Odd sized coils

[duaneage]

I need some coils that are a bit non standard. I am looking for 3.2 mh chokes but all I can find are 3.3 or 3.0.
I don't want to wind my own, don't have a source for magnet wire locally anyway. Could I use 3.3 mh coils and unwind them until I get to 3.2mH? I have a way to measure them to make sure I don't unwind too much.

Anyone have experience with this?

[Zilch]

I need some coils that are a bit non standard. I am looking for 3.2 mh chokes but all I can find are 3.3 or 3.0.
I don't want to wind my own, don't have a source for magnet wire locally anyway. Could I use 3.3 mh coils and unwind them until I get to 3.2mH? I have a way to measure them to make sure I don't unwind too much.

Anyone have experience with this?

Yes. I do it all the time with stock Jantzens. Hardly any crossover I build uses all stock values:

[Robh3606]

Most places will take a standard value and trim it for you if you don't have the means to measure yourself. You can also get them matched for a nominal fee. I believe Madisound does this. Take a look at there web site and on line catalog.

Rob:)

[duaneage]

madisound gets 7 bucks to trim them so i think i can unwind one.

[Baron030]

I also have some interest in making odd sized coil values. But, without test equipment, I have yet to attempt it. In following duaneage's example of modifying a 3.3mH coil to get a 3.2mH value. What about trying a little reverse engineering to create an accurate mathematical model to work from? From a 3.3mH stock coil, you could measure the coils height and diameter, and then count the number of windings per layer and the number of layers. Multiplying these two values would give you the total number of turns in the stock coil.

Since, there are formulas for winding your own coils and even some web sites with coil calculators. It would be a simple matter of plugging in your measurements into a coil winding formula and then compare the results. If the calculated value matches the 3.3mH stock value, then you know you have an accurate mathematical model to work from. It would then be a simple matter of crunching some numbers to calculate the number of turns that would have to be removed from a stock 3.3mH coil to create the desired 3.2mH value. Maybe, someone with some test equipment, could try out this idea and see if it really works or not.

I know it would be interesting reading to all of us, do it yourselfers out here. http://audioheritage.org/vbulletin/images/smilies/scratchchin.gif
Baron030

[Ian Mackenzie]

No.

You need an inductance meter or LCR Bridge.

[duaneage]

The Woofer Tester I have measures inductors, alternatively you could connect a 10 ohm resistor after a coil and using a signal generator determine the inductance.

I don't think pulling a few turns off a 3.3 mH coil to bring it down to 3.2 will affect the balance of the winding very much. Since we are dealing with audio frequencies not RF or video the ratio of coil height to coil diameter is not too critical, especially for a 1% reduction.
I did a little book research on this and found it to be a good plan. I hope to match the DCR as close as possible to what I am trying to get, there are small resistor values that can be added to make up the resistance if too small.

[WTPRO]

If you have the WT2, the impedance and phase meter display has been improved a bit in version 1.04 and will now automatically recognize lead or lag phase and from this the effective inductance or capacitance is displayed... after subtracting the DCR and re-computing from just the reactive component :-)

If you are wondering what can be resolved, here is a screen shot of the 36" test leads 'narrow' and 'wide' spaced to see the inductance change from 828nH to 2.174uH. Being able to clearly see this at audio frequencies raised a few eyebrows at the CES!

If the image does not post (I have never done this in this newsgroup before) here is a link

http://www.woofertester.com/testlead.jpg

Hope this helps