remarkably effective. Not as good as a full electrostatic shield, but very good and very simple to implement.If you use multiple coupling turns, this becomes less effective.John

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2022, 08:04:27 am
content: Quote from: Hydron on March 14, 2022, 08:30:41 pmThere is a switch, which is activated by plugging something into the socket.Anyway why not a shaver socket isolation transformer? The original poster didn't state the size. The title is ambiguous. I thought "very low capacity" meant small/low VA rating, until I read the replies and realised he was referring to capacitance. A shaver transformer is wound on a double bobbin and small, thus lower capacitance than most off the shelf isolation transformers.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 15, 2022, 09:01:07 am
content: Zero999 :thanks a lot for pointing that out, i took the freedom to edit that word in the topic "low capacitance"hope it is more clear now, what the main goal is..Kleinsteinthanks a lot for that idea, i have attached a picture to show if i understand you correctlytwo toroids are stacked, black on top of orange,i only drawn 4 seperate windings, spread out over the entire outer surface,like my follow up video, i will make 10 separate windings, and see how this performs,in the follow up video, i only care about power conversion, as the main factor, when that is solved, a low pF solution is wanted,lets imagine you need as much total copper, like in my 300VA project, all that copper outside those two toroids,will create a lot of capacitance.shortest possible wire length = low ohmas little trafo 1 to trafo 2 capacitance

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2022, 09:44:16 am
content: Everything is compromise: efficiency and regulation will suffer if two toroidal transformers are connected like that. I know it sound crazy but how about a motor generator set, rather than transformers?

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 15, 2022, 10:12:18 am
content: it is not crasy at all, that is the way to go, if we need to scale up to kW size.my main goals was:as low pF as possible, TOP PRIORITYcould afford to loose a bit regulation (most stuff i work with will handle a wide range of mains voltage variation)must be in sync with mains frequency (for easy scope trig)must be silent (motor-generator not possible)must be possible to make from normally accessible partsmust be compactmust be able to transportpower load range 0-300 wlow costno special mechanical parts, preferred all parts to be 3D printed

author: JohnG
author_rank: Frequent Contributor
timestamp: March 15, 2022, 12:39:04 pm
content: Quote from: oz2cpu on March 15, 2022, 09:01:07 amIf you want low capacitance between windings, you need to space the toroids apart, and make sure the spacer is mostly filled with air. 3D printing is your friend here. It will add leakage inductance, though.Also, the term capacitance can mean a lot of things. An electrostatic shield between windings reduces capacitance between windings, but can increase the total capacitance of the whole system. You can steer this around a bit by moving the shield closer to one or the other winding. Like I said, if you AC ground the coupling winding, you will reduce capacitance between windings a lot.John

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 15, 2022, 01:22:45 pm
content: exactly John..shield is one method to affect AC coupling between two objects,however adding a shield will lower the capacitance between the two objects,i want low total capacitance, this way ac currents between them, will be as small as possible,and it will give me the freedom to measure between what ever points i like to

author: TimFox
author_rank: Super Contributor
timestamp: March 15, 2022, 01:43:37 pm
content: With a large metal object, such as a transformer, substantial capacitance (to somewhere) is inevitable.If you really need very low capacitance from the secondary to everything, you must use an active guard, where the interwinding shield is driven by a buffer amplifier to minimize the voltages across the physical capacitances.

author: JohnG
author_rank: Frequent Contributor
timestamp: March 15, 2022, 02:29:34 pm
content: The total capacitance of a multi-conductor system, meaning the ability of a system to store energy in an electric field, will depend primarily on the size and separation of the conductors and the dielectric constant of the dielectric separating them. If the conductors are close, the shape of the conductors matters, but as they get farther away, it matters less and less.So, if you are trying to minimize total capacitance, your only option is to make the toroids small and far apart, and maximize the separation of the conductor used for the coupling loop from the toroids. Eventually, you run into a problem because if you make the toroid small, it has to get closer to the conductor forming the coupling loop. Also, you run into other practical concerns such as leakage flux and loss that gets higher due to the same things that make the capacitance smaller.So, just to say minimizing the total capacitance will lead to a dead end, so you need to find some number that is an acceptable minimum. The only way out is to forget the transformer and use electromagnetic waves in the far field.John

