Making a coil that doesn't exist...

squamishdroc

I'm working on a little 2 way desktop speaker and I need a .01mh coil for a notch filter. I have a couple .05 ones that I thought I would just unwind and measure with the DATS, only problem is DATS only measures down to .05. So I thought I would just unwind it and then try it out in the circuit - but how? Oh - the wife has a small kitchen scale I know it is not terribly accurate, but I figured I would start at half the weight and go from there. The difference in the circuit response is quite substantial from .05mh to .01mh so the closer I am to what I need the better. Will see how it goes

dcibel

Single layer:

Multiple layer:

squamishdroc

@dcibel said:
Single layer:

Multiple layer:

Math isn't my strong suit - so what would length be? The total length of wire?

squamishdroc

Oh I see - the rest of the pictures you posted didn't show in your original post - weird Thanks!

dcibel

FWIW I've never needed smaller than 0.1mH, 0.01mH seems absurdly small for a crossover part. Surely you can adjust a capacitor value to create the same notch with a larger inductor.

The reason something like DATS can't measure that small is because it determines inductance by simply creating an RL filter and determining the Fc. Values that small push that point beyond 20kHz.

You could use DATS to determine the inductor value, by placing the inductor in parallel with a large capacitor like 500uF, creating a resonant peak, however the result would be somewhat dependent on the accuracy of the cap value, but probably still more accurate than the kitchen scale

Ron_E

The formula for calculating the inductive reactance of a coil is: inductive reactance, or XL, is the product of 2 times p (pi), or 6.28, the frequency of the ac current, in hertz, and the inductance of the coil, in henries. XL =2p x f x L.

2p x 15000 x .00005 = 4.7 Ohms

Series a 4.7Ohm resistor with your coil and put 15K across them. When the voltage across the coil is the same as the voltage across the resistor the coil will be .05mh

That's the hard way. Inductance is the sum of inductors in series so I would measure a slightly larger coil and series it with the coil you're trying to make. When the value of the two coils in series matches the value of the known coil plus .05mh you have your new coil. Keep the coils separated by a distance so mutual inductance isn't a factor.

When I unwind coils I use an Exacto knife and a clip lead. Take some turns off and hold the knife edge against the wire and take your measurement.

Ron

squamishdroc

Thanks guys👍👍

Wolf

There have been times where a 0.025mH or 0.05mH have been used for a series (shunt) notch for breakup on a woofer in my case. However, if I get down to 0.01mH, I try the circuit without to see if there is a difference or not. Usually, it's minor enough that the extra component is eliminated.
I have never gone below 0.025mH or over 10mH in a design.

ScottS

I've also used small coils for two reasons. I have used them in the shunt leg of a low pass filter for notching out woofer breakup (although a small cap in parallel with the inductor is usually a cheaper way to accomplish the same thing). I have also used them to tame a rising tweeter response (in series with the tweeter in a high pass filter). I have never used anything smaller than 0.025 mH, although I did notice today when shopping for little stuff to get to $100 for free shipping that PE carries a Jantzen 0.01 mH inductor (part number 255-002).

jhollander

I just did this today. I used another .05 mH coil in series then wound my own on a plastic former. Let me know if you want one. I have spare .01 mH. Usually I'd do another circuit but I'm out of SPL in my tweeter circuit so the tiny notch is all the shaping I could afford.

Used some of Craig's magnet wire

PWRRYD

I have a lot more of that wire, and a lot of aluminum bars (read DIY out riggers). If we ever have another DIY GTG they will be there for the taking.

Wolf

What Al bars?

squamishdroc

This is for a large peak at 9k on a full range driver. I would get the ones from PE but I live in Canada so the duty and shipping would be silly for a $5 coil. I will try building the circuit w/o the coil and with my unwound one and see if it makes a difference. Thanks for the offer John but not worth it to ship to Canada.