# Winding a toroid inductor, is this calculation correct?

#### JMK

Summary
I am going to wind an tapped toroid inductor for a LCR circuit in an equalizer. I have calculated the number of turns I need for each winding.
Hi,
I am going to wind an tapped toroid inductor for a LCR circuit in an equalizer.
I have calculated the number of turns I need for each winding.
This is my first time winding an inductor so I need to get some things straight.
What to do with the 10 kHz tap, should I twist a wire with the 12 kHz tap in order to have two taps with 31.4 mH?

And did i do this calculation correct?

Info:
Core type MPP
OD 39.90 mm
ID 24.1 mm
HT 14.5 mm
AL value: 740 +/- 8% (nH)
MPP Permeability: 550 (μ)
Wire gauge: 34 AWG

16 KHz - 29.9 mH 195 turns (wire lengh for 29.9mH 195 turns = 8.829 m)
12 kHz - 31.4 mH add 5 turns
10 kHz - 31.4 mH twist a loose wire to 12 kHz ^?
8 kHz - 48.2 mH add 48 turns
5 kHz - 67.5 mH add 45 turns
4 kHz - 87.9 mH add 41 turns
3 kHz - 156 mH add 111 turns

I used this calculator https://coil32.net/online-calculators/ferrite-torroid-calculator.html

Help is much appreciated!
All the best

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#### Tom.G

Yes, that works. An easier way is connect the 12kHz tap to the switch then connect together the switch contacts for the 12kHz and 10kHz positions.

You can do the same for the capacitor at the 3kHz and 4kHz positions.

Cheers,
Tom

p.s. 34Ga wire is quite hard to work with, you might consider 30Ga, or even 28Ga.

Last edited:
JMK

#### JMK

Hi Tom,

Yes i would just do it on the switch of course.

Do you have any tips on how to wind this type of inductor?

I know that its best to wind this in a linear pattern
but when i am done with lets say 195 turns, should i then glue the end so its tight, or should i overlap the end with the new winding of 5 turns, so that would hold it in place?

Also when i am done winding the coil, should i then dip the coil varnish, or is this unnecessary, inappropriate for this type of inductor and use?

Any tip would be appreciated! :)

Thanks again
All the best.

#### Tom.G

Do you have any tips on how to wind this type of inductor?
Since it's for audio work things are not very critical, so here are a couple practical tips.
• The polarity of the windings is important. They are to be wound all in the same direction and the finish of one widing connects to the start of the next.
• The calculations are approximate. The number of turns is based on the theoretical, which is approximated when machine wound. Hand winding often needs a few percent more turns. Don't worry about it for audio stuff.
• You won't be winding as tightly or as uniform as a machine so don't cut the wire off the spool too short. 10% extra length should be more than enough. Just remember that as the layers build up the wire needs to be longer and adjust accordingly. For instance that last 111 turn layer, depending on wire size, could use almost as much wire as the first 195 turn layer.
• Nice even layers that go smoothly and completely around the core are best. Again, for audio work this is not particularly critical from an electrical standpoint. From a mechanical standpoint, the "scramble wound" approach takes up more room, uses more wire, and the overlaps are high-stress points where there is more likely to be a short between turns.
• A trick to make winding less tedious is to start winding from the middle of the wire. If you keep track of the wire length you started with, you will know how many turns you can get from that length, and how much to take off of the spool to complete the winding.
• As for dipping in varnish or using glue, I would avoid those for this use. Tape can be used if needed to keep things neat and out of the way. That is assuming the final assembly will mounted in the audio rack, not sitting on top of the 14inch woofers. If you do need protection from vibration, check that any adhesive/varnish you use does not attack the wire insulation.
• Have on hand: tape, pencil paper and eraser, tape measure, felt tip marking pen, wire cutters, a way to mark the wire ends so you know which is which when you connect them; keep in mind the poalrity as noted in the first tip.

Here are a couple links that may be useful.

In this video note that it was wound too loose. Under vibration that one is going to change characteristics and wear thru the insulation. It also uses more wire than if it were wound more tightly.

In this one, the TIP at the end of the article assumes that the inductor will be mounted standing up on a PC board so he leaves about 30° of the core free of wire.

And here are some links showing toroid winding machines in action.

Cheers,
Tom

#### JMK

Very very useful information Tom.
Thank you very much for this, really clears it up for me.

All the best

"Winding a toroid inductor, is this calculation correct?"

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