Toroidal Transformer Rewinding

Thread Starter


Joined Dec 22, 2012
Hello, I bought some time ago a 2x17Vac/200VA transformer from Indel. I was dissatisfied by the voltage of the secondary, because it was too high for my current project. So, I disassembled the transformer and I took off a few turns from one of the 17Vac secondary. After that modification, the output voltage from the 2 series secondaries was about 30Vac (17Vac from one secondary and about 13Vac from the modified secondary).
A few days ago, I needed a 26Vac transformer so I disassembled again the transformer and I made an output for 26Vac.
Now, the secondary winding wires are not equally distributed on the toroidal core and if I connect a load on the secondary, one half of the transformer is heating up more than the other half. I will make a test to check the maximum temperature of the transformer after a few hours of working with load. This kind of problem was something that I expected, and my questions are:
1. What are the problems that can appear if the transformer was rewound as I specified above ?
2. Should I buy a new transformer ?

Thank you in advance.


Joined Jul 18, 2013
Normally when you wind(rewind) a Toroidal, you should spread the winding so it covers at least 360°.
Maybe you have one piled in one spot.
One other mistake sometimes seen is where the Centre mounting bolt has includes a cover that extends down to the bottom chassis, this effectively is a shorted turn and heats up the transformer.


Joined Jun 17, 2014

Toroid transformers are hard to wind from start. There are complicated machines that are used to do them in production to watch one work is an experience.

To modify a bifilar winding you really have to take both windings off at the same time. Look up bifilar coil or winding.
If one secondary is put on first and the next secondary after that, then you only have access to the second secondary unless you take that off completely so you can access the first secondary. That's the way it usually works. If you add turns to only one secondary then strange things can happen if you try to use them in parallel.
If you add turns then you have to recalculate the primary current also.


Joined Jun 17, 2014
Although I have added overwinds many times and alot easier than a EI type.;)
I agree if a dual paralleled winding should be wound bi-filar.

Oh yes sometimes it is an easy add on.
The machines that wind these things in production are really cool though and amazing to watch.