Talk:Toroidal inductors and transformers

formula needed
This page ought to have formulae for calculating inductance of toroidal transformers. I have an air-core, toroidal transformer for which i want to know the inductance. The given external link, ostensibly a toroidal calculator, is no help; and since it is also littered with ads, i'm thinking it is too spammy to place on wikipedia. -- 99.233.186.4 (talk) 00:54, 16 September 2010 (UTC)

Proposing to delete the Sentence "The magnetic flux in a toroid is largely confined to the core, preventing its energy from being absorbed by nearby objects ..."
Energy does readily get out of a toroidal inductor into nearby objects. For example, the nearby object may be the secondary coil of a toroidal transformer. And it is a good thing because if energy could not get out of a toroidal inductor then a toroidal transformer would not work, because the primary is a toroidal inductor. But, there is a grain of truth in this sentence. It is easier to shield a toroid than some other shapes. A lot of the benefit comes from having a closed magnetic path. It is the axial symmetry (taking the axis to be the line through the center of the donut hole) and a restriction that the current does not have a "toroidal" component that leads to the conclusion that the "lines of flux" are axial circles of constant intensity. Combine that with Faraday's law and one can compute that the B field is zero outside the core of the toroidal inductor. But it depends on having axial symmetry. If there is a chuck of magnetic material next to the winding, the symmetry is broken there is a non-zero B field outside the core of the toroid. As a didactic example, the toroidal coil is a poor example because it leaves the student thinking that he has mastered the idea when all he really did was learn a non-general special case.

I propose that the entire sentence be deleted. Constant314 (talk) 00:56, 7 October 2010 (UTC)


 * The flux is largely confined in a high &mu; toroid. I would not delete the statement, but a modification might be appropriate. There is a B field outside of the core; toroids have leakage inductance.
 * BTW, typical procedure is to put a dubious or disputed tag in the article statement that points to this section.
 * Glrx (talk) 04:31, 7 October 2010 (UTC)


 * Regarding your comment “dubious or disputed tag” would you kindly point me to a page that tells how and why to do that?


 * WP:AD, WP:dubious. Glrx (talk) 15:57, 8 October 2010 (UTC)


 * The flux is largely confined to the core if the core is made from a high &mu material and has a closed magnetic path, even if the material in no toroidally shaped. The ideal axially symmetric toroid sufficiently far from anything that would break the symmetry actually has zero external flux.  And of course if it is far enough from everything, there is nothing to which couple energy.  But the sentence as written is more wrong than it is right.  Energy is not prevented from being absorbed by nearby objects.   It is not self shielding; two toroidal inductors near each other have a significant non-zero mutual inductance.  And it is only true that the flux “is largely confined to the core” in special cases where the axial symmetry has been preserved.  So, if the choice comes down to having this sentence or deleting it, I think it is a better article without it.  But, fixing it would be great.  I think a statement like the following would be correct. “The toroidal form is somewhat easier shield than other forms.  In some cases the form contains the magnetic field sufficiently enough to not need further shielding.” Constant314 (talk) 13:02, 8 October 2010 (UTC)


 * I disagree with your mod: consider a pot core. Shielding is about attenuating, and closed high &mu; mag path confinement does provide shielding. Glrx (talk) 16:12, 8 October 2010 (UTC)


 * I will take your suggestion and use the dubious method.Constant314 (talk) 06:52, 9 October 2010 (UTC)
 * It is interesting to note that the green torroids in the picture are not wound in a way to take advantage of B field confinement. Instead they act as a one loop coil in the plane to the toroid. Constant314 (talk) 16:31, 11 October 2010 (UTC)


 * I find your fix to agreeable. I am inspired to add a section about field confinement in a toroid. Constant314 (talk) 01:04, 19 October 2010 (UTC)


 * You are free to improve my fix; I don't think it is great. I'll look over the technical details of your field confinement edit. Your wonderful images may be appropriate for Rogowski coil -- where avoiding the residual winding is important. Glrx (talk) 03:06, 20 October 2010 (UTC)


 * if there is zero external flux, how does the Rogowski coil work? is that because the current generates a flux, which disturbs this property? i would welcome discussion of field confinement in the article, that includes cases when it does not apply -- 99.233.186.4 (talk) 03:21, 14 November 2010 (UTC)


