Air core inductor section shape

[rabbitz]

Going through Jantzen air core inductor information, there's a large variety of cross section shapes for the same inductance and wire size. Currently I am unwinding a 0.56mH down to 0.12mH for a crossover upgrade and lines up with shape (A) in the drawing.

What I would like to know, is there any electrical differences (such as pick up) between a tall smaller outside diameter (A) vs a short larger outside diameter inductor (B) with the same wire size? I know (A) tends to have a slightly higher DCR. I also imagine the variation in shapes available enables more flexibility when mounting so they can be low or smaller diameter to suit the final application.

[twest820]

An A winding and a B winding with equal inductances will have equivalent behavior to a first order approximation. If second order and higher effects turn out to matter that's basically saying inductor's not properly shielded (either by 1/r, orientation, or an actual shield). If this is an exercise in space optimization which aspect ratio provides better noise immunity depends on the details of the winding geometry and the structure of the incident field; if you want rigorous answers the best option's an electromagnetic field solver though working through analytic approximations has reasonable predictive power. One needs the crossover layout and, ideally, how it fits into the speaker for that sort of analysis.

If you really want to minimize crosstalk and effective source impedance an active crossover is more effective though. An LR4 and two chip amps to service typical listening powers will fit in less volume than most passive crossovers too.

[McCormack]

My informal observations from winding my own inductors is that, for a given length of wire, narrower, taller windings will give a higher inductance value.

[rabbitz]

It was more a question about why so many variations in the Jantzen catalogue for the same wire size and inductance value. Is one better than the other in electrical terms or are the options for layout purposes.

From their catalogue here's the variation for 0.22mH with 18AWG wire.
Numbers are AWG - mH - DCR - IDxHxOD in the pic.

For reference it's an AR series xo similar to one I built (shown in the pic) and space for inductor orientation is not a problem. The Jantzen inductor I unwound ended up being:
15AWG - 0.12mH - 0R09 - 25 ID x 30 H x 35 OD approx - 80 turns, so I was just comparing profiles to the Janzten sizes available. That's when I noticed the variations on offer.

[Conrad Hoffman]

Inductors need to be at right angles to each other to minimize crosstalk. IMO, physical position will have way more effect than inductor construction. The two things you'd think about with construction are DCR and self-resonant frequency due to internal capacitance. DCR isn't hard to calculate, but I don't know of formulas for capacitance. Usually it's high enough not to worry about, at least for normal inductors with round wire.

[AndrewT]

Wheeler's formula and similar, show minimum DCR when the section shape of the winding is ~square and that the OD of the square is roughly 4times the width and 4times the height of the square.

eg.
choose a core/bobbin diameter of 20mm.
Wind on 10 Layers of 10 Turns in each layer using 1mm diameter wire.
The 10 Turns makes the coil width ~10mm, the 10 Layers makes the coil height ~10mm and the overall diameter is ~40mm.

You will find that these proportions are very close to the theoretical best "shape" for the lowest DCR of an air cored inductor. These 100T will give ~0.27mH and ~0r22

Using the same bobbin and increasing the wire to 1.2mm diameter gives ~0.12mH and 0r1 using ~67T, 8layers with 8 to 8.5T per Layer.

[rabbitz]

Here's the inductor calculator results for Andrew's first example.

For ref, the pic shows the finished xo and the inductor that was unwound is the one on the left.

Thanks for all the replies and I'm aware of most of the information supplied. It's just the variety at Jantzen which had me puzzled.