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2nd January 2004, 12:48 PM  #1 
diyAudio Member

Unwinding my inductor to obtain lower inductance
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. 
2nd January 2004, 02:29 PM  #2 
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK


2nd January 2004, 04:27 PM  #3 
diyAudio Member
Join Date: Jun 2002
Location: USA, MN

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=((NT)/N)^2 rearrange to: T=N*(1sqrt(0.25/0.33)) T is the number of turns you must remove. BTW, the above only works for air cored inductors, AFAIK.
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2nd January 2004, 08:19 PM  #4 
diyAudio Member

thanks guys

3rd January 2004, 03:22 AM  #5 
diyAudio Member

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? 
3rd January 2004, 03:33 AM  #6 
diyAudio Member
Join Date: Apr 2003
Location: Montreal

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 
3rd January 2004, 03:46 AM  #7 
diyAudio Member

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

4th January 2004, 10:00 AM  #8 
diyAudio Member
Join Date: Aug 2003
Location: London

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. 
5th January 2004, 09:59 AM  #9 
diyAudio Member

we so need an inductance meter, perhaps if i do a big project where i need to adjust numerous inductors i will buy one.

5th January 2004, 10:58 AM  #10 
diyAudio Member
Join Date: May 2002
Location: Switzerland

Have you got a frequency counter and a sinewavegenerator ? If so you could easily measure the inductancechange by building a parallel resonant circuit by adding a capacitor ?
Regards Charles 
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