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From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Tue, 13 Jun 2006 18:11:15 +0100
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Subject: LF: RE: PA4VHF/p propblems with core/high voltage etc
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-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Dick
Sent: 13 June 2006 14:34
To: rsgb_lf_group@blacksheep.org
Subject: LF: PA4VHF/p propblems with core/high voltage etc


Aperently the N30 core seems to be not the best solution as an step-up
transformer.
Luckely I do have another 3c85 core (50-60mm) lying around.
Is it better to use this one instead of the N30 material??


Dear Dick, LF Group,

The peak flux density in the core is given by

B(peak) =3D Vrms/(4.44.f.n.Ae) (for a sine wave voltage)

Where=20
B(peak) is peak flux density, T
Vrms is the voltage across the winding, V
f is frequency (Hz)
n is number of turns
Ae is effective cross-sectional area in m^2(nearly the same as actual
cross-sectional area for a toroid)

(Be careful of the units - the manufacturer's data is often given as =
mm^2,
cm^2, square inches, etc...)

The flux density does not depend on the type of ferrite - so if you are
using cores of similar cross-section, and the same number of turns, the =
flux
density is similar also. What does change is the specific core loss, Pv,
which is a function of type of ferrite, flux density and frequency. For =
3C85
at 137kHz and 100mT (for 500W TX and so Vrms =3D 158V, and 16t primary), =
Pv is
somewhere around 160kW/m^3, according to the graphs in my old Philips =
data
book. The effective volume of a TX51 size toroid is 21500mm^3, (i.e. =
2.15 x
10e-5 m^3 - careful of units, again), making the total power loss in the
core about 3.4W. Obviously I am just guessing about what size the core =
is,
but it should be about right. To this you will need to add some watts of
losses in the winding resistance. Overall, this will be a substantial
improvement on the N30 material, but will require good ventilation all =
the
same.

There is a trade-off involved in the number of turns - increasing the =
number
of turns reduces the flux density and so the core loss, but increases =
the
loss in the windings. Also, having a large number of turns makes the =
leakage
inductance higher, and a small number of turns makes the winding =
inductance
too low.

Cheers, Jim Moritz
73 de M0BMU