About technologies for your digital home

For beginners and tinkerers

Designing/rewinding your own power transformer

May 20th, 2019

As you might already read in my previous post, I started to work on a new project – TDA1543 audio DAC project. After reading in different places, I decided to wind the power transformer myself. According to my understanding, almost all of industry produced transformers are under-winded and it barely matters where did you get the transformer itself…

Few interesting and useful links about winding the transformers

Most DYI’er like to salvage the old transformers and reuse their cores and the bobbins. So, your starting point could be to learn how much of the power you could squeeze out from old transformer. Of course then, you should be able to decide either that will be enough for your project.

Here is a nice website, where you could check the max power of the core and do some other calculations.

Another good website, where you will be guided through calculating different parameters for your transformer, can be found here.

Main transformer’s equation

Ai= area of core (in m^2)
F= operating frequency
Bm= magnetic flux density
Te= turns per volts

Just one short comment here, we can see that the area of core is inversely proportional to the lower part of the fraction: 4.44 x
operating frequency x magnetic flux density x turns per volts. Whatever you decide to do with your transformer design but this equation should be satisfied.

Normally, the magnetic flux density is determined by few factors: the steel quality and the application of the transformer itself (chosen magnetic flux density). For audio purposes, people tend to use quite lowish flux density value. Few clues on this will be given in next reference.

Absolutely fantastic read is the following, which covers everything, includes different comments related to different parameters and plus gives an Excel file with all calculations and to to check either your designed transformer will fit e.g. onto your salvaged core:

Once we are here and you decided is it worth to proceed, just double-check either you have an electric rotary tool (via eBay) as it is very handy during the disassembling process of the transformer and its bobbin.

Salvaging the transformer

It sounds that as quite simple task. You just take out the transformer and dissemble it. Well, not so fast. I will give my small statistics. I had four 10-20 VA’s transformers. I was able to disassemble two out of four.

Disassembling transformer
Broth soup with one of my transformers and turpentine substitute

Here is one of the examples. I kept the transformer for one week in the turpentine substitute. However, no luck. It remained as a stone.

If the transformer operates at pretty high magnetic flux density and has low turns per volt, it would vibrate quite a lot. I read that the vendors put the epoxy varnish in vacuum on such transformers to reduce the vibrations. Smart. However, it is close to impossible to disassemble such one. I think two of mine got this wonderful epoxy varnish.

However, it was not that bad in other case and I was able to disassemble the transformer.

Disassembling transformer

To disassembly the transformer requires to have right tools, which includes the electric rotary tool (via eBay). Once the calculations done and you are sure that everything will fit onto the bobbin, you only need to get right diameter magnetic wire.

In order to make nice symmetric wingdings, I am planning to get a mechanical transformer winder (via eBay) as I will rewind few more transformers for other projects.