Unwinding my inductor to obtain lower inductance

michael · 2nd January 2004, 12:48 PM

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.

sreten · 2nd January 2004, 02:29 PM

Details in this article :

http://sound.westhost.com/lr-passive.htm

sreten.

Ron E · 2nd January 2004, 04:27 PM

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.

michael · 2nd January 2004, 08:19 PM

thanks guys

michael · 3rd January 2004, 03:22 AM

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?

tool49 · 3rd January 2004, 03:33 AM

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

michael · 3rd January 2004, 03:46 AM

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

Nardis · 4th January 2004, 10:00 AM

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.

michael · 5th January 2004, 09:59 AM

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

phase_accurate · 5th January 2004, 10:58 AM

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

2nd January 2004, 12:48 PM	#1
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	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, 04:27 PM	#3
Ron E 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=((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. __________________ Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan* Armaments, universal debt, and planned obsolescence--those are the three pillars of Western prosperity. —Aldous Huxley

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2nd January 2004, 02:29 PM	#2
sreten R.I.P. Join Date: Nov 2003 Location: Brighton UK	Details in this article : http://sound.westhost.com/lr-passive.htm sreten.

2nd January 2004, 08:19 PM	#4
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	thanks guys

3rd January 2004, 03:22 AM	#5
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	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
tool49 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
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	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
Nardis 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
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	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
phase_accurate diyAudio Member Join Date: May 2002 Location: Switzerland	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