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Ferrites: L changes with current change.

A

amdx

Jan 1, 1970
0
I have ask before about changes of inductance vs. AC excitation in ferrites
before and didn't get a satisfactory answer.
We can use a particular potcore at 5ua or 10amps. but how much does the
inductance change over this range?
What current does the manufacturer use to make his curves.
I'm thinking about a transformer at the input of a radio receiver with a
signal measured in microvolts.
Say we built a 50 ohm to 200 ohm transformer using the manufacturers specs,
and made the primary
inductance 4 times 50 ohms at our design freq.. It would work fine at 100 ma
to many amps.
But at 5ua is there enough inductance in the primary to make it function as
a proper impedance transformer?
Mike
 
E

Eeyore

Jan 1, 1970
0
amdx said:
I have ask before about changes of inductance vs. AC excitation in ferrites
before and didn't get a satisfactory answer.
We can use a particular potcore at 5ua or 10amps. but how much does the
inductance change over this range?

Depends on the material. It'll change with temperature too.

What current does the manufacturer use to make his curves.

They sweep the current.

I'm thinking about a transformer at the input of a radio receiver with a
signal measured in microvolts.
Say we built a 50 ohm to 200 ohm transformer using the manufacturers specs,
and made the primary
inductance 4 times 50 ohms at our design freq.. It would work fine at 100 ma
to many amps.
But at 5ua is there enough inductance in the primary to make it function as
a proper impedance transformer?

Losses may be too high. Try winding one for the job.

And get a copy of Epcos's Ferrite Magnetic Designer tool.
http://www.epcos.com/web/generator/...errites/Page,templateId=render,locale=en.html

Most manufacturers make 'copies' of the popular ferrites. There are often direct
cross-references beteen say Epcos, Ferroxcube/Philips/Yageo and TDK for example.

http://www.pe-coils.com/MnZn_Feirrite/MATERIAL COMPARES.pdf
http://www.dianyuan.com/blog/u/2008-12/6123_1230623550.pdf
http://www.google.co.uk/search?hl=en&q=ferrite+cross-reference&meta=&aq=f&oq=

The cheap Chinese manufacturers tend not to have the latest low-loss, high temp
parts though but that's a PSU issue.

Epcos's data also advises different ferrites for different applications.

Graham
 
A

amdx

Jan 1, 1970
0
Eeyore said:
Depends on the material. It'll change with temperature too.



They sweep the current.



Losses may be too high. Try winding one for the job.

And get a copy of Epcos's Ferrite Magnetic Designer tool.
http://www.epcos.com/web/generator/...errites/Page,templateId=render,locale=en.html

Most manufacturers make 'copies' of the popular ferrites. There are often
direct
cross-references beteen say Epcos, Ferroxcube/Philips/Yageo and TDK for
example.

http://www.pe-coils.com/MnZn_Feirrite/MATERIAL COMPARES.pdf
http://www.dianyuan.com/blog/u/2008-12/6123_1230623550.pdf
http://www.google.co.uk/search?hl=en&q=ferrite+cross-reference&meta=&aq=f&oq=

The cheap Chinese manufacturers tend not to have the latest low-loss, high
temp
parts though but that's a PSU issue.

Epcos's data also advises different ferrites for different applications.

Graham

I should add this a general question, I don't have a specific job in mind.
Just wondering what happens to AsubL at 0.1 microamps.
Mike
 
A

amdx

Jan 1, 1970
0
I should add this a general question, I don't have a specific job in mind.
Just wondering what happens to AsubL at 0.1 microamps.

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/...F/PDF__N27,property=Data__en.pdf;/PDF_N27.pdf

--
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.
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).
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.
Since I have never seen this discussed and people build working radios
everyday,
I'm probably all wrong, but I'm not sure where.
Thanks, Mike
 
A

amdx

Jan 1, 1970
0
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
 
E

Eeyore

Jan 1, 1970
0
amdx said:
I should add this a general question, I don't have a specific job in mind.
Just wondering what happens to AsubL at 0.1 microamps.

You must have too much spare time on your hands !

Graham

due to the hugely increased level of spam please make the obvious adjustment to my email address
 
E

Eeyore

Jan 1, 1970
0
amdx said:
Ok Bill, I'm over my head on this because I don't understand all the terms.

So study the subject ! I had to.

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.
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.

Guesses are a waste of time. Either you KNOW or DON'T KNOW !

