Toroid transformer rewinding

Changing output voltage

Abstract

I have a huge 1kVA toroid transformer at home. The primary winding is for 230V and the transformer features two secondary windings with 66V each. The output voltage is too high for the intended application of the transformer and this is why I need to lower the output voltage. I document my experience with the modification in this article.

Modification

original toroid transformer
This is how the original transformer looked like. Overall diameter is 185mm, height is 60mm and weight is roughly 8kg. It looks fairly well made. Cost was around 100€ long time ago. I even have a second identical one. The transformer also features a shield winding between the primary and the secondary windings to reduce capacitove coupling.
Building an audio amplifier with this monster transformer would mean having roughly +/-90VDC supply voltage, which allows 550WRMS into 8Ω load or 1100WRMS into 4Ω load. Clearly, such a high powered amplifier is barely useful in a domestic environment, but back then I was crazy enough to design and build such a monster amplifier. I even planned to build the amplifier dual mono, but one such transformer is sufficient to power a stereo amplfier easily. So I have one transformer left over now and plan to build an amplifier with more reasonable power supply voltage.

There are multiple ways to lower the output voltae of a transformer:

Each option has advantages and disadvantages and some options may be more feasible with given transformer than others. I decided to reduce the number of turns of the secondary windings since they are easily accessible.

toroid transformer with cover tape removed
This is how the transformer looks like with the cover tape removed. The two secondary windings are wound on top of each other. Wire diameter is 1.8mm. I wonder why there is no insulation layer in between the two seondaries. With the wires being on top of each other, the weight of the transformer, the mounting torque compressing the transformer, the vibration during operation, the heat generated and all this over many years of usage, I wonder whether the thin layer of insulation of the wires can sustain good insulation. It may be common practice to do it that way, but I believe there should be a layer of insulating tape in between.
toroid transformer with secondary windings number of turns reduced
This is the transformer with the secondary windings turns reduced to provide an output voltage of 54V. After rectification, this results in roughly 75VDC. This is still plenty of voltage for up to 360WRMS amplifier output power into 8Ω load. Now there is just a region on the transformer with no secondary windings. It may be perfectly acceptable to leave the transformer this way and just apply the cover tape again. Another option would be to use the open space to add some low voltage secondary windings. Some additional low voltage windings are very useful, either for boosting the power supply rails for the front end amplification circuitry or to power any auxiliary circuitry. Since the power rating of the transformer has been reduced by lowering the output voltage, there is plenty of spare left. Putting the auxiliary winding as section winding into the empty region would be easy to do and the high leakage flux associated is no concern for the application.
toroid transformer with one secondary winding reduced
I decided not to take the easy approach and proceed in a different way instead. I wanted to redistribute the secondary winddings onto the full circumference of the core again. Since the windings were wound on top of each other, just stretching the windings would result in the whole windings to become lose and generate noise and afore mentioned risk of insulation failure. So I removed the top winding completely.
toroid transformer with one secondary winding redistributed
Redistributing of the bottom winding also tightened the winding around the core a bit. I covered the bottom winding using adhesive PET tape. I was looking for PET tape without adhesive like the cover tape the transformer had applied, but did not find any such tape anywhere. The adhesive tape application was much easier to do than I anticipated. The tape is pretty stretch, which allows to tightly apply the tape and the adhesive helps to secure the tightness of the tape already applied. The tape I used has 10mm width and a more wide tape would have been easier to process. Also, the clear tape is very transparent and it is difficult to visually determine coverage and overlap. Colored tape might have been a better choice. The tape is super thin, but a good insulator. the additional insulation is important for my peace of mind.
toroid transformer with secondary winding re-applied
The wire of the secondary winding that I removed was totally bent and buckled of course. I had to straighten the wire again prior to application. I went out on the sidewalk, tied a bench wise to a traffic sign, which held one end of the wire and gripped the other end using a pair of tongs. Application of some drag both straightened and lengthened the wire. I would the straight wire onto a piece of cardbord, which I used as shuttle. This made application of the winding much easier than dealing with roughly 15m of wire rioting inside my living room. It is just important not to bend the wire too tightly onto the sutlle, else the previous straightening was futile. Application of the winding was a lot of effort nonetheless. After the electrical test, I applied another layer of adhesive PET tape to secure the winding. At this stage, I consider adding some auxiliary windings. To be continued.

Conclusion

The project is still ongoing. No overall conclusion yet.