Get hold of a data book on ferrite cores. Most of them include flux
density versus field strength curves for the various ferrites used in
their cores.
EPCOS at least makes the data availlable on the web - here's the link
for their N27 material
http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Ferrit...
--
Bill Sloman, Nijmegen
Ok Bill, I'm over my head on this because I don't understand all the
terms.
If we look at page 3 of the link you gave, first graph, you will see the B
vs u(o) curve.
At 25mT it is already down 34% from the peak and I would bet a dollar that
the curve
drops faster the lower you go with the B field.
Look at page 4, which shows the field versus the magnetising current -
it looks more or less like a straight line at zero field, which
suggests that permeability doesn't drop all that fast below 25mT.
Presumably what is happening is that is that the "sticky" component of
the alignment of the magnetic dipoles responsible for the hysterisis
is falling out of the permeability at low fields - as you can see from
the curves on page 4 it goes away at high fields.
My guess (only a guess) is that if I wound my theoretical transformer
(described above)
and had my 0.1 microamp driving it, it would be no where near 25mT.
Somehow
this
B vs u(o) curve will relate to permeability which is then related to (A
sub
L). I would then
use (A sub L) to calculate the turns on the theoretical 50 ohm primary
(200
ohms inductance).
Every specific core pair has magnetic path length, specified in the
data sheet, and the exciting field (in Amperes per metre) is just the
current looped around that core divided by the magneitc path length in
metres. In your case this is just your 10uA times the number of turns
divided by the magnetic path length of your core.
It just seems that at very low currents the permeability will drop so low
I
will not have my
200 ohms inductance and the transformer will not work as designed.
Wrong.
Since I have never seen this discussed and people build working radios
everyday,
I'm probably all wrong, but I'm not sure where.
Your error lies in assuming that the permeability drops to zero at
zero magnetising current. It doesn't, and in fact in this case
probably continues to fall off roughly linearly to about 60% of the
peak permeability.
If you really want a more stable inductance, gap your core until the
inductance is around 10% of the ungapped figure - which usually takes
a layer or two of 60 micron transformer tape between the core halves -
and wind a new core with three times the number of turns (the square
root of ten times more turns, to be precise but it is difficult to gap
the core this precisely, which is why the manufacturers do it for you
by grinding down the centre leg of a gapped core set).
Since 90% of the magnetic path is now in the gap, current dependence
goes down by a factor of ten. Bigger gaps provide even more stable
inductances, if you can afford the extra series resistance and inter-
winding capacitance.
--
Bill Sloman, Nijmegen
Thanks, Bill
I did some quick calculations of B/H on graph number 4 to find u(o)u(r).
It varies from 1.7, peaks at 4 and then drops to 0.4, this is a 10 to 1
range
but the variance is not at the low end where I would have expected.
Again I don't know quite how B/H relates to A sub L but I think it figures
into it.
If I get some time I'll get numbers together for a transformer I made
for a Flag antenna. I'm curious about the B in my transformer compared
to the values shown on the graph.
Mike