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Ferrites: L changes with current change.

Discussion in 'Electronic Design' started by amdx, Dec 18, 2009.

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  1. amdx

    amdx Guest

    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?
  2. Eeyore

    Eeyore Guest

    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.,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. COMPARES.pdf

    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.

  3. amdx

    amdx Guest

    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.
  4. amdx

    amdx Guest

    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,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
    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
    I'm probably all wrong, but I'm not sure where.
    Thanks, Mike
  5. amdx

    amdx Guest

    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.
    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.
    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
    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.
  6. Eeyore

    Eeyore Guest

    You must have too much spare time on your hands !


    due to the hugely increased level of spam please make the obvious adjustment to my email address
  7. Eeyore

    Eeyore Guest

    So study the subject ! I had to.

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

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

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

  8. Eeyore

    Eeyore Guest

    Just call it Al. Everyone else does.

  9. amdx

    amdx Guest

    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,
  10. amdx

    amdx Guest

    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
  11. amdx

    amdx Guest

    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
    Hey Jim, I'm a little surprised at your response, is Phil rubbing off on
    Have a drink of fine wine on me :)
  12. Fred Abse

    Fred Abse Guest

    On Mon, 21 Dec 2009 21:24:43 +0000, Baron wrote:

    So did Marconi Instruments in the 1950s - VHF sweep generator using a
    "magnetic reactor"
  13. Fred Abse

    Fred Abse Guest

    Buddy, can you spare a dime?
  14. Fred Abse

    Fred Abse Guest

    Until the Supreme Court decision, Florida certainly thought so ;-)
  15. amdx

    amdx Guest

    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?
  16. Tim Williams

    Tim Williams Guest

    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.

  17. Fred Abse

    Fred Abse Guest

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