Ferrite inductors have a ferrite core. Ferrite is a class of ceramic material with strong electromagnetic properties such as high permeability in combination with low electrical conductivity. Ferrite inductors are deployed in a variety of electric circuit applications such as broadband, power conversion and interference suppression.
Make a simple ferrite inductor by wrapping at least 20 turns of wire around a ferrite rod. Using an inductance meter, measure the inductance of that rod. Record the inductance as "L" and the 20 turns as "N."
Calculate the AL value of the ferrite inductor. The AL value is the base relationship between the inductance, with a given ferrite core, and the number of turns. Calculate the AL value using the following formula: AL = [(100/N)^2)] x L.
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For example, if your measured value of L in Step 1 is 25 uH, the corresponding AL value will be: AL = [(100/20)^2] x 25uH =( 5^2) x 25uH = 25 x 25uh = 625 uH.
Use your AL value to calculate the inductance for various values of N using the following formula: L = AL/[(100/N)^2].
For example: If N is 15, L = 625/[(100/15)^2] = 625/[6.66^2] = 625/44.4 = 14uH.
If N is 25, L = 625/[(100/25)^2] = 625/[4^2] = 625/16 = 39uH.
If N is 30, L = 625/[(100/30)^2] = 625/[3.33^2] = 625/11.1 = 56.3uH
As you can see, the inductance increases as N increases. This is because by placing more turns of wire around a loop, it concentrates the magnetic field into a smaller space, where it can be more effective and create more inductance.