 * Not sure about the Rogowski coil.  I have a (stalled) work in progress on the torroidal transformer.  The basic story is this: the B field is zero, but the A field (magnetic vector potential) is non-zero.  The time derivative of A adds a component to the E field.  The E energizes the secondary.Constant314 (talk) 04:46, 14 November 2010 (UTC)


 * There is a reciprocity theorem. If, in a certain circumstance, a particular current in the primary causes a particular open circuit voltage in the secondary, then the same current in the secondary causes the same open circuit voltage in the primary.  In other words, the coupling inductance is the same no matter whether you drive the primary or the secondary.  So, you can analyze the toroidal transformer with a one turn secondary you can infer the action of the Rogowski coil.Constant314 (talk) 15:40, 14 November 2010 (UTC)


 * The Rogowski coil does a closed contour integration -- summing the induced voltages around the loop. Then Green's theorem gives the flow rate of electrons through the loop's surface. Glrx (talk) 02:17, 15 November 2010 (UTC)
 * I had a quick look at some of the papers on the UK web site. My impression  is that they are terminating the Rogowski coil with a low impedance so that it functions as a current transformer.Constant314 (talk) 10:31, 15 November 2010 (UTC)

More in-depth pictures
Pictures/Diagrams clearly showing primary and return winding terminus needed. As it is, it looks like the just dissapear. —Preceding unsigned comment added by 184.76.222.168 (talk) 02:43, 18 December 2010 (UTC)
 * which section?Constant314 (talk) 04:16, 18 December 2010 (UTC)

Better Article Needed
I doubt a lot of scientists come to Wiki for information on toroids. This should have a leading paragraph that states the information people want to know, which is WHY someone would want to use a toroidal transformer instead of a standard square unit. This article doesn't mention the word audio, or even radiation.--75.79.150.41 (talk) 07:10, 19 October 2011 (UTC)
 * We all look forward to your contribution to the article. cite it and write it!. --Wtshymanski (talk) 13:44, 19 October 2011 (UTC)


 * I think the article should tell what a toroidal inductor is before it tells why someone would want to use it. But telling why someone would want to use it surely belongs somewhere in the article.  Constant314 (talk) 20:51, 19 October 2011 (UTC)

Original research
The field confinement and vector potential sections of this are absolutely correct, beautifully illustrated, and clearly explained. However, the only citations are in the initial paragraph or so, and so I suspect these are original research. I also think they put undue weight on these topics which, while mathematically and geometrically elegant, are not of much practical importance. Perhaps they can find a home other than Wikipedia. What do others think?Ccrrccrr (talk) 00:18, 23 October 2011 (UTC)


 * Add references and keep them! Add more about other topics if you think these have undue weight.  Don't delete stuff. 71.167.58.122 (talk) 19:54, 21 November 2011 (UTC)

Lots of Useless Info.
This article s like an IBM manual; full of perfectly correct but entirely useless information. The basic equations are by Maxwell, and have been copied from an undergrad physics text book. Nothing to do with ferrite toroids, though. The word "saturation", highly pertinent for ferrites, is not mentioned anywhere.220.244.85.162 (talk) —Preceding undated comment added 01:55, 10 August 2012 (UTC)


 * The first sentence has links to transformer and inductor. They both discuss ferrrite. No need to duplicate that here. Constant314 (talk) 13:17, 10 August 2012 (UTC)

Misleading Illustrations, Inaccessible References, Confounding Explanations
Figures 5, 6, and 8 show an equal number of return wire turns to the primary winding in the opposite direction. Theoretically, that would neutralize (by shorting) all inductance. Further, the only references on this page are to books. Including internet references may enhance credibility. For example, http://www.cliftonlaboratories.com/toroid_and_solenoid_external_field.htm mentions the single turn effect. Finally, the sentence "No matter how many times the winding encircles the core and no matter how thin the wire, this toroidal inductor will function as a one coil loop in the plane of the toroid," implies the inductance is decreased without circumferential current compensation. Not the case. The afforementioned link better describes the "single turn effect."

