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The core symbol in below diagram indicates that it is an iron core material but Iam not sure whether it is correct. I see the same representation in many other circuit schematics as well.

enter image description here

Below ref says "To prevent saturation, the differential mode inductor must be made with a core that has a low effective permeability (gapped ferrites or powder cores). The common mode inductor, however, can use a high permeability material and obtain a very high inductance on a relatively small core"

"For the most part, ferrites are the material of choice for common mode inductors"

Were all the diagrams just a loose non- rigorous representation? Or was there any meaning in that core representation?

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3 Answers 3

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Depends on the working frequency range, of course. But for almost all applications, MnZn (Manganese Zinc) ferrites are used.

Sometimes symbols look confusing, yes. If you see a single inductor with two or three parallel lines on top it's an iron-core choke. If you see two inductors sharing the same parallel lines it means these two inductors are wound around the same core. For CMCs and DMCs there are different symbols such as two inductors having a Z (CM) or 90-degree-U (DM) or a circle in between with phase dots, etc.

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In the old days, transformers were just made with two materials for the core - air, and some type of magnetic material, iron being the most common back then.

So an air core transformer just used the coiled wire symbol, while the iron core transformer added the parallel lines to indicate the use of a magnetic material.

I don't think there was a standard as to how many parallel lines were used. Two or three seemed to be the most common.

The picture below is from the IEEE Standard 315 Reference Designators 1975:

enter image description here

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Material is as others have answered; as for construction, CMCs optimize maximum inductance at low current (since the common mode is supposed to be isolated, or "current compensated" as they're also called), and DM optimize energy storage (inductance at nominal current). The latter requires a low effective permeability, which is done in ferrite by adding an airgap (which averages out the high permeability of the bulk material), or "distributed airgap" materials like powdered iron (usually as a toroid, or molded around the winding).

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