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).
It just seems that at very low currents the permeability will drop so low I
will not have my 200 ohms inductance

At what frequency ? And that's the vector sum of Xj ohms + Rdc ohms by the way.

and the transformer will not work as designed.
Since I have never seen this discussed and people build working radios
everyday, I'm probably all wrong, but I'm not sure where.

Take the advice and get the Epcos program. It tells you all about saturation and change in inductance with amps.

Graham
 
A

amdx

Jan 1, 1970
0
Eeyore said:
So study the subject ! I had to.

Guesses are a waste of time. Either you KNOW or DON'T KNOW !


Thanks Eeyore, finally someone that KNOWS this stuff.
I have a 50 ohm to 200 ohm transformer, 4 turns to 8 turns on a FT37 toroid
of material 75.
I have 2 microvolts driving the 50 ohm primary at 1Mhz.
The effective volume of the core .163 cm^3
What is the B field?
You will KNOW what other data you need if any (I'd have to guess) so let me
KNOW and I'll
look it up for you.
It sure will be interesting to see how far off my guess was.

At what frequency ? And that's the vector sum of Xj ohms + Rdc ohms by
the way.

1Mhz, Xj=200 Rdc=0.025

Take the advice and get the Epcos program. It tells you all about
saturation and change in inductance with >amps.

I'll look at it, when you calculate the B field quantity, we'll see if
the Epcos program
goes to that low level.
Thanks again for your help,
Mike
 
A

amdx

Jan 1, 1970
0
Eeyore said:
You must have too much spare time on your hands !

Graham

Hey Graham,
I reposted a graph from Fred Bartoli on
alt.binaries.schematics.electronic, it shows
inductance dropping with decreasing drive voltage. It doesn't go low enough
though.
Mike
 
A

amdx

Jan 1, 1970
0
Jim Thompson said:
[snip]
Hey Graham,
I reposted a graph from Fred Bartoli on
alt.binaries.schematics.electronic, it shows
inductance dropping with decreasing drive voltage. It doesn't go low
enough
though.
Mike

Sonnova gun! I wonder what might be causing that?

Get out your textbooks ;-)

...Jim Thompson
Causing what ? This is my third attempt at finding out if my postulation
has any merit at all.
I've been told forget it, just build the transformer.
It is just a curiosity for me, I have used the same core at 300 watts power
levels and at microwatt levels.
Would it have been a better transformer at lower frequencies if I had added
more turns. I don't know and
I have know way to measure microvolts
Note his graph only goes down to 2Vrms, I'd like to see a graph down to 2
microvolts.
Hey Jim, I'm a little surprised at your response, is Phil rubbing off on
you?
Have a drink of fine wine on me :)
Mike
 
F

Fred Abse

Jan 1, 1970
0
On Mon, 21 Dec 2009 21:24:43 +0000, Baron wrote:

Rhode & Schwarz used that technique in some of their instruments.

So did Marconi Instruments in the 1950s - VHF sweep generator using a
"magnetic reactor"
 
F

Fred Abse

Jan 1, 1970
0
Mag amps are _cool_. Slow, bulky, and heavy, but bulletproof and highly
effective.

Until the Supreme Court decision, Florida certainly thought so ;-)
 
A

amdx

Jan 1, 1970
0
Thanks Eeyore, finally someone that KNOWS this stuff.
I have a 50 ohm to 200 ohm transformer, 4 turns to 8 turns on a FT37
toroid of material 75.
I have 2 microvolts driving the 50 ohm primary at 1Mhz.
The effective volume of the core .163 cm^3
What is the B field?
You will KNOW what other data you need if any (I'd have to guess) so let
me KNOW and I'll
look it up for you.
It sure will be interesting to see how far off my guess was.



1Mhz, Xj=200 Rdc=0.025



I'll look at it, when you calculate the B field quantity, we'll see if
the Epcos program
goes to that low level.
Thanks again for your help,
Mike
Graham where did you go,
To quote you "Either you KNOW or DON'T KNOW !"
Three days, no response, does that mean you don't KNOW?
Mike
 
T

Tim Williams

Jan 1, 1970
0
George Herold said:
Yup, I'd never heard of them. The wiki article was not much help. Do
you have any good references?

I have a copy of,

Paul Mali, _Magnetic Amplifiers / Principles and Applications_, John F.
Rider Publisher, Inc. (1960).

It shows operation and use well enough that you could put one together, but
it's not a nuts-and-bolts thing where you can design a mag amp from B-H
curves and circuit values and such. If you can find a copy (library or
online?), it's a worthwhile introduction.

Tim
 
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