I made some inductors with CF196T1305 ferrite cores ( http://www.cosmoferrites.com/Downloads/ProductSize/3defd392-c210-4f70-a6c6-70f5b60b1953_T1305%20_WS.pdf). According to the data sheet these should have an AL of about 1200 with +30/-20% tolerances. But the inductances of my inductors does not match the math.

Here are pictures of my windings.

  • Turns = 20, \$ I_{math} = 480uH \$ , \$ I_{actual} = 728uH\$, about 50% more than calculated.

enter image description here

  • Turns = 25, \$ I_{math} = 750uH \$ , \$ I_{actual} = 1194uH\$, about 59% more than calculated.

enter image description here

I do not posses a LC meter, so what I did is, I build a LC tank circuit, with a capacitor of some known valve. I give a square wave of fast rising and falling edges, now by watching the circuit resonate, I tried to calculate the incidence.

enter image description here

I don't know but such huge difference seen wrong. Are my windings OK? Or what could be the fault?

  • \$\begingroup\$ I'm going to bet that your testing method uses a fairly small current compared to the rating. Toroids saturate at a point, and are often rated based on what inductance they would have at a certain operating current. \$\endgroup\$
    – user39962
    Apr 9, 2015 at 20:55
  • \$\begingroup\$ Test method is all-important \$\endgroup\$
    – Andy aka
    Apr 9, 2015 at 21:21
  • 2
    \$\begingroup\$ When measuring inductance you often get a large value just from the test leads. It's not like measuring resistance. You need to lay out your experiment, then short circuit the inductor under test with a short straight wire. Record that inductance, remove the short without moving any wires, and measure the extra inductance. \$\endgroup\$
    – tomnexus
    Apr 10, 2015 at 2:49
  • \$\begingroup\$ Another way to check it is to measure the slope of the current ramp when you apply a known voltage. Obviously you have to limit the voltage duration to prevent blowing up the inductor. You can use a current probe if you have one, or measure voltage across a sense resistor if you don't. \$\endgroup\$
    – user57037
    Apr 10, 2015 at 21:19
  • \$\begingroup\$ @Andyaka I have updated my question to show my method of measurement. \$\endgroup\$ Apr 11, 2015 at 3:35

1 Answer 1


The core supplier states http://www.cosmoferrites.com/Downloads/Downloads/bcb70e7c-ae33-40ec-a881-0d5120db57b6_GTC.pdf that the inductance factor AL is calculated: AL = (Lf – L0)/N**2 where L0=inductance without ferrite core, Lf=inductance with ferrite core. For ring cores the Testing Conditions are: 10 kHz/150mV/N=1/L0 = 0/25˚C. If your calculation or your measurement is at different conditions, that may explain the big difference you find. But can you say what frequency is most relevant to your application?


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