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.

I'm confused about the bandwidth tap, how should I wind this on, just one turn to the core?

Also the 10 kHz tap, should I twist a wire with the 12 kHz tap in order to have two taps with 31.4 mH?


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 194.851 turns Wire lengh 8.829 m

12 kHz - 31.4 mH add 4.827 turns

10 kHz - 31.4 mH twist a loose wire to 12 kHz ^?

8 kHz - 48.2 mH add 47.761 turns

5 kHz - 67.5 mH add 45.325 turns

4 kHz - 87.9 mH add 41.324 turns

3 kHz - 156 mH add 110,982 turns


Help is much appreciated!

  • \$\begingroup\$ 1) Can you provide the circuit in which there inductors will be used? 2)I think you have your calculations for the number of turns wrong. You will never wind nearly 111,000 turns of 34AWG on that core. Will you please show your calculations. Do an online search for "toroidal inductor calculator" and note that the inductance is proportional to the square of the number of turns. \$\endgroup\$ – Peter Jennings Aug 24 '19 at 22:43
  • \$\begingroup\$ Hi Peter, thanks for your answer The inductor would be used in this circuit. gearslutz.com/board/attachments/geekslutz-forum/… The turns should be read like 111 turns (i just copied the values from this calculator coil32.net/online-calculators/ferrite-torroid-calculator.html \$\endgroup\$ – inductor Aug 24 '19 at 23:23
  • \$\begingroup\$ Note the mH values on the schematic is not correct. \$\endgroup\$ – inductor Aug 24 '19 at 23:36

In my comment I was misreading your question.

You cannot have partial turns, so they have to be rounded to the nearest whole turn. The table will then read 195+5+48+45+41+111.

I would suggest that you use one continuous length of wire for all 6 taps.

Take the length of wire that the calculator says you need for the largest inductance, 156mH. This is just over 20m, add a bit for the taps, say 1 m then start winding. After 195 turns create the 29.9mH tap by bringing out a loop and twisting it together. Add a further 5 turns and create a second tap in the same way. Repeat for the other taps.

Once you have wound all the turns, you can clean the insulation of each of the twisted taps in turn and tin them with solder to make the connections.

From the diagram I see that the taps are selected by a multiway switch, all you need to do is connect the two fixed contacts on S1b for positions 10KHz and 12KHz together and to the second tap on the inductor. You can do the same on the S1a switch for the capacitors that are shared between frequencies.

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  • \$\begingroup\$ Thank you very much Peter, this really clears it up for me, I will try it out, and see how it goes. Is it necessary to coat the coil when its done, or would this be unnecessary, inappropriate for this design? \$\endgroup\$ – inductor Aug 25 '19 at 3:30
  • \$\begingroup\$ I doubt that coating the inductor is necessary. However you will have to mount it somehow with the other components. You may have a mounting kit supplied with the core, in which case, use it. However probably not. In which case something like plastic cable (zip) ties (not too tight) to hold it onto a board and some silicone adhesive to prevent it from moving. Don't forget that you cannot use any form of metallic clamp that would complete a full turn through the core as this would act as a shorted turn and ruin the inductance. There are several videos on YouTube on how to wind toroids. \$\endgroup\$ – Peter Jennings Aug 25 '19 at 9:13
  • \$\begingroup\$ Just another thought. If you have any means of measuring inductors, maybe you should wind a test one to start with, say the first section of 195 turns, then measure it to check that the answer given by the calculator is correct. \$\endgroup\$ – Peter Jennings Aug 25 '19 at 9:16
  • \$\begingroup\$ Thanks for tips, i will stay away from coating then. I think i will seal the inductor with some tape. I did this circuit old school P2P, the inductor will be in an enclosure on the back of the cabinet filled with wax. Yes it would be best to do as you suggested, i will do a test-winding to figure out how close the value is. All the best! \$\endgroup\$ – inductor Aug 25 '19 at 14:37

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