Transformer design: how to size a core? - Page 1
Forum section: electronics / projects

author: Buk
author_rank: Contributor
timestamp: October 13, 2021, 02:08:47 am
content: I want step up Li-ion battery voltage to 100-125V @ 0.5A and ~145kHz.I'm thinking of using a toroidal transformer to get the voltage boost.The first thing I need to know is will a suitable size toroid physically fit into the space I have available.To that end, are there any rules of thumb for sizing a (toriodal) core to handle 50W of power at 145kHz?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2021, 02:11:35 am
content: What input voltage?How much experience do you have with switching power supplies, and Li-ion batteries?Why such a particular frequency?If it's just a few cells in series, a push-pull converter is a fine way to go, as typically used in automotive equipment. Matter of fact, you might get away with a 100W automotive inverter, just saw off the AC "modified sine" chopper circuit.Edit: oh, right!https://www.eevblog.com/forum/projects/heat-generation-using-piezo-elements/Tim

author: Buk
author_rank: Contributor
timestamp: October 13, 2021, 02:46:40 am
content: Quote from: T3sl4co1l on October 13, 2021, 02:11:35 amDid that answer all your questions, or should I supply further information?

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 13, 2021, 03:47:46 am
content: Probably the simplest, most practical explanation of DIY magnetics you’ll ever find:https://ludens.cl/Electron/Magnet.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2021, 06:28:04 am
content: I don't remember you mentioning what battery pack you were going to use so let's start with thatTim

author: Buk
author_rank: Contributor
timestamp: October 13, 2021, 07:26:14 am
content: Quote from: T3sl4co1l on October 13, 2021, 06:28:04 amI'm hoping to get away with a single 21700 4200mAh cell with a (claimed) maximum discharge rate of 45A.spec sheet (pdf)They are from Moli One Energy Corporation of Taiwan and have a good reputation.At a pinch I might consider a parallel pair of 18650s, say Sony VTC5As with a 25A rating, but I'd rather avoid the extra weight. It's not just the 30g for the batteries, but the extra weight in the bigger case.

author: Thomas
author_rank: Regular Contributor
timestamp: October 13, 2021, 10:07:11 am
content: http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html#smps

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2021, 07:19:11 pm
content: What will you use to hold the cell?My experience with snap-in holders has been, they have quite high resistance, topping out at not many amperes.If you'll have a spot-welded lead frame, or just something better than what I've used, it should be feasible.Still, the very high current seems unnecessary. How about a small 3S pouch or something?In any case, you will probably need a Baxandall (or "ZVS", often erroneously "Royer") oscillator with a pair of MOSFETs:https://adammunich.com/zvs-driver/At the low voltage, the top circuit is actually sufficient; I would add series gate resistors, and maybe clamp zeners to protect them some more.If plain on-off control is sufficient, then also a switch to supply that, along with logic to manage the battery (basically cutting out below 3V, while not drawing many µA in the process). Maybe an LED too? If you want adjustable power, the switch can be PWM'd (don't forget the catch diode, you're making a buck converter with the oscillator's series supply inductor), varying the amount of current into the oscillator (down to a minimum, as it needs some voltage to begin oscillating).The oscillator will run at whatever resonant frequency it's tuned to, partly a function of the transformer inductance, load capacitance, and the load itself (piezo stack). This is probably the best option to start out with.The next step up from an oscillator, is basically a free-running oscillator with PLL and some means of sensing the load tuning (current or voltage phase usually), or some means of forcing it to the operating frequency (selectable capacitors?) -- quite a bit more complicated, as you can see. So if the oscillator is good enough or can be made to work, it's the preferable option.Tim

author: Buk
author_rank: Contributor
timestamp: October 14, 2021, 03:30:57 am
content: (Not sure what this has to do with the OP question, but thanks for the interest.The plan for the cell contacts looks like this:The copper colored component is an 8mm diameter copper rod with a 10mm x 1mm contact face (all one piece). The white thing is a silicone insulator. At the base, there is a screw in lid, with a 10mm raised face that connects negative to the case. The circuit will have reverse polarity, under voltage etc. detection.The current plan for the circuit is a VCO chip driving a mosfet (or H-bridge pair if a tapped primary is beneficial) connected to the primary side of a toroidal transformer; in the form of a flyback converter. A second inductor in parallel with the secondary coil provides the energy storage. The piezo is connect in series with the secondary coil via a diode. Probably a limiting resistor, and maybe a cap is needed there also.I've determined that a 20x10x10 toroid can handle the load, maybe a 16x8x8 will also depending on its permeability curve.

