On May 3, 9:17 pm, "Leland C. Scott" wrote:
Did you look at the B-H curves for the material you're using? Any magnetic
material will be non-linear to some degree. Normally the lower the peak flux
density you operate the material at the more linear the B-H curve. Generally
ferrite will have lower losses compared to powered iron. However powered
iron has more tolerance to small DC bias in the core without saturating the
material. Which brings up another point you should not operate the core with
a DC current flowing through the winding if you can avoid it. The best you
can do is find the combination that provides the inductance you need while
operating the core material at the lowest flux level you can get away with
using which tends to suggest using a high permeability material using the
least number of turns to get the needed inductance. The flux density is
proportional to the number of turns on the core and the current in the wire
(ampere-turns which is the "B" field on the B-H curves). As you noticed the
higher permeability material seems to work better, that 's what I would have
expected. That's my 2 cents worth of advice.

--

I had no particular interest in looking at the B-H curves, since it's
distortion in filters I'm interested in, and that's easy for me to
test directly.

With respect to flux density: there seems to be a bit of a disconnect
between what I know and what you've written. From Faraday's law of
magnetic induction, I expect that for the same inductance with fewer
turns, the flux density must be higher: if the inductance is the
same, the voltage must be the same for a given current at the test
frequency. The same voltage with fewer turns around the same cross-
section core implies higher flux density. No?

So far, my tests suggest that you can't draw any conclusions one way
or the other from only the relative permeability of the core
material. Ideally, you should do tests in the actual application. My
testing is slightly derailed (I hope temporarily) because the cores I
got that were supposed to be FT50-67 are clearly something different,
almost certainly FT50-61. I am testing all the other low-to-medium
permeability (F)T-50 cores I have available at the moment.

Cheers,
Tom