Type 43 material for common-mode choke

That's a 5943000201 in the Fair-Rite catalog. Al = 375nH/n^0.5.

You'd only need 16 turns x2 to get a 2x100uH common mode choke.

It's the right material for the application.

RL

Yes you're right, 16, thank you. Nice

Yes.

Jeroen Belleman

Too many turns can eventually give diminishing returns due to too much intra-winding capacitance.

You can work that out. Sixteen turns on a toroid sounds like a single layer winding, where the parallel capacitance tend to be around 1pF, which would make the coil parallel resonant at about 8MHz (very roughly).

As soon as you start stacking up layers of windings, life gets more difficult and you have to starting thinking about partitioning the winding into successive multilayer chunks, where there isn't a lot of voltage difference within any one chunk.

Very roughly indeed, an octave out - I make 1pF 100uH resonant at 16MHz

piglet

I would say an octave is good enough for a SWAG. Even a decade would pass in most of the cases.

Best regards, Piotr

You are right when some information is missing but in this case all the data was available to calculate better.

piglet

Oops. I wasn't reading carefully enough. I got quite the wrong inductance, and seem to have plugged in the wrong capacitance too.

Repeating the calculation gives me 15.9MHz - which certainly isn't worth reporting as anything different from 16MHz.

A common mode choke is going to use a bifilar winding, which is to say a length of twisted pair, which has a very well-defined interwinding capacitance, which I'm no longer game to try to work out.

Thanks for the correction .

I agree with piglet. I'm not in the least pleased with myself.

There is no such thing as material 'Q'.

When you assume subjective properties in magnetics, you make a fool of you and me.

RL

I think the crossing point is where the resistance is equal to the reactance. No?

If so, then the Q at that point is 1. No?

The material itself doesn't have reactance. It does have a bulk resistivity, but that isn't directly related to core loss effects, though lossy materials can often be identified by their low bulk resistance.

"It was my understanding the that the point where u' and u" cross on apermeability curve is where the material Q = 1."

- Refers to a graph constructed using measurements performed on a predifined core shape and turns count, as comparative reference information for the core material in question. The graphical contents would change with a different core shape or winding configuration - shifting this specific 'Q' point, without altering the material composition of the core.

Another common graph is for the single-turn bead, which attempts to reduce winding effects on measurement of the loss characteristic. Each bead size and shape will still have its own curves, without alteration in the core material type.

Anyone can stipulate that a Q factor exists for the relation between any two measurable quantities. providing that its definition is presented within the scope of the work.

In a non-resonant circuit, the term has little meaning, as losses in the core are (non-linearly) dependent on the amplitude of the peak flux excursion, and many other inter-related physical factors of the core and winding.

RL

A lot of instruments to measure inductors and capacitors report Q, by which they mean the ratio of reactance divided by resistance at the chosen test frequency, well away from resonance.

Joe Gwinn

I've tested some commercial AC-line CM chokes. I was surprised by how little normal-mode DC current it takes to saturate the ones I tried. They seem to assume that all AC loads are exactly DC balanced.

As long as they tell you what they're talking about, then they can call it anything they want.

Q is not a characteristic of the core material.

RL

Perhaps not. But as soon as you put a wire through the hole it has Q at some point. What good is the core if it not used?

Many LCR meters report Q as a ratio of reactance divided by resistance.

For example, the Agilent / Keysight U1733C 100Hz/120Hz/1kHz/10kHz/100kHz Handheld LCR Meter:

Graphs ARE available for parts under that specific condition, and for that use.

The OP, however is refering to a graph for 43 matl, and is only applicable to a 17mmOD /10mmID / 6mmH test speciment with an unknown number of turna. Fairite, the mfr of 43 matl, doesn't even bother to offer this part size for sale. It is a lab test specimen.

The size of the test specimen can vary from one matl data sheet to the next. None are necessarily commercially-sized parts, though some manufacturers do use a recognizable part number.

For core material there IS a 'relative loss factor' which is specified as a maximum limit value at a test frequency. There ARE graphs for that - check Magnetics Inc matl data.

RL