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 14, 2021, 04:53:19 am
content: For one offs, you’ll find pot or E cores with a plastic bobbins are far easier to DIY than toroidal cores. Obviously it depends on the number of turns required and the gauge of the wires. The two piece type cores are also easier to deal with when you need to create air gaps if saturation becomes a problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2021, 04:55:53 am
content: So like a flashlight?I wouldn't recommend making your own. Like I said, spot welding is probably the only way for that kind of current. (But, others who have more experience with cells, do chime in!)50W at 150kHz will probably need more like a 25mm OD toroid, depends. In that ballpark anyway, yeah.I would recommend a nice clean cylindrical bobbin style, like E, EP, ETD, and use copper foil for the primary. Push-pull (as in the ZVS circuit) is preferred to reduce RMS current per transistor. The two halves of the primary should be tightly coupled together, which is done by using two strips of foil together, and connecting them in series outside the winding. (Or put down two turns but solder another foil connection in the middle, same idea. Probably about the same bother to assemble, honestly.)You can use enameled wire just fine, but you'll need to use many strands in parallel, both to handle the current (ala litz cable), and to interleave enough strands to get reasonable performance. (Pro tip: make pairs or quads of wires for both halves of the primary, then wire a bunch of those in parallel for ampacity.) The secondary probably doesn't need to be too tightly coupled to the primary (leakage inductance is acceptable, to some extent) so can just be a single block.No diodes or anything, maybe some combination of inductor and capacitor to match the secondary winding to the piezo stack. (A diode would just... charge it up to DC, and, sit there doing nothing?)Tim

author: Buk
author_rank: Contributor
timestamp: October 14, 2021, 06:53:30 am
content: Quote from: T3sl4co1l on October 14, 2021, 04:55:53 amThe battery must be removeable, so spot welding isn't an option.For my prototype, I'll be making the body out of copper. The tube, which is one part of the full case, will be made from 22mm heating pipe. The rest will be made from copper sheet silver soldered together.If it ever got to being made commercially -- extremely doubtful -- then probably a Zamak (Zinc alloy) die casting would be the solution.Quote from: T3sl4co1l on October 14, 2021, 04:55:53 amI've finally got a FEA simulation to run at 145kHz, and a 16x8x8 core gets to 0.47T; which unless someone tells me I am wrong, it what I will be using.

author: Buk
author_rank: Contributor
timestamp: October 14, 2021, 06:55:07 am
content: Quote from: WattsThat on October 14, 2021, 04:53:19 amEasier, but they take more space; which is the big limitation.

author: sandalcandal
author_rank: Unknown
timestamp: October 14, 2021, 11:48:14 am
content: Quote from: Buk on October 14, 2021, 03:30:57 amWord of caution with the cell holder design. Slightly less of an issue with flashlights/torches but highly enclosed designs like this circumvent cell venting safety features and turn the cell+holder into a pipebomb/shotgun. There are a numerous deaths due to lithium ion battery explosions with vapes; DIY, modded, misused or poorly designed. For this reason, most companies don't like selling their cells (directly) for vape applications and official material forbids their use in vapes/e-cigs. If you must use this form factor design youneedto add pressure relief/venting.Edit: Even with pressure relief, there is still significant danger for any highly enclosed design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2021, 04:50:35 pm
content: 0.47T, with what ferrite?0.2T may be more comfortable, with respect to core loss at that frequency. Maybe less. Definitely look for a low loss type like 3C95 or better.Tim

author: Buk
author_rank: Contributor
timestamp: October 14, 2021, 05:48:33 pm
content: Quote from: T3sl4co1l on October 14, 2021, 04:50:35 pmI believe that is true for EI and pot transformers,less so for toroidal.At a pinch -- and considerably more money -- I could opt for anNanocrystalline amorphous metal corethat can run upto 1.56T and 1/5th the losses; but I'd like to avoid that cost if I can.

