I was just curious if the effect inductance of a branch of a circuit would cancel if you put two identical but oppositely wound inductors in series. It's not something I have come across.But looking at the change in signs I imagine this to be the case. Someone mentioned it to me regarding wound resistors in a circuits, and splitting them into 2 resistors in series, with opposite winding to reduce/attempt to cancel the inductance.

I have also recently needed to place several surface mount inductors in series, and I have no way of identifying which end is which, or the winding directions. They are not all the same make or model.And I have had difficulty measuring their inductance.

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    It would depend primarily on how well the two inductors are coupled. A practical application of this is used in wirewound resistors to reduce their parasitic inductance, but in this case the windings are done on one form bifilar or Ayrton-Perry winding topologies are used. They tend to be a niche application because it's expensive to produce than a simple helical coil. – isdi Nov 8 at 16:15
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    Manufacturers do put polarity marks on inductors for some applications to reduce such mutual coupling effects. – isdi Nov 8 at 16:29
up vote 10 down vote accepted

It depends of the extent of their mutual inductance. Roughly speaking, if they are wound on the same ferrite core, they have a large proportion of their inductance that is mutual (as in a transformer for instance), so they would mostly cancel. If, as it seems to be in your case, they are separate components, they are wound on separate cores and have little mutual inductance. In case of air inductors or resistors as mentioned, the distance between them and exact geometry has to be taken into account and only an electromagnetic simulation software can tell how much is canceled, but that would be much less than a complete cancellation.

If you put two inductors in series with a common axis, there will be little cancellation. If the two were put into close proximity side-by-side you'd get more, but probably not much.

It is possible to get non-inductive wire-wound power resistors. These essentially take a single conductor, fold it in half, then wind the resulting doubled wire on a core (not a magnetic core). Because the halves are physically close, the voltage rating of the resulting resistor is less than with normal construction (winding the wire on a core), but the inductance is much less.

In reality, it will never cancel completely! The equivalent inductance is:

enter image description here

Mutual inductance is always less than the sum of the inductance. When two inductances are identical (L1=L2=L) and perfectly coupled (M=L), then it will be completely cancelled but I don't think it happens in reality.

To find the polarity of your SMD you can measure the total inductance in series connection in both polarities and if there is at least a small mutual inductance, you can see the difference in the measurement. (if series connection in the same polarity, the total inductance will be L1+L2+2M)

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