Are R core transformers a thing?

[Alex Eisenhut]

https://www.aliexpress.com/item/1005004566890764.html

Never heard of or saw a transformer like this. Are they common?

[jonpaul]

Design and manufacture magnetics since 1970s, never heard term R core,transformer, perhaps a brand or trademark.

The linked Chinese mains transformers are a copy of the common 1960s design using a round cross-section tapewound cut  core, in a form to take a pair of core tubes for the windings.

Results are low C P..S, low external flux leakage.

Same as a tapewound toroid but easier and cheaper to wind.

Popular in audio amplifiers and preamplifier with classic linear power supply.

Nowadays the toroïdal mains transformers are common and wide range so these are seldom seen

Jon

[Alex Eisenhut]

Yeah it looks like a 19th century design with wrought iron in the core, right off Michael Faraday's desk! I mean it probably works well enough, I just never saw anything like it for sale now. And I never heard of R cores before either.

[jonpaul]

Dear Alex: Bravo to recall the toroid of Michael Faraday....

Recall also Nikola Tesla and Charles Proteus Steinmetz as transformer and electrical engineering pioneers.


The use of laminated, tapewound, cut core is indeed old school but still very relevant today, as certain high perm or low loss materials are only available in thin metal plates, lams or tapes.

It was Bell Laboratories that pioneered such materials as supermendur.

More recently amorphous steel was used in power util dist xformers to reduce core losses and save huge energy.

Though many apps use high freq/SMPS/Ferrie or powdered iron cores, the hgih Bmax and ow mains freq loses of steel, silicon steel, grain oriented steel, and the above material dominats in linear 50/60/400 Hz mains transformers.

I had design/mfg/consulting  experience with all of these shapes and some of the material

1/ 1971..1975 Sequerra (ex Marantz) FM-1 tuner PSU

2/ 1983..1986 12 kW cinema arc lamp ballasts 40 kG tapewound cut core.

3/ Isolation/transient/balanced power: 1990s Furman Sound.

I can assure you all of these materials and shapes are still designed and manufactured nowadays.

Happy to post images or links if anyone is interested.

Just the ramblings of an old retired EE!

Bon Soiree,

Jon

[SiliconWizard]

Yes they are, I've used a R-core transformer years ago in an audio amplifier design in place of a toroidal one.
Nothing new, I did that over 10 years ago. As I remember, they started to appear commercially around the beginning of the 2000's or so. They were probably based on older designs although I am not aware of them in this exact form in older available products.

You can see one manufacturer that's not a dodgy chinese one: https://www.custommag.com/r-core-transformers

They share the low leakage characteristics of the toroidal, but in a nutshell, the real benefit of R-core compared to toroidal is that the two windings are separated with a significant distance while on toroidal ones they are not - so pretty much the same benefits, but with better isolation.

[amc184]

One of the big benefits of the R-core layout is ease of manufacture compared to a toroidal type.  With a toroidal core you need to snap together a bobbin around the core, wind wire onto the bobbin, then wind the wire from that bobbin onto the core.  With an R-core the parts where the windings sit are cylinders, so a carrier can be placed there and spun, making the winding in a single step.

They have the same low flux leakage and compact footprint of a toroid.  If wound with the primaries and secondaries each on a separate carrier, they can have fairly low interwinding capacitance.

I've seen a few used in the Japanese audio industry, and China more recently.

[coppercone2]

I wonder if you can cut EI core transformers up to make R core transformers . I think so. Turn the E's into L and make L into boxes .

[jonpaul]

copper, sorry no...

R trsf has no gap, circular cross section, special tapewound core.

jon

[David Hess]

I wonder if you can cut EI core transformers up to make R core transformers . I think so. Turn the E's into L and make L into boxes .

Transformer like that are already made.  They have a rectangular shaped core made from stampings and separate bobbins for primary and secondary.  I usually see them in PC mount transformers.

