I was wondering why there are different symbols for common mode chokes and if/how they affect the way I have to connect them.

Common Mode Choke Types

(Upper image is incorrect but left here for understanding the comment from @Felthry)

Common Mode Choke Types

Most often I see type 1 and there is no doubt on how to use them. But with types 2 and 3 I'm not so sure. They must have different meanings as I saw them simultaneously in one circuit. Can anyone enlighten me or refer me to a decent source on what the difference is?

  • \$\begingroup\$ That's odd... I'd thought types 2 and 3 were alternatives to the dot notation for showing relative polarity, but if they have dots as well as the core symbols, I suppose it isn't. \$\endgroup\$
    – Hearth
    Commented Nov 19, 2018 at 14:08
  • \$\begingroup\$ @Felthry You are right. I uploaded a new image. So Type 2 and 3 are equal? \$\endgroup\$
    – Peder
    Commented Nov 19, 2018 at 14:29
  • \$\begingroup\$ I think (don't quote me on this) that types 2 and 3 are just a method of showing the relative orientation of the windings. 2 and 3 are not the same, but if you swap the polarity of one of the windings, 2 turns into 3 and vice versa. \$\endgroup\$
    – Hearth
    Commented Nov 19, 2018 at 17:33
  • \$\begingroup\$ Does this answer your question? Inductor Symbol? What does this symbol indicate? \$\endgroup\$ Commented Apr 3, 2023 at 20:47

1 Answer 1


The dot is what governs the polarity and it certainly matters how they are connected. Seeing both symbols in the same circuit (most likely as the input to the power stage of a switchy power supply or inverter for EMI suppression) indicates some are common-mode chokes and the others are the differential-mode chokes. The common-mode chokes have the dots on the same side as power coming in or power going out. The idea is that the inductance cancels with equal and opposite polarity of the (differential) current while providing high inductance to mis-balance currents (common-mode).


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