The material may have merit but is badly presented. The presentation is so confusing that other users in ##electronics @ freenode IRC dismissed it as "free energy genius", and did not assist my understanding of it. — Preceding unsigned comment added by 76.104.2.80 (talk) 23:58, 10 September 2012 (UTC)


 * I can assure you that the inductors shown in figures 5, 6 and 8 do not have any shorted turns and function as normal inductors. As for your conclusion that the sentence "No matter how many times the winding encircles the core and no matter how thin the wire, this toroidal inductor will function as a one coil loop in the plane of the toroid," implies the inductance is decreased without circumferential current compensation, perhaps you can explain the reasoning that leads to your conclusion.Constant314 (talk) 04:17, 11 September 2012 (UTC)


 * As for the shorted turns, my mistake, sorry. Perhaps the article should clarify the return windings actually rotate the same direction as the primary, and therefore contribute a magnetic field in the same direction.


 * As for the wording of "No matter... one coil loop...", there are two distinct effects at work in toroids. As my reference states, "The typical toroid inductor can be considered to be two inductors. One is the traditional "circular solenoid" where the magnetic flux follows the core and the second is the one-turn loop..." Presently, the wiki article might be read to indicate both theoretical inductances are neutralized by lack of return winding. — Preceding unsigned comment added by 76.104.2.80 (talk) 16:19, 11 September 2012 (UTC)


 * I understand your point that the article may need clarification that the neutralizing winding only neutralizes the inductor in the plane of the torroid.Constant314 (talk) 00:14, 13 September 2012 (UTC)


 * I agree that the article has some flaws; my gripe is that it discusses theory as was mentioned above (textbook..) but says little about practical matters such as why it is used in certain applications. As was mentioned, Wikipedia serves a broad audience and tech and math theory doesn't serve much of that broad audience.  I use ferrite toroids all the time and it's nice to know some theory (in layman's terms) but does that really add meat to what the typical reader is looking for when he hungers for knowledge about this?  Even I'm not a typical reader so I think others should help answer that.  Perhaps the article should be labeled as incomplete.  What I would add is that toroids are found in many places that people don't even realize, such as on the ends of computer cables, as RFI/EMI suppressor sleeves.  But they are often used in power supplies, CFL lights and communications equipment in the form of transformers that are sometimes hidden inside of a package.  Why do designers use toroids instead of open core inductors such as bobbin cores?  I'm not a designer so I can't add any insight to that.  Signed by Watson 208.127.16.197 (talk) 12:55, 20 December 2012 (UTC)
 * Go ahead and add the information that you think should be there.Constant314 (talk) 16:17, 20 December 2012 (UTC)

This is actually quite a good technical article. One that slipped past the wiki police. That is lucky for the author because usually the forces of ignorance remove anything written by a domain expert. — Preceding unsigned comment added by 113.190.166.113 (talk) 02:21, 5 August 2013 (UTC)


 * No, what usually happens is that some other editors come along and try to clean up the mess left by the original editor, who couldn't be bothered to write a coherent introduction, or for that matter a coherent sentence of prose English, because he was too busy trying to impress us by regurgitating a marginally notable derivation in one tiny area of the subject matter, which he copied off the whiteboard in EE201:Introduction to Electromagnetics last quarter. Which is what's going on with this article.  -- Chetvorno TALK 07:04, 5 August 2013 (UTC)

how are they made?
disappointed not to have found in the article anything about the development of the toroidal device for use in power supplies (surely where most of them live), how they were invented & how they are constructed. to the uninitiated, it looks to be an impossible task to create a machine that will wind both primary & secondary onto a solid ring of ferrite without 'letting go of the wire' on each pass through the hoop. a picture, ideally... but a description. the article is incomplete without it. duncanrmi (talk) 14:35, 23 May 2017 (UTC)
 * There are machines that wind them, though I don't have any information. When winding by hand, you wrap the entire winding on a shuttle (think of a Popsicle stick with a notch on each end) and then pass the shuttle though the center hole for each turn.  It is slow and tedious. Constant314 (talk) 16:24, 23 May 2017 (UTC)
 * Here is a you-tube video https://www.youtube.com/watch?v=oBdJvT7tpSM     Constant314 (talk) 02:54, 26 May 2017 (UTC)
 * FYI User:Glrx/sandbox, but it is OR. Glrx (talk) 20:12, 2 June 2017 (UTC)

"Feynman" reference is missing
The reference to "Feynman" is missing from the bibliography. NotYourFathersOldsmobile (talk) 21:29, 3 February 2018 (UTC)