author: Terry Bites
author_rank: Super Contributor
timestamp: March 15, 2022, 02:43:20 pm
content: Toroids by their very nature have the worst leakage of all!Look for a split bobbin with shield windings

author: JohnG
author_rank: Frequent Contributor
timestamp: March 15, 2022, 04:38:20 pm
content: Leakage flux has more to do with winding arrangement than anything else. If there are two windings and they are far apart, then there is more room for flux from one winding that does not travel through the other winding, i.e. leakage flux. On the other hand, if the windings are properly interleaved, wound bifilar, or the conductors are coaxial, then the leakage can be low no matter what kind of core.For example, if I wind a toroid with a lot of turns of low-impedance transmission line, I can build a low leakage transformer. If instead I wind each winding with the same number of turns, but each winding is in a small bunch on opposite ends of a diameter of a toroidal core, the leakage will be much higher. Same core, same turns ratio, different leakage.The problem is that reducing the leakage means reducing the space between windings so that the easiest path for flux is a path through both windings. This means putting them in close proximity, which in turn means increasing the capacitance between windings.John

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2022, 05:36:00 pm
content: You need a core type transformer, for low capacitance. 300VA is on the small side, but they do exist.http://jamestransformer.com/en/transformer/CD_core_transformer.htmlhttps://nicoreindia.com/transformer-core/

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 15, 2022, 05:59:44 pm
content: thanks Zero, not a bad idea, looks easy.I have made quite a few Shell types too, but i newer made a CD Core as you link too..cant really see it will deliver a lower capacitance ?as you saw in the first video, my two toroids ended with 7pFThis one is 22pF, and it is my site:http://webx.dk/oz2cpu/230V-trafo-isolation-low-capacity/230V-trafo.htm

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2022, 06:41:21 pm
content: The CD core can be slightly lower capactiance than a shell type core, but no a low. In both cases one has capactance from the winding to the core. The direct winding to winding capaciance can be a little smaller.CD cores can use grain oriented core material and this way use a higher magnetization. So the same size transformer can transfer more power (e.g. 1.5 times). The better core material is also often a factor with torroid transformers.The 2 stacked torroids could still be a good solution if the balance is more on the low cap side. Anyway it used 2 cores and thus naturally has more loss and also more winding resistance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2022, 08:04:14 pm
content: Quote from: oz2cpu on March 14, 2022, 06:25:55 pmHah, I've done that, incidental to making an induction power supply:Was just the easiest way to get copper tubing (a couple hundred amperes at 50kHz -- water cooled), two turns through a primary.As for low capacity, I don't know why the hell you need it this low,but sooner or later you'll find iron-core transformers can't go any lower. The only real solution beyond that, is to grow size and shrink distance. See where this is going? Do it with an SMPS. With carefully shielded windings and balanced operation, EMC can be made reasonably low, minimizing required filter capacitance (or EMI requirements can be relaxed or waived if this is a one-offy sort of thing).In general, increasing the frequency shrinks the transformer. You can do it with air-core transformers and loose coupling at some MHz up, or compact antennas at 100s MHz, or hell, do it at optical frequencies with LEDs or lasers and photovoltaics (albeit at quite low overall efficiency, unfortunately). The latter method is the choice for certain obscure, highly demanding applications: HVDC power converters (~1MV links between 1000s km) use stacks of SCRs (or relatives), with integrated drivers synchronized and powered optically. Or Tektronix has (and maybe others, I don't know) an isolated probe that sends power as well as data via optical fibers; the head unit is essentially the whole-ass front end of a digitizing scope, powered by laser beam, and sending high bitrate data back.Tim

