Winding your own inductors

Jason

Powder Monkey
Paid Member
2000-10-08 1:19 pm
Melbourne
www.diyaudio.com
I'm in the process of winding my own crossover inductors, with varying results.

Compared to professionally made inductors of the same gauge and length, they tend to have lower inductance. The tighter packed the coil the better the results, so it seems and would make sense.

I had a look around the web for a coil winding reference/tutorial and didn't find much decent information other than that at http://www.lalena.com/audio/.

I was wondering if anyone had another reference or personal tips and tricks on the subject regards optimum sizing (in this case using 14ga wire and aiming for 0.24mh).

Thanks in advance,

Jason
 
Jason,
Keep in mind that tighter packed means more stray capacitance between the windings, but yes, higher inductance.
The trick about winding inductors is that there are scads of variables. Worse yet, there are no standardized formulas. Sound weird? It's true. I've seen papers comparing five or six formulas and pointing out that the results were inconsistent. (I touched on this briefly in another thread, but didn't feel like making an issue of it; people look in a textbook, find a formula, and assume that it is *the* formula. There's no such thing. Not to mention, you'd think that [seeing as how we're now in the 21st century] we'd have a reliable formula for winding inductors. T'ain't so, unfortunately.)
Anyway, find a book with a formula and wind an inductor, but be prepared to spend a while fine-tuning it. Or drop by http://www.sound.au.com. I believe there's some info in the speaker section about inductors for passive crossovers (note that he also mentions, in passing, problems with inductor formulas).
As far as optimum, generally speaking you want the cross-section of the inductor to be approximately square. It'll be more efficient that way, since the magnetic flux will be more confined.
Broomsticks or other wooden dowels make good cores for transformers. There are plastic coil bobbins for sale, but it's difficult to find the size you need.

Grey
 
the highest inductance comes when the coil crossection is nearly square.
This just happens to be the closest packing of the turns in a winding.

If the packing does have that effect of reducing the inductance, then extended coils whether they are single layer or triple layer (leads at opposite ends) will tend to have more inductance than loosely packed coils.

If you want least resistance then the square form of the cross section is important.
Look at your old fashioned VHF coils in radio sets etc. The coils on the former/s are all bunched into squarish packets.

Once one makes the decision to use a long coil rather than a square coil one may find that looseness of packing begins to have a lesser effect on reducing inductance.
i.e. a hand wound single layer coil may be less sensitive to packing ratio, than a mullti-layer coil.
 
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You might be interested to read this:- http://www.solen.ca/pdf/solen/stdind.pdf - from Solen on what they call 'perfect lay hexagonal winding'.

Note that there are 2 (at least) types of enamelled solid copper wire out there, annealed copper and 'hard drawn' copper wire, the former being comparatively soft and workable and the latter being quite springy and usually supplied in large loose coils.

Annealed copper wire is often stretched before use to put what is called a 'set' on it, i.e. it becomes straight and a little stiffer due to slight work hardening taking place. The wire is secured at one end and the other end is pulled until a slight give of a few percent is felt. This is usually hand-done by 'feel' and results in neat-looking wire links on PCBs.
 
the highest inductance comes when the coil crossection is nearly square.
This just happens to be the closest packing of the turns in a winding.

If the packing does have that effect of reducing the inductance, then extended coils whether they are single layer or triple layer (leads at opposite ends) will tend to have more inductance than loosely packed coils.

If you want least resistance then the square form of the cross section is important.
Look at your old fashioned VHF coils in radio sets etc. The coils on the former/s are all bunched into squarish packets.

Once one makes the decision to use a long coil rather than a square coil one may find that looseness of packing begins to have a lesser effect on reducing inductance.
i.e. a hand wound single layer coil may be less sensitive to packing ratio, than a mullti-layer coil.
Absolutely true. I used A. N. Thiele's (yes, that Thiele) JAES article "Air-cored Inductors for Audio". If the cross-section is square, and the innner diameter is 2x the coil width, this geometry gives minimum resistance for a target inductance. The formula gives the wire diameter and length, the coil dimensions, but not the number of turns. I made several crossover coils in the mH range, and they all were within 1%...
 
