Inductor polarity question

[eh la bas ma]

Hello,

I am building Schalltechnik-04 Omnilooper :

https://schalltechnik04.de/en/instructions/omnilooper/6

p.6 they say the 2.2mH has to be aligned correctly, dot to dot (there is a dot printed on the pcb).

Unfortunately the inductor musikding provided has no dot, and both legs have equal length. All i can notice on the part is the number 36, a "c" small letter, and an "R" capital letter reversed, like the russian Я. These symbols are located on the side of the part, at equal distance of both legs, so it doesn't look like some orientation marker.

Looking at the part from every angle, i can't see any marker anywhere...

The two other inductors were in a plastic bag labelled "Bourns 222 K2" and have dots. This inductor, twice as big as the two other 1mH, same color, is on its own, no label.

I wonder how can i figure its orientation ?

I tried to check the datasheet of an inductor that could look like this one, from Bourns, but i couldn't find any clue.

https://media.distrelec.com/Web/Downloads/_t/ds/rlb-series_eng_tds.pdf

Looking online for a way to test its polarity, it looks like a multimeter won't be enough, i would need an osciloscope or some special meter that i don't have.

Any help would be welcome !

[Rob Strand]

In normal circumstances you don't care about the "polarity" of such inductors and in fact it is rarely given.   While the Bourns inductors show a dot for the start of the winding that alone doesn't determine the "polarity", it also depends on the winding direction.

You can get magnetically biased inductors, which look similar, and these do have a polarity and have magnets attached to the core.  These were common in CRT monitors.   The Bourns units are not this type.

Polarity would be defined, for example, as the magnetic field pointing in some direction when position current is applied to a certain pin.   There is no standard for it.   You can't be certain all brands of inductor are wound with the same direction so determining the start of the winding isn't sufficient.

To cut to the chase:   I suspect the designer has found orientating the inductors a certain way helps reduce noise induced by the inductors.  You will find in practice this will be the case.   All you have to do is put one inductor in arbitrarily then see which orientation of the other inductor gives the lowest noise.  Maybe you can't tell the difference.

If the external field is a problem the solution is to use magnetically shielded inductors.

Based on the assumption that the Bourns units are wound in the same direction it might be possible to set-up an experiment with an oscilloscope, or with an LCR meter, to work out the relative polarities of connections.   In some cases you might be able to eyeball the winding start and winding direction.  You might even be able to use a compass (apply a current an choose leads which cause the same direction of field out the top of the inductor.).

[Elektrojänis]

I wonder if in addition to the magnetic field direction and magnetic bias, the circuit impedances on the ends of the coil could have an impact on the induced noise?

I mean if you connect the end of coil thats connected to the outer layer of the coil to lower impedance point, would it have similar shielding effect as with capacitors, when you connect the end that connects to the outer foil layer to the lower impedance point? With inductors this would only work on those that have several layers of copper wire.

[eh la bas ma]

To cut to the chase:   I suspect the designer has found orientating the inductors a certain way helps reduce noise induced by the inductors.  You will find in practice this will be the case.   All you have to do is put one inductor in arbitrarily then see which orientation of the other inductor gives the lowest noise.  Maybe you can't tell the difference.

Thank you very much fort these clarifications !

I soldered the 2.2mH in a random orientation, and i will change it if i have some noticeable background noises. I wasn't sure if i could damage the part or the circuit with some wrong orientation, so now i can proceed with the build without doubts.

Thanks !