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 15, 2022, 08:51:01 pm
content: man you rock !! 2 windings :-) i just love it..watercooling, that is cheating !>powered by laser beam, and sending high bitrate data back.WHATthat is from another planet, right ?by the way here is the (hopefully) last follow up videohttps://youtu.be/H4WUNACZ-rA

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2022, 09:37:50 pm
content: https://www.tek.com/en/products/oscilloscopes/probes/isovu-isolated-probesTim

author: JohnG
author_rank: Frequent Contributor
timestamp: March 15, 2022, 09:47:17 pm
content: Quote from: T3sl4co1l on March 15, 2022, 09:37:50 pmYes, these are really nice probes! Too bad they only work with one brand of scope. Better take a deep breath before you look at the price tag...I do not think they digitize in the probe head. I think they use a Mach-Zehnder modulator and transmit analog data. But, I could be wrong. I am not going to do a tear-down...John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2022, 11:37:31 pm
content: Yeah, that one's probably analog. I was probably thinking of a combination of things. I also saw Lecroy has one but it's battery powered (link is FM, so, arguably digital?); and there was that scope with high isolation inputs (in the base unit, uses conventional probes) with short distance optical links, which I think was digitized on the high side).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2022, 07:27:02 am
content: At a low power level there are also acoustic transformers: The energy is transmittet via ultrasound through a solid (AFAIK usually some quartz glass). It is kind of mechanical, but still low noise and low wear. There are some references on this but I could not find any commercial offers.

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2022, 02:22:13 pm
content: At university, a project involving 1 MV on the high-voltage terminal used a dielectric (ceramic) waveguide to beam power from a CW magnetron up to the terminal where it was rectified for DC power in the terminal. Still some capacitance due to the dielectric waveguide, however.

author: branadic
author_rank: Super Contributor
timestamp: March 16, 2022, 06:20:14 pm
content: Let me just point you to a replica of the Metron transformer mentioned before:https://www.eevblog.com/forum/metrology/power-supply-for-voltage-references/msg3411708/#msg3411708-branadic-

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 16, 2022, 07:16:07 pm
content: wow : see this picture :-)so now you are impressed with my 7pF ??

author: branadic
author_rank: Super Contributor
timestamp: March 16, 2022, 08:03:21 pm
content: Not really, I think I made the measurement wrong, as the screens were just floating.-branadic-

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 16, 2022, 08:39:25 pm
content: i think you are right, since such a tiny dual toroide constructionshould have much less pFcould be fun to see this design tried, with "my" way to orient the two toroids ??any takers ?at least if you got a few extra toroids and the 3D printed parts, try to measure pF on both constructions ?with exactly the same wire, and turns, and total wire length.any takers for this challenge ?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2022, 10:05:08 pm
content: The 2 torroids stacked on the axis has the advantage of less inductance (this gets more important with higher frequency) and less magnetic emissions.There are usually also 2 extra shields to reduce the injected current after shielding. Often this current is the more important parameter than the total capacitance / capacitance between the shields.To get less capacitance a aspect ratio if the transformer with more hole and less thickness would help to allow a little more distance.

author: JohnG
author_rank: Frequent Contributor
timestamp: March 16, 2022, 11:23:51 pm
content: You can determine the capacitance of an isolated sphere in free space by C=4*pi*e_0*r, assuming an empty universe with no net charge.You can approximate the capacitance of a simple object to free space by picking the smallest sphere that the object will fit in (as long as the aspect ratio of the object is not too large), and determining the capacitance for that. So, if one of your toroids is 2.5 cm in radius, that's about 2.8 pF to an empty universe for a single winding, and you can never be less than that. You have a second core much closer than the edge of the universe, so 7 pF is not bad without a shield between the two windings. You can see that getting below a pF will need something that is small and far from anything.John

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 17, 2022, 08:24:54 am
content: exactly John, i did see a bit over that, but I now know my conductive table contribute to my measurements more than i first realized,I am in the business of acquiring a thick solid copper bar,so if this happen soon, i will make a new follow up :-)