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[Towertalk] Frying doesn't hurt ferrite.

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Subject: [Towertalk] Frying doesn't hurt ferrite.
From: (Chuck Counselman)
Date: Fri, 10 Jan 2003 22:27:56 -0500
Rick Karlquist N6RK wrote:
>If you put an ampere turn or so through a high permeability
>ferrite, it will magnetize and permanently lose a lot of inductance.
>You can of course fix this by heating above the Curie temperature.

You are absolutely correct, and I knew it when I wrote what I did, 
which is why I hedged by writing that they do not retain ***much*** 
magnetism (in comparison with "hard" ferrites).  I probably should 
have been less vague.

I have experimented with ferrite cores, deliberately putting DC 
ampere-turns through them to see what it did to their incremental, or 
small-signal, permeability.  Just as you say, they magnetize 
permanently and much of the "choking" inductance is lost.  How much 
is lost depends on the material.  You can find B-H hysteresis curves 
in the Fair-Rite catalog.

Whenever I had magnetized a core I always marked it so that I would 
not use it later without having demagnetized it.

>I cringe when I see people putting ferrite beads on one
>leg of a 20A power supply.  The 20 ADC magnetizes the bead
>into saturation the first time you turn it on.  What you want to do
>is run BOTH wires through the bead so there is no net DC.

I agree completely.  If you must put a ferrite bead on a wire or a 
cable carrying a significant net (common-mode) current, whether it's 
DC or 60-Hz AC or whatever, you must use an appropriate type of 
ferrite.  The Fair-Rite catalog will help you select one. 
Unfortunately, such ferrites don't have huge permeabilities.

BTW, I learned while experimenting with 60-Hz AC that high-mu 
ferrites such as type 64 are slightly piezoelectric or piezomagnetic 
or magnetostrictive, or something.  When the magnetization reverses, 
the core clicks.  I think that it was the core itself clicking, not 
the wire (which of course experiences an electromagnetic force).

-Chuck, W1HIS

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