Question about coils

[Jonasz]

I have a 0.5mH coil and I need to unwind it to 0.44. How many rounds do I need to unwind to get the right value?

[Dennis H]

turns1/turns2 = squareroot(L1/L2)

[Jonasz]

Thanks a lot Dennis!

[engr_dave]

That formula only works when you know the number of turns in the original coil. And it is not 100% accurate. Dimensions DO make a difference:



If we let S1 = (c/2a)2 then the inductance, in henrys, can be approximated by

L = 4×10-7[pi]a N^2((0.5+S1/12)ln(8/S1) -0.84834+0.2041S1)

where a is the mean radius of the winding in metres.

Example: If the mean radius is 2.4 cm, the width is 9mm and N is 350 then L = 6.977 mH

Clearly, the easiest way to do this is to buy a cheap multimeter that measures inductance like this one

and just unwind until you measure the right value.

Dave