In the first picture you can see 45 turns of standard 0.4 mm copper wire. The impedance analyzer (e4990) shows an inductance at 20 kHz of 11.5 mH. Which is inside the tolerance of around 12.8 mH +- 20%.

The impedance analyzer measures Ls and Rs, which is the equivalent series resistance and inductance of the impedance at a given frequency.

45 turns of 0.4 diameter copper wire on an E100

However, using the following litz wire coil:

Rupalit 3.05x3.05 safety profile, 1260x0.071 (overkill for 20 kHz)

Measured the same way on the same core:

E100 Core with litz wire 45 turns

Gives only around 8.2 mH. How is this even possible? I have gone through simulations, lots of models that could possibly cause this, asked some Phds but I can't answer such a huge deviation.

Here is what I already thought of:

Any capacitance can't be the cause of this, since for lower frequencies we see the same inductance value (also evident in circuit simulation).

The air between the core legs can cause huge deviations, but when it is pressed down with a clamp we get the same results.

Even if I missed a turn on the litz wire coil, it wouldn't cause such huge change (44^2/45^2).

Very high quality litz wire (3.05x3.05 mm 1280 x 0.071mm rupalit safety) -> impossible for shorts, which could have explained lower inductance.

Has anyone seen this before or are there papers on this?


Here is the same measurement at home giving 11.17 mH.: enter image description here

Now I added 45 turns of thinner copper wire but with huge gaps to increase leakage as requested by @tobalt: enter image description here

As you can see leakage is increased by about 0.4 mH. Which in context to the original problem makes the litz wire measurement further implausible.

  • \$\begingroup\$ That impedance analyzer should be doing well at that frequency and that value of inductance. The only thing I can think of- and I don't see why it changes the measurement- do you have good solid connections to all the Litz strands? What are the measured Rs values? \$\endgroup\$
    – Fred
    Commented Mar 16 at 0:11
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    \$\begingroup\$ Would the down-voter care to share the reasons for the downvote? Seems like a valid question and plenty of investigation before posting IMO \$\endgroup\$
    – Frog
    Commented Mar 16 at 0:29
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    \$\begingroup\$ I can't see anything in the first picture; could you submit larger pictures, or more closely cropped to show the subject? \$\endgroup\$ Commented Mar 16 at 1:17
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    \$\begingroup\$ @Fred Good point, for some reason I turned a blind eye on the resistance measurement. I looked up the E4990 Help Page: "Equivalent series inductance Equivalent series resistance Ls - Rs". Then intuitively the Rs measurement of 5.2 Ohm seems wrong for the 0.4mm copper wire version. I will think about it for a minute. \$\endgroup\$
    – cu 29p
    Commented Mar 16 at 3:02
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    \$\begingroup\$ @SpehroPefhany yes you are correct. I'm mainly just talking about the series circuit. My argument is, that the equivalent inductance can never be lower if you account for parasitics at low frequencies (below resonance). \$\endgroup\$
    – cu 29p
    Commented Mar 17 at 0:13

2 Answers 2


For 0.4mm magnet wire, you have tight winding over the core with no gap. The litz wire looks 10x thicker, has additional jacket, and comes on a bobbin with thick walls. So the geometry of wires is totally different, coil-to-coil coupling is different. Nearly everything is different. Why would you expect the same inductance?

If you try a copper wire of the same as litz diameter over the same bobbin, I am sure your results will be much closer.

  • \$\begingroup\$ Well if anything I would expect the litz wire coil to have higher inductance. You mentioned the coil geometry, which because of air gaps should increase the leakage inductance. The total inductance is then the sum of leakage and magnetization inductance, which should be very slightly bigger than the 0.4 mm version. \$\endgroup\$
    – cu 29p
    Commented Mar 16 at 3:57
  • \$\begingroup\$ @cu29p, not sure about your "leakage inductance", but from simple physics it should be obvious that the outer layers of your litz bobbin are far from magnetic core, so the effective inductance factor (AL) for these outer layer turns is lower than if the wire would tightly wound on the core (as it is in 0.4mm copper). That's why your litz inductance is lower. \$\endgroup\$ Commented Mar 16 at 5:21
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    \$\begingroup\$ @cu29p can you make a cross-check: Take any wire, and make a deliberately bad/loose winding -> check if inductance drops even more. \$\endgroup\$
    – tobalt
    Commented Mar 16 at 5:38
  • \$\begingroup\$ If all "things" were the same, "Litz wire" would have a lower resistive part and Q would be higher for the same "inductance" ... \$\endgroup\$
    – Antonio51
    Commented Mar 16 at 9:10
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    \$\begingroup\$ @tobalt I added pictures for it. Thanks. \$\endgroup\$
    – cu 29p
    Commented Mar 16 at 12:32

As core was inserted into winding with 3d printed bobbin, some insides of the 3d printed material was scraped off and landed right between the core halfes creating an air gap lowering the effective permeability. Next time print the bobbins with more space inside!


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