Common Mode Choke As Regular Inductor

[Ice-Tea]

Anything keeping me from using a 2mH CM choke as a single 4mH inductor? Anything specific about their construction that would make that undesirable?

[johansen]

virtually no energy storage.

they can handle the same ac flux as any other ferrite core, but the core permeability is so high they can't handle any dc flux in the core.

[Ice-Tea]

Ah.. That makes sense.. So, no free lunch, then

[johansen]

my spreadsheet suggests 17 microjoules of energy storage for a common mode choke with 4mh inductance and 50 turns and a 5 mm by 5mm square core.

--which only requires a permeability of 1000-2000, and zero air gap to get that high a number at only 50 turns.

edit: 100 should be 1000-2000, depending on the magnetic path lenght.
some common mode chokes have very long and thin cores for some reason.
others us a solid block of ferrite, and "spin" the coil on a rotating bobbin.
-no air gap at all

[Sch3mat1c]

Somewhere around here I wrote down some specs on a tiny Panasonic CMC I measured. Since the core is ungapped, inductance is high (with a wide tolerance) and saturation current is poorly defined. Much better to look at saturation flux, which is independent of saturation current and inductance (which are very sensitive to gap). For a 5 x 5 mm ferrite core, that would be about 7.5 uV.s per turn. The one I have is around 3 x 3 mm, and I think it did around 40uV.s per winding, which suggests maybe 14 turns. Looking at it, that sounds about right, up to maybe 20-30 turns if there are more layers I can't really see on the thing.

In other words, as transformers, they aren't terribly horrible, but you aren't going to transmit more than a few watts at any frequency. Too high, leakage kills you; too low, the core saturates.

As a series choke, you'll be looking at milliwatts instead of watts, it saturates very easily.

There is one thing -- I once hi-potted a side-by-side bobbin wound type choke, hardly a half inch across, at 15kV. You can't get cheaper insulation anywhere else. "Reinforced" pulse transformers start at $5 and go up from there (e.g., RECOM DC-DC converters with >6kV isolation for around ten bucks per watt).

Tim

[Ice-Tea]

*Turns on the schematic-to-human-translator-machine*

*DING!*

" No free lunch, son!"

Yeah, guess I'll be plundering the Wuerth catalog again..