I want to make an LC filter resonant at 150 Hz using a 2.5 uF 400 V CBB61 capacitor and inductor. They are shown in the image below. L and C The toroid core looks like an RF toroid core but I don't know the AL. The toroid core length is slightly less than 30 mm and its outer diameter is 17 mm.

From calculation I need 450 mH inductor to make a resonant circuit with 2.5 uF capacitor at 150 Hz. So I planned to make 450 mH inductor from some of this core.

Is it possible to make an inductor from this core (or many of this type of core) that has optimum resonant circuit at 150 Hz (3rd harmonic frequency of 220 V / 50 Hz power system)?

If not, what kind of inductor should I use?

Its use is to make an shunt path for 3rd harmonic current.

  • \$\begingroup\$ Do you have three phase or single phase? Because for three phase is an easier approach using three current transformers in series - this is a really shunt path. \$\endgroup\$ Mar 15, 2016 at 10:53
  • \$\begingroup\$ It may be possible to make a circuit that resonates at the desired frequency. But will it handle enough power to have any measurable effect on a live 3-phase circuit? Certainly not. \$\endgroup\$ Mar 15, 2016 at 12:27

1 Answer 1


Easy check: Google/search Farnell/Digikey for some 450 mH inductors, see what these look like. Is that anywhere near what you can make with this ? My guess: no.

Also note that not all toroid cores are the same the properties of the material can vary widely. Some material gives you a high inductance but saturates at low fieldstrengths. A saturated coil behaves as a wire (the inductance is gone) !!

Unless you have specialized equipment to characterize the coil, I would forget about it and just buy a component that has a datasheet.

  • 1
    \$\begingroup\$ If theoretically, I put in series 450 pieces of 1 mH air core inductor commonly available for speaker crossover, I can use that inductor because it won't be saturated, right? \$\endgroup\$ Mar 15, 2016 at 8:14
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    \$\begingroup\$ An air core inductor cannot saturate under any circumstances - you just can't go below the vacuum permeability. \$\endgroup\$
    – jms
    Mar 15, 2016 at 9:15
  • \$\begingroup\$ Oops ! You're right, I missed the "air" bit ! My bad. \$\endgroup\$ Mar 15, 2016 at 9:21
  • \$\begingroup\$ And yes you can put 450 1mH inductors in series and get 450 mH. \$\endgroup\$ Mar 15, 2016 at 9:30
  • \$\begingroup\$ But the DCR of this massive coil combo will mean that the filter wont work.The air cored speaker crossover coil may have more resistance than reactance at 150HZ .If you made a BIG coil it would work. \$\endgroup\$
    – Autistic
    Mar 15, 2016 at 10:15

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