Re: Toroid core material

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <teslalist-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
 >
 > Original poster: "J. B. Weazle McCreath by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <weazle-at-hurontel.on.ca>
 >
 > Hello Coilers,
 >
 > I've some info and a question for my fellow coilers regarding various
 > materials used in making toroid cores and their usefulness as chokes
 > in TC service.
 >
 > As an RF filter in the hot lead of my PDT was a three turn choke that
 > I'd wound on a toroid core removed from an old computer power supply.
 > After a minute or so run time of the coil, the core material was hot
 > to the point of being uncomfortable to hold onto.  This indicated it
 > was pretty lossy, so I tried another core which had started life as
 > an 88 mH. "telephone choke coil" and noticed no heating even after
 > several minutes run time of the coil.
 >
 > It would seem that we need to choose the core material carefully for
 > this type of use.  The big question is, what is the core material of
 > the computer PS toroid and of the 88 mH. toroid?  They are obviously
 > quite different in composition, no doubt due to the frequencies they
 > where chosen to work at in their original uses.  I suspect that the
 > computer toroid worked in the 10's of kHz. range while the telephone
 > one likely was for voice frequencies of less than 3 kHz.
 >
 > 73, Weazle, VE3EAR/VE3WZL

	You'll probably see a lot of answers before this, but anyway the core
in the PS is a high saturation flux density ferrite and the core in the
telephone loading coil is 125 permeability molybdenum permalloy.  BUT,
neither is any good for the intended purpose.  You need lots and lots of
turns to avoid core saturation and if you don't use them you won't get
the desired filtering action.  Formulae for calculating flux density and
turns have been posted here many times and I'm sure they are available
in Terry's archives.

	Second thought.  Even if the cores don't saturate the use of inductive
filtering is potentially destructive to the transformer and resistors
are a lot simpler.

Ed