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Newbie questions about winding transformers

I've bought a pair of mono amplifiers, hoping to restore and modify them slightly. Unfortunately, one has been heavily modified already and the output transformer has been rewound.
I would like to rewind both OTs to make them a matching pair. Besides the task at hand, transformer winding is a valuable skill I've always wanted to learn.

I have had a look around on this forum and also around the web, but information seems scarce on this topic. I have a seen a few videos online and I think I can pull off the mechanical work. The few questions I have regard the theory and the design of transformers:
  1. I'm planning on buying a rather cheap manual winding machine from Amazon. While I'm at it I will buy the enameled copper wire as well. In order to do that, I understand that I need to know the number of turns and max current for the given winding. I understand I can also parallel multiple wires to obtain a bigger gauge, but given my inexperience and cheap equipment, I'd rather avoid that.
  2. I'd like to receive recommendations for cheap winding insulation materials. I'm interested in both scotch tape (I have seen yellowish transparent tape used as insulation) and pressed paper. I'm looking for something that's easy to get and fairly cheap, hopefully available at a local shop. Does insulation have an influence in design choices? (As far as I've understood yes, but I don't know how)
  3. The main issue I need to sort out is designing. I'd like to understand how to design a transformer. I understand the basics of how transformer work, and I would probably be able to compute the necessary winding ratio for a power trafo. Then, I'd need to decide the wire gauge based on the allowed current, the core size, etc.
    Guidance, references or even books on these steps are extremely appreciated. If there is some sort of spreadsheet or tool to compute these numbers I'd happily use that as a safety check before proceeding with winding.
  4. Winding output transformers is a totally different ball game, as I see oftentimes windings are interleaved, and I'd need to understand when and how to apply that technique. The OPTs I'm going to rewind are push pull, so they're at least easier (and I hope more forgiving) then single ended.
I'm going to rewind both power transformers and output transformers, so I may want to start from the first as they seem easier and more forgiving. Sine I'm rewinding all the iron, I can also change the amplifier design quite a bit. Given the amount of freedom I have in this build, I prefer to learn how to design transformers and not receive a pre-made design without explanation. Any suggestion or reference to a guide is appreciated.
If you want to explain by giving an example, my goal would be to produce 2.5k PP output transformers rated for 60w (I don't know the type of the EI laminations and core, I'd still need to measure them).
 
Well I guess any dielectric adds capacitance, but kapton tape is possibly on the heavier side.

Anyway, besides what isolator to use, my main issue is calculating the number of turns and wire thickness I will need, as I still need to purchase everything.
 
Kitchen silicone paper has pretty good insulation properties and it isn't hygroscopic. But it doesn't hold glue/tape well, so you need some practice for tight insulation turns. Its thickness is 45-55um
You could also use some virgin kraft paper, but remeber it is hygroscopic and I would be cautious using in moist climates. You also need to soak a sample in water and measure the pH to make sure it is neutral.
Yellow mylar self gluing tape can be bought online from many dealers.
 
Edit time ran out .... Never thought about the pH of insulation but I guess highly acidic paper wouldn't be good.

Aside from easy to get papers like grease proof paper there are proper electrical insulation papers like Nomex, Mylar & DMD to name but a few. These have far superior insulation properties as well as being tough. Finding a source for them like finding a source for bobbins and laminations can be problematic unless your want ££££'s worth.

Re how to work turns etc drop me a PM with your email address & I'll send you what I have about OPT & tfmr winding.

Andy.
 
Thank you everyone for the kind suggestions!

For the insulation material:
I do have some baking paper which from what I understand from a quick google search is covered in silicone too, so I may try using that one.
I do have a semi-local (in my same country at least) source of bobbins and laminations. This place sells both built transformers and materials for self construction.

Also, another question about insulation:
I have heard that optionally one can put dielectric between every layer of wire, as well as between each winding. Should I use two different materials? what else should I vary if I decide to do this optional thing? I have seen a bunch of videos on youtube of people winding transformers and OPTs but they never do it, probably not to waste time. Would you guys recommend it?

Have you tried this excellent software? http://www.dissident-audio.com/OPT_da/Page.html It was written by a member.
I had downloaded it before but I wasn't really sure how to use it. What I'd like to have is some sort of datasheet where I can input:
  • my EI lamination and bobbin size
  • my windings requirements
  • (possibly) the required frequency range
And I get out:
  • the number of turns for each winding (I assume I would get multiple windings for a single primary or secondary)
  • the interleaving instructions for the various windings

If this software can achieve that, I would love to use it (but I'd still like to understand the math behind it so I can sanity check my work).
In the meantime I'm going to give it another shot and hopefully I will understand how to use it.

Re how to work turns etc drop me a PM with your email address & I'll send you what I have about OPT & tfmr winding.
I would absolutely love to know how to do the math myself! I PMd you my email.
 
The amount of time and work to wind transformers is large enough that I'd never go cheap with anything. Not much useful will be available at any local shop anyway, so order the best materials you can. The math isn't too tough, but it's the details of construction that are hard to find. I've wound a fair number of transformers, but always for switchers or power, never audio.
 
When I wind toroid power tfmr's I insulate every layer and double insulate the primary from the secondary. I've tested the toroids I've wound up to 10kv, they pass.

The problem with EI tfmr's is that your window or space is finite unlike a toroid, every thou or 0.1mm counts and you soon find your running out of space. I've had to re-wind some tfmr's up to three times before I've got it right.

There is a simple bit of free software that is easy to use to wind mains tfmrs, see - https://sklaic.info/electronics/ that may serve as a starting point. You input your core size, primary voltage and secondary voltages, it gives you how many turns and what gauge wire to use, that's it. You have to calculate if the wire you have will fit etc & obviously a mains power tfmr isn't an OPT. However, I'd recommend getting hold of some old mains tfmr's, taking them apart to get the laminations and bobbins then having a go at winding a power tfmr before you try winding an OPT. It will teach you the basics of winding a tfmr.

I'll be in touch, Andy.
 
I had downloaded it before but I wasn't really sure how to use it. What I'd like to have is some sort of datasheet where I can input:
  • my EI lamination and bobbin size
  • my windings requirements
  • (possibly) the required frequency range
And I get out:
  • the number of turns for each winding (I assume I would get multiple windings for a single primary or secondary)
  • the interleaving instructions for the various windings

If this software can achieve that, I would love to use it (but I'd still like to understand the math behind it so I can sanity check my work).
In the meantime I'm going to give it another shot and hopefully I will understand how to use it.


I would absolutely love to know how to do the math myself! I PMd you my email.

The software does exactly what you describe. You input your impedance and wattage requirements along with core information and it gives you winding and interleaving numbers. There is a bit of a learning curve involved. My only annoyance is the wire gauges being metric.