Many errors in English
There are a lot of errors in the English in this document, for example the magnetic field emerging from one end of the core have a long path through air to enter the other end. NotYourFathersOldsmobile (talk) 21:38, 3 February 2018 (UTC)
 * Be Bold!.--Wtshymanski (talk) 00:45, 4 February 2018 (UTC)

Very little of any use
A great deal of this article talks about 'circumferential current' and 'B field containment', which is (for mains transformers) largely misleading and useless. The important factors are (for all intents and purposes) avoided altogether, such as VA ratings, saturation, regulation (with linear and nonlinear loads) and everything else that people want to know. Circumferential current is left-field and inconsequential to 99.9% of users, and is almost never a consideration for mains applications. RF circuits are a different matter, but when most people look up 'toroidal transformer' it's usually to find out about standard mains transformers. Esoteric parameters are interesting, but should not dominate the content.

Many of the references are to books that are not accessed by the reference, some are circular (referring back to the page where they are shown), and IMO the page is a hijack - pushing a particular agenda and ignoring the things that most readers would want to know. It has already caused FUD (fear, uncertainty and doubt) amongst readers, for no useful purpose. There is no mention of the use of an external 'flux band' (copper ring around the outside of the toroid), which was common with E-I transformers but seems forgotten for toroidal types.

As it stands, the article is of no use to man or beast.

Rode666 (talk) 01:47, 19 July 2020 (UTC)


 * Yes, it's kind of a nice discussion that is hard to find most other places but it's bizarre to have that much focus on such a tangential issue (no pun intended) is this article. Ccrrccrr (talk) 13:31, 30 April 2023 (UTC)
 * Greetings, I am the editor that added all that stuff about B field containment and circumferential currents. Before I edited it, the article incorrectly claimed total B field containment for all toroidal transformers.  I felt like that needed additional information.  Maybe I went overboard.  I do take exception to " misleading and useless."  As for the stuff that is not in the article; it wasn't my choice.  Anyone is free to add the missing information. Constant314 (talk) 14:03, 30 April 2023 (UTC)
 * Thanks for the prompt reply! What you did there is really nice work.  I'm just not sure it belongs here.  I think the proper solution on Wikipedia when people are making false claims is to ask them to back them up with reliable sources, and when they can't, remove the false material.  On balance, I'd personally prefer to keep this material here, particularly given the great photos and other graphics, but I'm not sure it's proper per Wikipedia policies.  But maybe we can actually find some sources that discuss the effect of circumferential current.
 * Note that the use of discrete turns also leads to non-zero external field, even with the circumferential current is cancelled. That's only a near-field effect near the individual turns, assuming they are evenly distributed.  Non-uniformly distributed turns are also common in practice, as seen in the first figure in the article with the little green-core inductors. Ccrrccrr (talk) 15:22, 30 April 2023 (UTC)
 * Here's a source: https://iopscience.iop.org/article/10.1088/1742-6596/476/1/012053/meta
 * Thus, there exists a single-turn toroidal current which creates poloidal flux outside the toroidal core and a multi-turn poloidal current which creates toroidal flux through the toroidal core.
 * Ccrrccrr (talk) 15:38, 30 April 2023 (UTC)
 * I was new and inexperienced and eager to make a contribution. Today, I probably would not have added it.  However, a think it may only go against a guideline rather than a policy, in which case it only needs a consensus.  I agree with what you say about discrete turns.  I can never remember which way is poloidal so I have always opted for circumferential but I have no objection to changing the terms. Constant314 (talk) 17:49, 30 April 2023 (UTC)
 * Oh, I agree that circumferential is a clearer term! I do like "polloidal", but it's not exactly a term you can expect the average reader, even the average expert, to know.
 * When I get a chance to put some time into this article, I'll probably focus more on bolstering the other sections--in a longer article the length of the discussion of that issue would seem less like it's being overemphasized. And with a few references it will seem less like original research. Ccrrccrr (talk) 18:38, 30 April 2023 (UTC)
 * Sounds good. No objection from me. Constant314 (talk) 21:05, 30 April 2023 (UTC)
 * Rogowski coils are often wound like figure 5. Constant314 (talk) 01:02, 1 May 2023 (UTC)