author: Buk
author_rank: Contributor
timestamp: October 14, 2021, 05:58:37 pm
content: Quote from: sandalcandal on October 14, 2021, 11:48:14 amI'm aware of the requirement; but the warning is appreciated.Quote from: sandalcandal on October 14, 2021, 11:48:14 amThe way to deal with it is the same way electrolytics caps and aircraft :

author: uer166
author_rank: Super Contributor
timestamp: October 14, 2021, 06:27:04 pm
content: You're not going to go far with a nanocristalline core at that frequency, the losses would be massive. I'm not sure where you got the 1/5 loss number from, but I doubt you can run it at any more than 10s of kHz.

author: Buk
author_rank: Contributor
timestamp: October 14, 2021, 06:38:07 pm
content: Quote from: uer166 on October 14, 2021, 06:27:04 pmFINEMET® FT-3K50T and FT-8K50DThese are brand new materials produced by applying a controlled magnetic field during annealing to industry leading thin nano crystalline ribbon. This provides a material that satisfies both high saturation magnetic flux density and high permeability. Other standard features: Low core loss, Low magnetostriction, Excellent temperature characteristics and small aging effects, Excellent high frequency characteristics and the Flexibility to control magnetic properties " B-H curve shape " during annealing process.

author: Buk
author_rank: Contributor
timestamp: October 14, 2021, 06:49:48 pm
content: Quote from: uer166 on October 14, 2021, 06:27:04 pmMore info:

author: uer166
author_rank: Super Contributor
timestamp: October 14, 2021, 07:32:11 pm
content: Seems like you're right, I did some math for a few cores, e.g. MP2010:100kHzVin=3.6, Vout=125, Iout=0.5Ipk = 22.5A, Iripple=10ASuggested L = 3.6uHNumber of turns = 7Bpeak = 0.3T, Bripple=0.14TInductance at Bpeak = 90% of zero, Core loss is ~2W at 0.14 Bpeak (about 3% efficiency lost in core losses)It's a little surprising to have such low core loss for laminated metal, so it would be great to see real life results. At such high currents and frequency you'd do well to use some form of litz as well, although a 1-layer windup here helps massively.Edit: above is for a straight boost which at this ratio would be impractical. Also these cores are not suitable for transformer due to very low permeability (they're gapped), so you'd be better off finding ungapped Metlgas cores, or regular low loss ferrite.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2021, 07:55:36 pm
content: Quote from: Buk on October 14, 2021, 05:48:33 pmThose are mains frequency transformers; the core losses are already low. Nanocrystalline materials are promoted in that space simply to achieve even lower idle losses. (When the replacement schedule is half a century as can be the case for distribution (pole/pad) transformers, it's worth spending a few thousand bucks up front, to save a few cents a day of electricity over that time frame!)Core materials fall off at high frequency. Typically there is a cutoff frequency where the impedance changes, from largely inductive (impedance proportional to frequency, ~90° phase shift) to resistive (impedance constant, no phase shift), or something inbetween. As it happens, sheet metals have a diffusion characteristic (impedance ~ sqrt(f), 45° phase), which takes over when skin depth equals sheet thickness, more or less. (At still higher frequencies, the capacitance between layers dominates, and impedance falls with increasing frequency.)For most nanocrystalline materials, this cutoff is in the 50kHz range. Which is not to say it's automatically useless up there -- but it is a hint that losses won't be as low as you might've hoped, and it's worth considering other materials too.Ferrites drop off in the 100k to 10MHz range, depending on material and size.Tim

author: sandalcandal
author_rank: Unknown
timestamp: October 14, 2021, 11:44:41 pm
content: Quote from: Buk on October 14, 2021, 05:58:37 pmGood to know you are aware.

