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saturable inductor

Discussion in 'Electronic Design' started by Michael, Jul 28, 2008.

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  1. Michael

    Michael Guest

    I thought I posted it already, it did not go through. Grrrrrrrrrrrrr
    Suppose I have two toroids sandwiched together. One is powder with
    single turn inductance (A1)=96nH. Another is high mu ferrite with
    A1=15uH.
    I apply 10V to 1 turn. The ferrite will saturate (~200mA) pretty soon
    (1/4 us). By that time the inductor will have ~0.26uJ stored in it....
    How will current look?
    I haven't got samples yet, I cannot try it...
     
  2. Joerg

    Joerg Guest


    There will be that classical shark fin, followed by a loud bang, molten
    solder splattering about, stuff flying around, smoke alarms blaring ...


    Be careful and make sure there si something that will limit the current.
     
  3. Michael

    Michael Guest

    Spark, smoke, sirens... I failed to ask why it doesn't happen
     
  4. Joerg

    Joerg Guest


    Well, if you have a current limit it won't ;-)
     
  5. Joerg

    Joerg Guest

    Some of the guys also overlook the fact that there is a rather large
    tolerance on ferrite core specs. Later, some folks in purchasing think
    that a ferrite is a ferrite is a ferrite. Phssst ... *BANG*.

    Don't ya love it when that happens? I once snuck into a mechanical
    design review. I wasn't supposed to be there but urgently needed to pass
    a message on to one of the guys. Looked at the PowerPoint screen and
    just had to ask: "Looks like there is way too little airflow. And what
    happens if that fan up there fails?" ... "Uhm ..., well, ... s..t!"
    Major redesign.
     
  6. Michael

    Michael Guest

    OK, I kind of guessed (qualitatively) that at first dI/dt is "slow"
    then there is a step (?? how long does it take for mu to drop??). The
    current steps up by sqrt(mu) and then keeps ramping up mu times as
    fast.
    Does it make sense?
     
  7. Joerg

    Joerg Guest

    Depends on the core, it's in the ferrite datasheet. Usually the "grace
    period" ends at some point where the inductor basically acts like an air
    coil, as if the core wouldn't be there anymore. For most practical
    circuits such as switch mode supplies that comes pretty close to a dead
    short with all its consequences.
     
  8. Michael

    Michael Guest

    I am not worried about "dire consequences" - I am sandwiching two
    cores actually (ferrite and iron powder), what I need to know is HOW
    FAST ferrite "disappears". They should use something like this in
    series trigger transformers.
     
  9. Joerg

    Joerg Guest


    All I can tell you is "really fast". For example, it can take just a few
    nanoseconds to drive Fair-rite #43 material into complete saturation.
    BTDT, got broom and shovel and cleaned up the mess ;-)

    The voltage applied to a transformer usually determines that. The higher
    it is the faster the primary current will rise and will core saturation
    be reached.
     
  10. Joerg

    Joerg Guest

    Good explanation. "Pretty darn fast" means just that, it gets down to
    nanohenries and the current (hopefully) pegs against some limiter.
    Michael should keep in mind that B-H curves and such are just typical
    data. There can be huge tolerances in core behavior, I have seen in
    excess of 30%. Us analog/RF dudes usually stay a good 40% below the
    "pretty darn fast" range.
     
  11. BTW, the magnetically controlled inductance is the old trick for
    electronic tuning of the LC tanks while keeping good Q and linearity.
    The parameters attainable are better then what can be done with varactors.


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  12. Joerg

    Joerg Guest

    It was also a nice method to achieve a mostly frequency-linear tuning
    range for oscillators. IIRC Drake and Collins used that. The downside
    was that the worm gear needed constant attention. So the usual setup was
    the receiver or tranceiver itself with a screwdriver (no, not the drink
    ....) and a bottle of rifle oil sitting next to it. In Germany we used
    the Ballistol brand which supposedly was also good to cure other
    ailments such as saddle sores after too much horseback riding. I don't
    know about that and would not do it but the stuff sure is quite
    universal in use:
    http://www.ballistol.com/uses.htm
     
  13. Joerg

    Joerg Guest

  14. legg

    legg Guest

    Composite cores are available from a few mfrs, inluding some parts
    that are basically two toroids glued together. The most common
    application is in a swinging choke, used to reduce minimum continuous
    load current levels in power supply unregulated auxiliary outputs.