I have wound quite a few inductors. This is just personal experience, but it worked well for me. I used a wooden dowel with stiff disks epoxy-ed to it to make a spool. I got a long drill bit and drilled from the inside of the spool out through one of the sides to feed the wire into the spool. I made a crank mechanism to wind the coil. I had a block of wood with some pieces of dowel sticking out of it to wrap the wire through to give it drag so I could pull the wire tight onto the spool. When I thought I might have enough wire on the coil to reach my target inductance, I used a razor knife to scrape off just a bit of the enamel so I could test the coil at that point. If it didn't have enough wire on the coil, I'd cover the scrap with some clear plastic box tape and keep winding. When I got to my target inductance, I wrapped the whole inside of the spool with several layers of electrical tape.

Metal by James Lehman ~ Extra Stimulus Inc.

You can measure a coil by putting a known resistance in series with the coil. Calculate the freq at which the coil's impedance and the known resister would be the same. Drop that freq across both the coil and resister in series and measure the AC voltage drop across each part individually. When you measure the same voltage drop, the parts will be the same impedance. You will have hit your inductance value.

I used my computer and sound card to generate sine waves of the right freqs.

You can use the same technique to add caps together to get exactly the right values.

James. :)
 
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Does the "looseness" of the coil affect the inductance?

What type of coil is more sensitive to "looseness" errors?

I suspect that a stretched single layer coil is very insensitive to "looseness" once you have the correct formula.
and the corollary to that, that the ideal "squarish" coil is most sensitive to "looseness".

But the inductance of all coils are extremely sensitive to diameter !!!!!!!!!!
 
I believe the highest inductance per length of wire (for an air core inductor) is where the width equals half the center hole diameter and the cross section is square. It is probably a good idea to overwind by 20% and take turns off incrementally, checking the inductance each time. I've used tape to temporarily hold windings in place while taking breaks, or, alternatively, hot or medium CA glue or epoxy to hold turns in place (the CA acts like a potting compound also, without completely filling interwinding gaps which is probably a good thing dielectric absorption wise) with an electrical, mylar or kapton tape overwrap when finished winding.


I have made removable side disks so that, once 'potted', my inductors are minimum size/volume with them removed. Facing materials that don't stick to the 'potting' compound need be used while winding the inductor with this technique, of course.

I've found PVC pipe caps to make pretty handy center forms as long as they don't have much of a radius at their ends. They're available cheap in a variety of sizes, are strong enough to do the job, and only require a hole to be drilled at their center to enable single screw mounting.
 
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Does the "looseness" of the coil affect the inductance?

What type of coil is more sensitive to "looseness" errors?

I suspect that a stretched single layer coil is very insensitive to "looseness" once you have the correct formula.
and the corollary to that, that the ideal "squarish" coil is most sensitive to "looseness".

But the inductance of all coils are extremely sensitive to diameter !!!!!!!!!!

It might make a difference if the coil is in the magnetic field of a speaker magnet. In general, loose windings can move and therefor absorb energy and throw off the value of the inductor.
 
Hi Jason,

The specs to wind a choke for 0.24mH out of#14 wire via the linked 'lalene' program, and using a centre diameter starting point of 2" and 1" length (between side cheeks, that is) gives approx 4 layers of 16 turns/layer, and trying to bend a #14 wire into a 2" dia and keeping it under control in a 1" gap without a winding m/c is no joke and is not something you'll want to do twice.

However, if you use 3 parallel wires of #16 wire, much easier (trifilar). For the low mH, and a "better" sound indeed, use enamelled Ohno copper wire.

As Oshifis indicated above (#11) the Theile design is the one for "best" sound but I don't have a link - can send a scan of original article if you want. Low impedance is only 1 of the parameters for a better sounding choke.