author: Buk
author_rank: Contributor
timestamp: October 15, 2021, 04:49:59 am
content: Quote from: T3sl4co1l on October 14, 2021, 07:55:36 pmI may have linked the wrong page or part of the page. That website has very long pages and lot of different things per page and many of the #targets are vague.And I can only go by the specifications provided by the Metglas/Hitachi. I have B-H curves for several samples of products from both of them -- now co-owned -- and if you look at thecore load v frequency curve for 2506SA1, they go up to 150kHz.And I am led to believe that the newer FINEMET® FT-3K50T and FT-8K50D products, used correctly, can produce even lower losses on small toriods at higher frequencies. But I am no expert, and can offer no first-hand experience here. I do know that most peope do not realise quite how different amorphous metal --MetallicGlass -- is from regular transformer steel.For a start, it is an order of magnitude thinner -- 0.015mm versus ~ 0.18mm -- than the best silicon steel sheets you can get. Add to that a surface texture that means that even when wound together without additional interleaved insulation, virtually no current can flow between layers. And the final piece of the puzzle appears to be the application of magnetic fields during annealing above the Curie temp. that freezes magnetic domains into the cooled product in much the same way they do with NeFeB magnets.But now I'm just quoting stuff from spec. sheets, and quite possibly mixing stuff from several different material specs; but I did do some (FEA) testing of various MetGlas/FineMet specs a few years ago for a different project -- a transverse flux motor design -- and I am convinced that for my application of a no-gap toriod, size for size versus a ferrite (FeNiZnV), at 145kHz, I can get 1/5th the losses, or use half the turns, or some balance between the two. If only I was prepared to pay the cost. For now, I do not think I need to.This is a 16 od x 8 id x 6 ferrite core with a 5 turn/15A primary , 155 turn/0.5A secondary @145kHz.The calculated losses are so low that I will need to recalibrate and re-run the simulation using individual wires -- this is a 10/330--rather than block level simulations, before I am convinced the results are correct.; but it is looking feasible.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2021, 08:21:22 am
content: Truly it's amazing stuff, amorphous/nanocrystalline materials; not only low loss, but some offer permeability rivaling supermalloy. The now-obsolete ISDN transformers, using the stuff, are just about unmatched in bandwidth. The cores are still around fortunately, so you can still make your own, just not off-the-shelf ready made transformers.That's quite extreme flux density for ferrite, are you sure your loss model is correct? Also, how exactly are you going to get 150 total amp-turns through a 1/4" hole?I'd like to see core loss more like 0.5W for a core that size, at least for continuous operation, let alone copper losses as well.Never heard of FeNiZnV, what's that?Tim

author: Buk
author_rank: Contributor
timestamp: October 15, 2021, 10:52:01 am
content: Quote from: T3sl4co1l on October 15, 2021, 08:21:22 amYour mention of "ISDN transformers" led mehere. They have cores in the size range I'm looking for. Of course, they don't list prices, which is never a good signQuote from: T3sl4co1l on October 15, 2021, 08:21:22 amNot entirely. I'm talking (waiting for) a magneto-harmonic FEA guy to sanity check my model; but I have some confidence it is correct.Quote from: T3sl4co1l on October 15, 2021, 08:21:22 amI have a whole8mm to play with not 6.35mmThe picture of the model above shows 315 x 0.25mm turns in the secondary in a 10mm ID, with room to spare. Whether I can physically wind it is another matter; but I do have some history rewinding motor cores. It is slow, painstaking work, but 0.25mm is easy to deal with.Quote from: T3sl4co1l on October 15, 2021, 08:21:22 amThis is (maybe) where experience trumps best endevours. My target is burst operation; say max. 30 seconds at a time. The body of the device can be a heatsink; though interior airflow could be a problem.Quote from: T3sl4co1l on October 15, 2021, 08:21:22 amIt is a material specification "soft iron ferrite" that comes pre-installed in the materials library of the FEA software. (Best reference I have isthis. An Iron Nickel Zinc Vanadium nanoparticle ferrite produced using electric arc deposition.I believe it may once have been market under the brand name 'Sinimax Softiron Softferrite". Very little reference to it.I used it for the sim because the material specs were available. Trying to get the specs for commerical cores is hard because they tend to be proprietary mixes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2021, 07:16:25 pm
content: Quote from: Buk on October 15, 2021, 10:52:01 amWell, they're also just a Chinese distributor (if not just window dressing for some bigger no-name distributor, who knows how many companies are at the same address?). AFAIK, HMG and VAC are the only two sources of the stuff, though it may be China is making it now, I don't know.QuoteThe primary is the stinker here: if it's solid, it's all surface. If it's stranded (litz) it'll take up way more space (about double?).And you don't have much space to spare: at a typical 8 A/mm^2, the winding factor is a whopping 24%. 10% is recommended for toroids. 50% would be pushing it for a bobbin style winding, but you get more core. I don't know how high you want to push the current density. Maybe more is acceptable given the size and duty cycle (as below).Also -- if your core losses actually are that low, it would be a valuable trade to reduce winding length, trading copper for core loss.The secondary, bunched up like that, will also suffer from proximity effect; easy enough to reduce by interleaving the windings.QuoteAh, if the duty cycle is enforced to be low, that might not be so bad. Put a thermistor in the middle?Potting with a nice moderately conductive compound would also help for internal temperature, considering you'll have basically no airflow inside there.QuoteAny way to craft your own from curves?Tim