    A part stressed over the full flux range (+/-) will show a change of
    permeability at fairly low flux levels - a monitored current waveform
    produced by a fixed alternating voltage will show 2 different current
    di/dt slopes, before saturation.

    RL
     
  15. I meant pure electronic solution by affecting the permeability of the
    core by the magnetic field of the external solenoid. This variant can
    have much better linearity and Q then attainable with varactors, which
    can be important for LO and input filters. AFAIK some early FM
    transmitters used this idea, too.
    Speaking of the motor tuning:

    http://www.armyradio.com/publish/Articles/William_Howard_Russian/Pictures/R-155A-01.jpg

    IIRC this R-155 receiver has 8-section variable capacitor bank tuned by
    the electric motor. The input section is built using tubes (total of 6),
    and the rest of the schematics is made from the discrete BJTs and
    crystal filters. In the old times, they knew how to do the real stuff.
    I put a notice on that. If there is PITA for whatever reason, the
    ballistol should help, is that right?


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  16. Joerg

    Joerg Guest

    They sure did but they were blissfully ignorant when it came to power
    consumption. Power just came out the wall outlet and was free ;-)

    According to the movie "My Big Fat Greek Wedding" Windex would be the
    answer. Very worthwhile to watch BTW, great fun.
     
  17. At the old times, it was the different engineering paradigm: design for
    the best performance whatever it takes. With the use of the well
    designed stuff we defeat all enemies so the power in the wall outlet
    will be completely free.
    I don't watch TV or movies, and I am not subscribed for the cable.


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  18. Joerg

    Joerg Guest

    That goes back all the way to Caesar and the Romans. They clear-cut
    pretty much all of what's now Italy to build one armada after the other,
    thinking that'll make them own the world. It didn't and now the soil on
    all those mountains has eroded. Nothing big will grow there anymore.


    This one's worth it. You can rent it at video places and watch it with
    friends that have a VCR. No idea whether it made it onto DVD. We have
    friends who said their (Greek Orthodox) wedding was an almost verbatim
    repeat of that story and we all laughed really hard because all of us
    had seen that movie. To this day the husband has refused to show us
    pictures of his baptism right before the wedding. Too embarrassing, he says.
     
  19. legg

    legg Guest

    A quick estimate would assume that during the first interval, only the
    ferrite path stores energy. As it's ungapped permeability is a number
    or orders of magnitude higher than the compounded material, this is
    valid.

    This duration can be derived from the B= Vt/NA equation.

    deltaB = ( V . t ) / ( N . A )

    deltaB= flux density change in Teslas ( 1T = 10,000gauss)
    V= Voltage applied across the inductor winding (volts)
    t= period of applied voltage (seconds)
    N= number turns in the coil
    A= physical cross-section (ferrite only) of the magnetic medium
    (meters^2)

    If the applied voltage is unipolar and reset to ~zero, the period to
    saturation corresponds to a B value of between 0.3 and 0.4 T (.35
    nominal), depending on material grade and temperature.

    If the applied voltage is AC, the period of the ferrite operation will
    be for double this value, as the flux will swing from + to -
    saturation levels.

    RL
     
  20. whit3rd

    whit3rd Guest

    My favorite old keyboard (it was a Burroughs, which should
    give a hint of its age) had keyswitches that moved a small
    magnet to saturate a core, which had two one-turn windings
    (looked like staples), one to excite and one to sense.
    It had great key feel...no mechanical damping or hysteresis
    like a buckling switch. Hall switches exist, too, but
    this was ... elegant.
     
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