Choosing an output transformer

[LinuksGuru]

Originally Posted by munorc

For those that wind their own transformers, where do you obtain the materials, cores, etc.?

I have read through many past threads and I do not find much info on sources.

May I ask why do you want to do this? Good winding equipment cost a plenty of money, high-quality DIY transformer design required digging arcane material in several academic-level engineering books 400 - 700 pages each, + ability to be a decent-level programmer.

If this is for fun only its OK, but if you are interested in good result IMHO better to order from factory or small specialized firms like Electra-Print, Lundahl, or Bud if he has something in the price range you would like to pay for.

[Ultima Thule]

Originally Posted by munorc

For those that wind their own transformers, where do you obtain the materials, cores, etc.?

I have read through many past threads and I do not find much info on sources.

Originally Posted by LinuksGuru

May I ask why do you want to do this? Good winding equipment cost a plenty of money, high-quality DIY transformer design required digging arcane material in several academic-level engineering books 400 - 700 pages each, + ability to be a decent-level programmer.

If this is for fun only its OK, but if you are interested in good result IMHO better to order from factory or small specialized firms like Electra-Print, Lundahl, or Bud if he has something in the price range you would like to pay for.

LinuksGuru,

cut the ** will you!
It's the second time I see you standing on a pedestal frowning at people asking 'noob' questions by being counter-product and highlighting just the negative aspects as you did in my thread (Bare R Cores and bobbins, where???), and oh.. programming, now step down from your high horses and don't be such a snob, I tell you what I have worked as a SMPS designer involving transformer designing for 2 world MAJOR Telecom companies with an arsenal of measuring equipment you can just dream of but I don't go and pull of peoples dreams for that!

Look at your self asking all kinds of delicate questions about transformer designing, why would you have the right to ask and get all the answers but at the same time frowning at others who are behind you in knowledge, nobody tells you it's impossible, the word 'leecher' comes to ones mind.

And winding machines costing $$$, there do exist small machines driven by hand perfectly useful for couple of transformers and people have even built their own winding machines of a scraped hand drilling machine attached to a wood box all in total for maybe $30 or so, one guy even did a winding machine for toroid transformers from child bicycle rim and all this exists in video form uploaded on YouTube and plenty of, and for some it's not a question of money but because its called DIY and giving a bit of challenge.

Shame on you!

Cheers Michael

[BudP]

Ultima Thule,

I agree with what you point to, though I have no reason to think Links was being negative in quite the manner you portray.

I am also quite negative about diy folks winding there own transformers. Not those used in telecom, but those used as power and output transformers, ones that see the power grid or high DC plate voltage. My concern is that it is flatly impossible to pre warn a new designer / winder about the very extreme safety issues involved. My tutelage lasted 7 years, under the careful eye of Nathan Grossener, the inventor if Metglass, Albert Fisher, a well know Chicago based transformer designer and Leroy Carson the chief designer at Better Coil and one of the most respected designers in the world of transformers. Also the inventor of black box thermal modeling that is used by ALL reputable transformer designers. This sort of understudy, with people of this caliber was possible only because almost no one was interested in learning about what looked to be a dieing field and they were very willing to spend the time and provide their expertise. I was not confident of my skills for about 4 years of 8 to 10 hour days, working as a junior designer in a Chicago transformer company.

That ordinary folks, EE's and other newly interested parties want to learn is splendid and I do try to provide some guidance, but just the thought of them making transformers in these classes scares me badly. I suspect that Links was just trying to provide some notion that this might not be an easy and straight forward thing to learn and I will also caution anyone involved in trying this out that it is not a sport, but a potentially deadly business and the modes of death are not obvious.

Bud

[LinuksGuru]

Originally Posted by Ultima Thule

LinuksGuru,

cut the ** will you!
It's the second time I see you standing on a pedestal frowning at people asking 'noob' questions by being counter-product
don't be such a snob,
the word 'leecher' comes to ones mind.
question of money but because its called DIY and giving a bit of challenge.
Shame on you!
Cheers Michael

Wow, so much emotions for so simple questions "why DIY and not ..." !


BTW, when I'm was in need for custom audio transformers, winding it myself was the first option. I dropped it almost instantly since no material was available at meaningful price, 100 kg was min for even quite expensive amorphous alloys. I had (20 years ago) manual winding machine borrowed for some time. It was built smart way by a former submarine officer, yet not capable if layering thin wires in nice orderly fashion as factory equipment does (a drawback of any DIY machinery).

The second option was to order it from specialized company. One which would make it for sure asked $3000 for the design and samples. Others wouldn't do it fine for sure since they used primitive calculus like the one from MRB.

Third option is to pay 2 x 300 - 500 EURO for a pair from Tamura, Hashimoto or Plitron. After all, even the price is crazy, I will obtain working products.