author: Buk
author_rank: Contributor
timestamp: October 16, 2021, 07:30:20 am
content: Quote from: T3sl4co1l on October 15, 2021, 07:16:25 pmFrankly I don't care where the stuff comes from if it does what its meant to do. I read somewhere about testing cores by winding a couple identical coils, a few turns each of a known wire; feeding in AC sweep into one and comparing the output from the other on a scope. I don't remember the details; and I don't have a scope.QuoteI don't know those initials?The only companies that I know of producing metallic glass are MetGlas in the US and Hitachi in Japan andHebei Shenke in China.My understanding is that the first two 'merged' (Hitachi took MetGlas over) to counter the growing threat from the latter once it became clear that what they produce is of very high quality.(QuoteFor my own reassurance, I clipped a 1/2"OD x 1/4"ID x 3/16" core of unknown type from a piece of scrap electronics; dug out a some scraps of used 0.5mm and 0.25mm wire and attempted a wind.This is what I got in about 20 minutes:The primary is 5 turns of 4 parallel 0.5mm to get 1.0mmØ equivalent. The secondary is 120 turns (ish, I may have lost count a couple of times) of 0.25mm.Not the neatest coil you'll ever see, but given new rather than pre-kinked wire; a somewhat larger core; and the incentive to spend more time to get it neater, and I think I can make a better job.And with a little adaption,QuoteGo find your 'smug face'... I'll wait.You were right!Whilst my model is correct, the material definition has a subtle error that means my simulation runs are effectively producing garbage numbers. It'll take me a while to sort that out.QuoteBoth good ideas. Thanks.QuoteYes, but extracting good data from those stupidly small graphs in the datasheets is very hard.When I was creating material definitions for MetGlas & Hitachi foils, I had to be very pursuasive before they sent me the raw data produced bythese guys who do their certification testing for them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2021, 08:35:07 am
content: Quote from: Buk on October 16, 2021, 07:30:20 amSorry, Hitachi Metals Group and Vacuumschmelze. The distributors are... neither here nor there, probably, just that the way Chinese distributors tend to work in general: there might be dozens of seemingly the same or very similar company(s), and you have to wonder which ones are representing prices fairly for example, let alone the providence of their products. Maybe that's overly paranoid, I don't know.Heh, on that note, at least amorphous type stuff is very hard to fake. You won't get very far into production before finding out you've got nothing but transformer iron in there!QuoteHuh, searching on a couple keywords, this only turns up under another domain (which might not mean anything, or even that Google is being terrible again at finding real results). Not really much data there, at a glance, but I also don't read Chinese and the auto translation isn't impressive, so, eh.If true, that's cool -- always good to have more sources of things!QuoteYeah I mean it fits, you can pull off a winding factor of 50% -- but it's going to look like, well, that; and all that crowding increases proximity effect, while the turn length greatly increases as well. So the efficiency falls; the price of compactness.QuoteI mean, no need for smugness, just didn't pass the sniff test -- compare with typical power and B-H curves, ferrites just don't go that high for one and if they do it's going to be many watts (I think?), you're deep into saturation there. Checking models is a key part of simulation!Uh, maybe saturation anyway? Is that Bpk, or like, p-p or RMS or something? It's usually Bpk, but better to ask to be sure.QuoteBut that aside, that's cool!Tim

author: Buk
author_rank: Contributor
timestamp: October 16, 2021, 09:46:07 am
content: Quote from: T3sl4co1l on October 15, 2021, 07:16:25 pmTurns out that TDK make theraw data availablefor a whole bunch of their core materials.That makes life *much* easier.

