i needed a 0.25mH inductor and all that was available was a 0.22 and a 3.3mH, i bought the 0.33 because i thought that i could unwind it until i reached 0.25mH.
Can someone tell me how i many winds till i get to 0.25 or another way of getting there without spending $60 on an inductance meter.
Can someone tell me how i many winds till i get to 0.25 or another way of getting there without spending $60 on an inductance meter.
Firstly, the 0.22 is just on the verge of being close enough, and may work ok, depending on what type of circuit it is going into.
To unwind the other one...realize that inductance is approximately proportional to the number of turns squared:
1) Count the number of turns in the 0.33mH coil - call it N.
( - just multiply the number of layers by the number of turns per layer)
2) calculate 0.25/.33=((N-T)/N)^2
rearrange to: T=N*(1-sqrt(0.25/0.33))
T is the number of turns you must remove.
BTW, the above only works for air cored inductors, AFAIK.
To unwind the other one...realize that inductance is approximately proportional to the number of turns squared:
1) Count the number of turns in the 0.33mH coil - call it N.
( - just multiply the number of layers by the number of turns per layer)
2) calculate 0.25/.33=((N-T)/N)^2
rearrange to: T=N*(1-sqrt(0.25/0.33))
T is the number of turns you must remove.
BTW, the above only works for air cored inductors, AFAIK.
okay im about to unwind the inductor and i want to check if im working it out right, firstly what is an aircored inductor???
secondly i worked that formula and it gave me 16.6 as the T value so now i unwind a single row as there are 16 winds per row. and there are 8 rows. am i doing this right?
secondly i worked that formula and it gave me 16.6 as the T value so now i unwind a single row as there are 16 winds per row. and there are 8 rows. am i doing this right?
Aircore simply means that their is no conductive material in the center of the bobbin. I calculated the same thing as you, so my guess is that you are doing it right!
Sébastien
Sébastien
okay thanks for the reply, 1 row is now unwound, now i still need the rest of the drivers so that i can do some real testing...grrrrr damn couriers taking a christmas break still
I just had the same problem.
I measured the size of the bobbin and counted the number of turns. Then I used the online calculators at http://www.colomar.com/Shavano/inductor_info.html
and http://lalena.com/audio/calculator/inductor/
to work out the lengths of the wire in the value I had and the value I wanted, then unreeled the difference in length.
I measured the size of the bobbin and counted the number of turns. Then I used the online calculators at http://www.colomar.com/Shavano/inductor_info.html
and http://lalena.com/audio/calculator/inductor/
to work out the lengths of the wire in the value I had and the value I wanted, then unreeled the difference in length.
we so need an inductance meter, perhaps if i do a big project where i need to adjust numerous inductors i will buy one.
Have you got a frequency counter and a sinewavegenerator ? If so you could easily measure the inductance-change by building a parallel resonant circuit by adding a capacitor ?
Regards
Charles
Regards
Charles
If your original value was accurate, and you worked through the formula properly, and your inductor was air core, you don't need to bother with measurements.
Variations of 5-10% are not a big deal when dealing with most crossovers.
I checked with my inductance winding formulas, and the formula I gave you works _very_ well (like less than 1-2% error) even if you were to unwind half of the coil.
If you _want_ to measure, you don't need a frequency counter and sinewave generator, you can just use a sine generator program on your computer, but you would need a resistor and/or an accurately known capacitor - and an accurate Multimeter. Most cheap multimeters have flat response from 40-500Hz, but roll off beyond those points quite rapidly.
Variations of 5-10% are not a big deal when dealing with most crossovers.
I checked with my inductance winding formulas, and the formula I gave you works _very_ well (like less than 1-2% error) even if you were to unwind half of the coil.
If you _want_ to measure, you don't need a frequency counter and sinewave generator, you can just use a sine generator program on your computer, but you would need a resistor and/or an accurately known capacitor - and an accurate Multimeter. Most cheap multimeters have flat response from 40-500Hz, but roll off beyond those points quite rapidly.
If the original value of the Inductor is accurate, not even the measurement capacitor has to be accurate then. Just make sure you take the same one for both measurements ! The frequency change will be the inverse of the square root of the inductance change. So you just have to figure out by what ratio the resonant frequency has to increase.
Regards
Charles
Regards
Charles
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