I dig quite deep into this issue just to find how complex it is when it comes to REAL professional design & quality. I wrote my own CAD program and discovered later sad fact - there is no ferromagnetic's material data publicly available to feed into this software. Academic material I have (huge 400 - 700 pages books) is of Russian/Soviet-era origin, and all data sheets are for the alloys manufactured back in these days.

Just a basic problem. In real life push-pull output stage *never* fully balanced, there is *always* DC current flows over primary coil. Under certain circumstances, 10% discrepancy of idle current of output tubes may lead to the 2x decrease of primary coil inductance and 5x increase of nasty 3rd order harmonic distortions. Now the question Ultima Thule (aka Michael), from (as you say) some snob on the pedestal: how would you *precisely* re-calculate transformer properties if there are no manufacturer's data - how permeability is changing under flux caused by *both* DC and AC??? The required "mu/aw0/flux" chart is nowhere to be found from today's manufacturers! Air gap is not used in PP trafos like the ones found in SETs. I assume there is another way apart from the one described in Russian books (BudP certainly knows and uses it), yet it will require digging formulas from another 500-page book!

Originally Posted by BudP

Ultima Thule,
That ordinary folks, EE's and other newly interested parties want to learn is splendid and I do try to provide some guidance, but just the thought of them making transformers in these classes scares me badly. I suspect that Links was just trying to provide some notion that this might not be an easy and straight forward thing to learn and I will also caution anyone involved in trying this out that it is not a sport, but a potentially deadly business and the modes of death are not obvious.
Bud

Bud, you got it 100% right.

[BudP]

I am going to go slightly off topic here and post an insulation system design we did for Mackie Designs a number of years ago, to replace a noisy power toroid in one of their medium sized mixers. This is the document submitted to CE for international approval of the safety insulation system.

You will have to use your imagination to see the insulation materials as they would be applied to a coil. This does not utilize a pre-molded CE rated bobbin, as they loose too much winding volume. Instead the creepage and clearance requirements (6mm across any surface and 0.040" across any gap) are made solely with dielectric tapes. This design also had very tight thermal, regulation and emitted field characteristics. This will give you a general idea of just how complex a robust, totally safe linear power transfromer is to make.

All of our high voltage power transformers are made just like this and are then vacuum impregnated with a polyester resin that seals the windings from oxygen, to keep corona at bay and also turns the transformer into an obsidian hard lump.

Bud

[munorc]

My interest in transformer materials was one more of curiosity. I have seen references to people winding their own transformers. I have looked around and it seems like obtaining the core material, in small quantities, was not really all that possible. The direct interest relates to the desire to build something that is transformer related (a motor growler). For that application, it looks like my best source of materials will be scrap industrial transformers. I posted the question here since it was an active discussion and there was some mention of transformer materials. I did not want to resurrect an old thread where similar things were discussed in the past. I apologize for the hijack.

For my current needs, transformers that are commercially available will satisfy my needs, for the point that I am at in the world of tube amplifiers. I have been following this thread since I found it quite interesting. I have been enjoying BudP's input to these discussions.

[BudP]

As an addendum to the previously posted insulation system layout. The basic relevant CE 6500 criteria are as follows'

1.) A minimum of 6 mm of creepage and clearance (8mm for units shipped to Finland). This means that for all adjacent or coil crossing wires from windings of opposite polarity insulation must be provided on all sides of the wire in such a way that there will be 6mm of distance from any part of the wires to any part of a winding of opposite polarity. This also requires that any coil wire that is bound within the windings of opposite polarity has a 6mm creepage and clearance from any other bound wire of a winding of opposite polarity. Insulation coating on magnet wires is specifically ignored here, just as if there was none at all.

2.) All dielectric barriers must be constructed of three separate layers, any two of which will be able to provide the full dielectric withstand of 3750 vac for one minute without breakdown. Breakdown limit does allow a small amount of current to flow across the surfaces of the insulation, anywhere from 5 micro amps to 125 micro amps, depending upon the standards for the particular type of equipment the transformer will be used in. Any dielectric barrier that is not three layers of material, as in bobbin flanges, must be a contiguous material with a minimum thickness of 0.040" and be able to withstand the above voltage stress tests. All of these barriers must INSURE a physical separation and a 6mm creepage distance. This means they must be physically fixed and sized in such a way that a poorly trained manufacturing technician cannot corrupt their purpose. All dielectric stress tests will be performed after the transformer has been run for eight hours, at 10% above it's rated load in all secondary windings.

3.) All secondary windings must withstand a direct short circuit, with all other windings loaded to plus 10% of rated load, without emitting molten material (much less catching fire) or in any way dislodging or distorting the dielectric withstand materials by more than 5% in any direction. These short circuit tests are to be applied consecutively, with no more than 10 seconds between shorting of one and un-shorting of the previous and the previous winding is to be taken back to it's 10% overload and must provide 90% of it's original voltage for the duration of the tests on other windings.