author: Buk
author_rank: Contributor
timestamp: October 16, 2021, 07:48:57 pm
content: Quote from: T3sl4co1l on October 16, 2021, 08:35:07 amYou might find this:Designing Low-Power Flyback Inductors Using Tiny Toroids (Part 1): A Boost Converter Applicationand this:Designing Low-Power Flyback Inductors Using Tiny Toroids (Part 2): Calculating Lossesinteresting; or not.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2021, 08:49:58 pm
content: What a strange article; lambda is used for flux, the application is not introduced so there's no motivation or direction behind the subject, and no mention is made of a suitable controller -- the most particular it gets is naming a 2N7000, a TO-92 device of all things. And he goes directly to #26 powdered iron, literally the worst choice available, without explanation. This feels so... vintage? Yet the copyright is proudly 2019.I think I get the idea of what's being done there -- it's hard to review because it's all over the place, the core selection is unexplained, then the core loss flux density formula is given, then the power loss is given -- it's like it's completely in reverse logical order.So in the end he's got 8uH Lp, rated for 210mA. Sounds very much like one of these:https://www.coilcraft.com/en-us/products/power/coupled-inductors/1-n-shielded-coupled/lpr/lpr4012/lpr4012-103d/which is a smaller outline at a tiny fraction of total height. They also have core loss calculations on site; though I'm not sure if the coupled inductors are supported?Inputting similar numbers, Micrometals recommends very far from #26; T37-6 and T30-2 are typical examples, giving excellent low losses (43, 35mW), though mind that these are single-winding inductor figures, and the copper losses would be about 40% higher in a flyback transformer.To his credit, the core loss calculations seem consistent with what Micrometals' calculator says. It would seem the inexplicable core choice, or lack of motivation, is the only real problem; the calculated results are sound.Tim

author: Buk
author_rank: Contributor
timestamp: October 17, 2021, 07:22:01 am
content: Quote from: T3sl4co1l on October 16, 2021, 08:49:58 pmI don't see any reference to "2N7000" or "TO-92" in either of the articles I linked? (Sorry one of the links was broken, I'll correct it shortly.)QuoteI can't argue with your logic; but 3 observations:I suspect the choice of core for the example was deliberately obscure by way of demonstrating that it could be done, not necessarily that is was a good solution to anything in particularIn support of that suggestion, I offer another article of his which might clarify some of the motivation:Single-Bundle Windings Make It Easier To Build Custom Magnetics In-HouseI haven't seen the references to TO-92 packaged transistors you saw; but when it comes to electronics, I likevintage. Components big enough for me to see with my tired old eyes; recognisable component values; no need to for an STM, nor the dexterity of an eye surgeon, to fit (and remove) from boards. You can keep your smt ...I think the guy likes to explore the obscure and forgotten topologies; and to prototype. (Which makes him useful to me for this.)(BTW: I got really excited aboutthis, before I realised I'd need a stack of 20 in parallel)QuoteThat's good to hear.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 17, 2021, 09:16:34 am
content: The paragraph on page 3, "For a small converter, a small MOSFET switch such as a 2N7000 is used. ..."It's interesting, subtle; merely change a single digit and it seems current enough (2N7002 is the SOT-23 version, still very much in use for jellybean purposes, alongside BSS138, BSS84 and others). Just as well, it could mean the "original", generic part, and there's not actually anything to read into. But so too, the choice of #26 (and yeah, a possible motivation is to "make do" with it). We can only speculate, short of e-mailing the guy, heh.I saw the reference to, but didn't look at, the "single bundle" article. I don't have a problem with that, at least from what I gather from its use in the first document. Indeed, I've done that plenty of times myself, and actually I'd recommend you do it as well, if you can. The biggest benefit is: getting excellent transmission line transformer type coupling between windings. Especially so, if you pair primary and secondary wires (as pairs or quads), and then bunch up however many it is you need to wire up in series and parallel, for primary and secondary. The biggest downside is: you have to painstakingly ohm out every single goddamned wire, to make sure you're getting them all properly in series and parallel..! (Even with color coding, you'll only have so many colors to choose from, but for your case, dozens of strands to cover, so...) But yeah, fine for one-offs.BTW, if you're curious about off-the-shelf possibilities, shop around for inverter transformers. Nothing wrong with using a, say, 240V to 5V, flyback transformer in reverse! Or maybe a forward type would be more suitable (do you really know what inductance you need, as yet?), or multi-secondary so you can wire them for a CT'd primary, etc. They'll likely be bulkier than what you may be looking for, on account of the squarer shapes (mostly ferrite E-core styles), bobbins with rows of pins, and reinforced insulation, and they tend to be expensive (in part just because you're buying tiny quantities through a distributor, prices are kind of irrelevant -- but anyway, the alternative is your labor, potentially a big savings?).CCFL transformers are also interesting, but probably much too high a ratio for what you want, and they only come in so many sizes. I guess if you wanted to rewind one you could. They tend to have a low profile and high aspect ratio, so might still fit inside your cell-sized enclosure, and I think the power level is about right, give or take size selection that is.Tim