I had a core crack once due to overheating. It was in a "2KW" 4:1
balun. It didn't care for the particular impedance my zepp presented
on 75 meters. I guess I should have saved the two pieces and used
them for a "clamp-on" choke.
73 de Mike, W4EF.............................
----- Original Message -----
From: "Chuck Counselman" <firstname.lastname@example.org>
Sent: Friday, January 10, 2003 3:16 PM
Subject: [Towertalk] Frying doesn't hurt ferrite.
> An astute Towertalk reader asked me off-list whether the ferrite that
> I had salvaged from my burned-out common-mode choke/balun was still
> good. He said he had read that frying ferrite ruins it.
> I replied that I did not know how ferrite could be damaged in a
> garden-variety fire unless it was mechanically fractured by rapid
> differential thermal expansion, because ferrite is a ceramic
> material, and it is refractory.
> Ferrite (like all ferromagnetic materials) loses its ferromagnetism
> above its Curie temperature, but AFAIK it recovers fully when it is
> cooled. (A "hard" ferrite loses its "permanent" magnetism, but it
> can be remagnetized. The ferrites used in chokes, inductors, and
> transformers are "soft" and do not retain much magnetism.)
> But there's no substitute for a good experiment, so I rewound my
> choke and measured it.
> When the choke was brand-new, I had measured its inductance to be 37
> microhenries and its series resistance to be zero, within the roughly
> one-ohm precision of my Autek RX Vector Analyst. The measurement
> frequency was 1 MHz, where the measured reactance was about one-half
> of the range (500 ohms) of this instrument.
> The ferrite rod that I salvaged from the incinerated choke/balun was
> a blackened mess, but I had scoured and scrubbed it clean. Despite
> my scouring and scrubbing, it still bore a visible imprint of the
> original winding, which appeared to have had 26 or 27 turns. So I
> rewound it with 27 turns, spaced like the original winding, and used
> the same Autek instrument that I had used originally, to measure the
> impedance of the rewound choke. I measured it at f = 0.75 MHz and
> also at 1 MHz. Both measurements yielded the same inductance, of 40
> microhenries, and the same series resistance, of zero.
> So frying this ferrite did _not_ change its magnetic properties.
> When you burn out a choke/balun/transformer with a ferrite core,
> salvage the ferrite, which is usually the most expensive part.
> 73 de Chuck, W1HIS
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