4.) At no time during any of these stress tests is the transformer to exceed it's maximum rated insulation temperature classification in any winding, which depends upon the lowest rated material in the dielectric structure, or adjacent to those structures that might cause deformation of that, or other dielectric materials. This means using a thermal fuse in the primary, and also that a low VA winding of opposite polarity must be wound out at the last winding position and a thermal fuse placed on it too, if the primary VA under short circuit of that low VA winding, is not high enough to cause the primary to heat up and trip the thermal fuse for the primary. Resettable fuses with proven life times are acceptable, but they are bulky and expensive. There are some exceptions to this requirement, for units used in "professional" circumstances. Oddly enough Guitar amps fall under this classification, but they must be current fused so that they will not fail the abuse tests and those are not any less stringent. We build all of our Guitar amp power transformers in such a fashion that they cannot ever have a turn on, over current inrush, due to core or coil offset voltages, for just this reason.

There are other tests for softening of lead wire under compression and a very rigorous test of all of the materials found in the coil to prove that they will not deteriorate each other or the insulation on the magnet wire. And then there are subsequent tests of these same abused units to ensure that they meet specific requirements for their eventual usage. There are no exceptions to any of this, if you fail one part, the test halts, you are charged the full fee and get to start all over again the next time.

In my world, the above is everyday stuff, minimum basic requirements for being in business. Then there are the FDA regulations and after that the medical component industry and beyond that are space rated applications. This is why having folks just decide to make their own transformers, for high voltage or even mains connected use, scares me so badly. Without the above proof of suitability the usual 300 k hours of mean time before failure that is industry standard, cannot be guaranteed. To not build to that lifetime standard almost certainly guarantees failure and subsequent fire and deadly shock hazard.

So, you have been warned, this is not a playground.

Bud

[SY]

Bud, terrific post. Many thanks!

[Ultima Thule]

Thanks Bud for your reply, informative and well appreciated!

The transformer designing I worked with was no sc. "Telecom transformers", SMPS stands for Switch Mode Power Supply and they were connected directly to the mains voltage and have to fulfil all the standards as any generic transformer you are making (creepage, clearance distances, flame class, pollution degree) and yes I have done the CE/UL certification and that did incorporate not only the transformer design and parts used in it but the whole product so I am very familiar with all that you describe here just that it's couple of years ago since I worked with it as I am doing other things these days thus have become a bit rusty.

That said I would like to say Bud you are really providing golden information for many here without going of with a notion or assuming whether somebody is incompetent or not (and that's how it should be in the first place), keep on doing that, I do appreciate at lot what you post here though I haven't taken part actively in this thread before.

I am still waiting for an answer from LinuksGuru on the question I had in the R-core thread where he could order just a couple of custom made transformers from China (or did you have to buy 100 Kg/$3000 from China??), and as Bud thinks it's a laborious task to make own transformers I'm sure many appreciate to know where LinuksGuru can buy them.

BTW, if anyone knows where I can buy C-cores (I have skipped the idea of R-cores as they seem to be even harder to get hands on) in small amounts I would be very happy to know as I need transformers for my upcoming PP KT88 amp, I am taking part in one group design going on over at this link: "Mullard 5-20 KT88 PP blocks! You are all welcome with constructive ideas and support!

Cheers Michael

[BudP]

Here is another insulation system submission document that is a bit more on topic for those who must build their own. This is a guitar output transformer that is in the products of a major manufacturer.

The CE6500 standard for output transformers differs from the power transformer only in that the creepage and clearance is defined by the AC RMS voltages of windings of opposite phase added together. The range is 1 mm for up to 30 vac rms combined, and 3 mm for 60 to 900 vac rms. Creepage and clearance does climb to 6 mm and more as combined voltages rise above this point. This means that most sensible outputs need to have 3 mm of creepage and clearance, fixed just as it is in the power transformers.

These output transformers are viewed as secondaries to the power transformer and so, hazardous voltage and safety low voltage requirements are applied. This means that there must be two layers of insulating materials, each of which which must withstand the dielectric stress voltage of 2750 vac for one minute.

As you go through this document, notice that the secondaries are physically offset from the bobbin walls by a tape that has NO effective dielectric strength. It still must meet the temperature class and pollution class of the rest of the materials. This 3 mm wide tape is the key to a number of benefits. The space left unused does not generate leakage inductance. The secondary windings are subject to linear antenna field events and the core phase discontinuity that occurs close to the window walls at low flux levels does not seriously disturb the proper phase relation ship of primary to secondary at high levels of flux density as it moves farther out into the window area and into the windings. And finally it meets the CE requirement for fixed creepage and clearance.

The secondaries are not short circuited for this class of approval but they are loaded to 10% over current. The coil materials must also pass a very rigorous test of all of the materials found in the coil, to prove that they will not deteriorate each other or the insulation on the magnet wire.

Transformers, both power and output, that are built to these standards are very tough to destroy and when failure does occur they will still protect a user from shock hazard and, because all of the materials are self extinguishing, from a fire hazard once the power is removed from the transformer. Naturally, with a set of thermal fuses in place even this limited hazard is removed.

Bud