[AndyC_772]

R-core transformers were used in Technics amplifiers around the mid-late 90s.

I still have my old SU-A600 and use it every day, it's a great piece of kit and, given the amount of use it's had, probably one of my best purchases ever.

[Kleinstein]

The magnetic leakage of the R-core depends on the windings. With primary and seconday of separate sides for good isolation, the stray field is no longer that good. For a good stray field if would more need 2 x a split bobin.

The advantage of a tape wound core or toroid over the EI core is that one can use materials only available as thing foils (e.g. amorphous) or grain oriented (anisotropic) silicon steel. The grain orineted steel has quite some advantage (higher useful magnetization, less loss, lower magnetostriction) over isotropic steel used for EI cores.

[tooki]

I know Sony was using them in some of their premium consumer audio gear, as well as in broadcast audio gear, back in the 1990s. Of course I couldn’t afford any of those things as a teenager, but I remember seeing them in catalogs. (And now in service manuals.)

My Tascam rack-mount headphone amp uses two of them.

[T3sl4co1l]

The magnetic leakage of the R-core depends on the windings. With primary and seconday of separate sides for good isolation, the stray field is no longer that good. For a good stray field if would more need 2 x a split bobin.

Yeah, there are some misrepresentations in this thread... If you wind P and S both on each bobbin, you get flux cancellation per bobbin, low leakage (comparable to, or maybe a bit better than, a shell-style transformer), and modest capacitance (less than toroid, but maybe same or more than shell).

If you wind P and S on separate legs (whole bobbin P, whole bobbin S), you get maximum leakage, which leaks out into the space between windings.  Capacitance is minimum, though still not terrifically low because the winding-core capacitances will be modest, and you get half that because they act in series.  (Maybe with a special bobbin that gives more airspace between winding and core, you could make a medical-style low capacitance unit?)  Grounding the core acts as an electrostatic shield (and an additional shield could be placed between windings in the middle, to get ~total shielding), reducing transfer capacitance but increasing individual winding capacitances (since they act to ground, rather than in series with each other).  A heavy copper flux band around both windings would be highly desirable to short out leakage, improving regulation and reducing external fields.  Mere copper foil will not do the job: this will carry some amperes at full load.

Compare the leakage path between bank-style windings on an EI core: P/S are side-by-side on a single bobbin, so the leakage is the space between them (and some of the space around, mainly in the volume where the windings stick out from the core).  A flux band helps here, and modest thickness copper is adequate to short out the relatively small volume.  This style has usually 10 or 20% regulation (worse in smaller sizes, I think?), so the separate windings R-core style (or sectorally divided toroid, equivalent to it) will have even worse regulation (30, 40%??) without a flux band.

I've seen a few R-cores myself; they may be more common in Europe, not sure.  As far as shape, it's a low-profile design, but unlike a toroid, the rectangular form may give other options in a tight layout.

Also once saw a low-profile shell-style transformer, EI lams, but very stubby 'E's so it was quite short, and a long stack of them to make up the difference.  The winding window was nearly square.  Kind of like an ELP ferrite core and bobbin you'd see these days, but in laminated iron instead.  Probably more expensive due to the wasteful* cut, not to mention the low production quantity (just judging by how infrequently I've seen the style, I mean).

*Default laminations are "wasteless".  Consider the 'E's butted together tip-to-tip, and the negative space between them (what will be the winding area) equals the 'I' piece. https://www.seventransistorlabs.com/Images/Wasteless.png  A bit larger winding window is usually desirable (accounts for the extra insulation needed in a build), hence ferrite shapes usually have a relatively wide winding area for example; but it's not such a big deal for steel, and is cheap enough I guess, compared to having to scrap the difference.

Tim

[Alex Eisenhut]

Thanks all, I guess I'll try a Chinese R core xfo, it's just for a C64 power supply, even though the one I made 35 years ago is perfectly fine.