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?
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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- Bottom
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