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