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From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: 3C85
Date: Thu, 1 Feb 2001 12:55:09 -0000
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> Does anyone know the permability and spec of 3c85 material? I  cannot
find
> this info in any of my tech info books. The T200 series seem very low
like u10 powdered iron whereas 
> the 3c5 ferrite  type are as high as u7000 Seems important to get the
right mix
> g3kev
> 

For ferrite material used in transformers the  Ur is irrelevant so long
as it is 'High' enough to ensure magnetising current is negligable
compared with that current transferred through to the secondary.
Permeability of high Ur ferrites is a very unstable parameter anyway,
and shifts wildly with temperature and flux density.  Once a suitable
ferrite for power and frequency is identified,  the important thing is
Bmax which is a function of Volt-seconds, core area and turns.   The
parameter affecting upper frequency performance is the hysteresis losses
in the material, and overall losses with increasing power mean that
total ferrite volume scales with power transferred across the
transformer.  'Better' ferrites have lower hysteresis loss at higher
frequencies, but very rarely can Bmax ever much above 0.2 Tesla with
modern materials, so provided this vital parameter is never exceeded,
transformers using a ferrite appropriate to the frequency, and size
suitable for the power will work OK.

To get an idea of what core size can do what, look at some SMPSU
transformers as used in computer power supplies.   Modern 200W supplies
use a surprisingly small core and they operate in the region of 100 -
200kHz.  When carrying a sine wave I have found that a given core is a
lot more efficient than when carrying a squarewave, and can use less
turns,  so a core from a 200W SMPSU is probably going to be adequate for
a 400W  137k sinewave signal.

For Iron dust (or ferrite dust, or ferrite with an airgap) the material
serves only to 'conduct' the magnetic field into a small airgap and this
defines the total inductance.  For a gapped ferrite core the gap
dimensions are exact and can be measured.    Inductance can then be
determined quite accurately from the width and area of the gap alone
PROVIDED the ferrite has a high enough Ur to not influence the total
value (analogy: small and large resistors in parallel)  and is not
saturating.  The result is that a standard sized gapped core then has a
specific inductance specified in Henries per (turn squared).    

In dust cores, of course, the gap cannnot be identified physically and
the specification is then replaced by the specific inductance value
only.   Permeability can be a bit more accurately defined now as it
depends mainly on the gap rather than the magnetic material, hence is
defined in manufacture by particle size, mix density etc, but is not as
useful a parameter as Al.   
The frequency limits are defined by the losses in the material making up
the body of the core.   Bmax rules still apply, so Vrms = 4.44.F.N.A.B
should still be used to derive the minimum number of turns needed to
prevent saturation, and again power handling scales with the material
volume.   Don't add too many extra turns to try to reduce B too much, as
then copper losses begin to creep up offset against diminishing returns
on core losses.   For professional large PSU transformer designs,
considerable effort is spent carefully trading off copper losses and
core losses to maximise efficiency, primarily to reduce heating.   

Iron dust 200 cores mean 200 (units of 0.01") in diameter and the
following number (and colour of the coating) shows which frequency
family they belong to.   The ARRL Handbook (all editions for the last 25
years) contains an excelent set of Iron dust core specifications.

Andy  G4JNT

> 


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