Re: [EE] kapton thickness for transformer

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On Thursday, January 23rd, 2025 at 6:31 AM, Chuck Olson <[email protected]> wrote:

> Forgot the link to the November 1976 issue of QST with the Ugly
> Duckling article :
> 
> https://www.worldradiohistory.com/Archive-DX/QST/60s/QST-1976-11.pdf
> 
> Best Regards,
> Chuck, WB9KZY
> 
> On Thu, Jan 23, 2025 at 6:47 AM Chuck Olson [email protected] wrote:
> 
> > I only rewound one transformer, never wound one from scratch. Lew
> > McCoy had an article in November 1976 QST on the "Ugly Duckling" where
> > he rewound an old TV set transformer for a low voltage (18 volt)
> > output (can you say E I ?). He left the primary alone and also the
> > insulation over the primary. The secondary windings were gauged by
> > the number of turns on the 5 volt winding.
> > 
> > My victim was a non-working Knightkit tube amp (KG-240 ?) - it still
> > works but it buzzes and the laminations do kinda warm up :)
> > 
> > In these modern times I'd consider a design for a 3D printed plastic
> > bobbin - usually the primary and secondary are two windings side by
> > side separated by plastic - check the data for Tamura (or other
> > brands) of transformers for ideas.
> > 
> > If this is a toroid, yikes, good luck on that !
> > 
> > There is a book on tubebooks.org on transformer design:
> > 
> > http://www.tubebooks.org/Books/Lee_1955_Electronic_Transformers_and_Circuits.pdf
> > 
> > Best Regards,
> > Chuck, WB9KZY
> > 
> > On Wed, Jan 22, 2025 at 10:53 PM madscientistatlarge
> > [email protected] wrote:
> > 
> > > I'm building a large isolation transformer (1KW+-) on a laminated steel core. How much kapton is typically used for insulation between the windings and the core, i.e. how many 1 mil layers (or 2 or 5 mill is also available).? I know that surviving power spikes requires planning for 10KV+ surges. I'm thinking 4 or 5 layers at 1 mill thickness? Lets say the voltage is 250V (I'll also be using the output in series with the outlet to get "220" V). I'm in the U.S. so power line is about 127V (they keep creeping it up to increase power levels). Thank you.


  I'll read that.  I intend to be conservative on this, and apply some derating.  I may make a bobbin, though I could just put a plasic disc between primary and secondary.  I was planning on side by side with a gap/insulator.  The whole point is safety, I really, really hate getting shocked, I've had some good ones (BAD!).  One nice thing about kapton/polyimide is that it doesn't track much, i.e. an arc usually doesn't make a conductive carbon path on the surface when heated by an arc.  The arc heating is actually why you feel static electric shocks with door knobs and such.

I'm thinking of gluing a bobbin together.  Transportation is a problem for me, but there is a local hackers gild with all the neet stuff (They were just recently gifted a 2.4kw laser that's probably been refurbished by now.  More importantly 3D printing both with filament and liquid polymerization.  I could probably find one online.  A bobbin would definately work best, and makes it easy to wind off the transformer, I could hack together something to turn it slowly pretty easily.  I am sure it will tend to deter me from rewinding other transformers but I have time and little $$ Though I can now spend something on my hobbies.  I won't be ordering materials until the first part of next month so I have plenty of time to think and try not to be stupid.  I've worked in life safety, we all make dumb and dangerous mistakes more than most people realize.



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