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Magnetic field of a solenoid

Discussion in 'Electronic Design' started by Klaus Kragelund, Jul 25, 2007.

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  1. Hi

    I need to transfer some energy accross a platic barrier. I will be
    using a solenoid on both sides of the barrier, one to transmit and one
    to receive the energy

    First thoughts is to use an air core to provide maximum stray field
    (in a ferrite core solenoid the field will be concentrated in the
    core, but somehow I think the air core is better over longer

    The barrier is 2mm thick, and may use almost any circuit on both
    sides. An HF oscillator on the primary and a simple diode
    rectification on the secondary side, preceeded by a capacitor to
    adjust the ressonance to get optimum perfomance.

    But, I am in US right now, so I have none of my books and the internet
    have not helped me in this matter.

    The B field of a solenoid is:

    B = u0 * uR* I * N

    But how do I calculate the B field at say 10mm from the core in order
    to be able to calculate the current in the recieving core.

    Any ideas?


  2. Joerg

    Joerg Guest

    Hello Klaus,
    I'd suggest pot core halves, assuming the available diameter in that
    area is 10mm or more. 2mm isn't much of a gap, should not be a big deal
    to bridge.

    The classical way to transfer energy over a gap is a series resonant
    converter. This basically "notches out" most of the leakage inductance
    and results in a good net energy yield on the other side. How much
    energy to do have to get across?

    Like with all things there is a caveat: The regulatory guys. With a
    series resonant converter you usually cannot stay within an ISM band
    like 13.56MHz because it's only a few ten kHz wide. So it'll have to be
    a rather low frequency.

    Calculations are difficult. Could be done with lots of data from the
    core mfg but it's better to test it in the lab and provide a really
    healthy net energy margin.
  3. Uncle Al

    Uncle Al Guest

    How about 60 watts net power transfer at 7 meters separation with 40%
    It's been done to a fare-thee-well over a couple of meters separation,

    Soljacic WiTricity 22,600 hits
  4. I actually already did a test with two pottet cores. See pictures:

    This one is wound:
    (bad mobile phone picture)

    That can transfer plenty of power, but it will be costly (the turns
    are not PCB traces, but a cobber wire)

    I was looking for perhaps using a standard part like this:

    At least for the primary side. The stray field would frindge around
    the top, but I can get the manufacturer to make it more like a
    straight rod to get an even field.
    Then the secondary side could be either a coil like that or an air
    core inductor (it has to be 90 degrees angled to the PCB to that

    Actually I would like to be able to calculate the field, since I may
    be using it in another project where the end goal is to transfer only
    a signal with very low frequency. So perhaps a Hall sensor could do in
    that app.

    But close to the core it is the near field that decides the magnitude.
    So its difficult to calculate


  5. That sound almost Star-trek like.

    I just need 100mW


  6. Bert Hickman

    Bert Hickman Guest

    The MIT folks used about 400W from a 10 MHz vacuum tube oscillator to
    perform this feat. For your application, two coils on silicon steel or
    ferrite "C" cores should work as a simple transformer at line frequency
    (albeit with relatively high leakage inductance from the air gap). Dual
    resonant circuits are not necessary but could be used, at line or higher
    frequencies, to improve overall performance.

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  7. John Larkin

    John Larkin Guest

    I've done this with parallel ferrite rods, with a tuned coil on each.
    If the rods are several times longer than your 2 mm gap, and close to
    the barrier, coupling should be pretty good.

    A pair of C-cores would be even better, but bulkier.

    How much power are you trying to couple?

  8. Hello Klaus,

    If the barrier is only 2mm, the best solution would be probably a magnetic
    circuit made of the two half cores.

    How to calculate:
    We assume that mu of the core material times gap width is much higher then
    the length of the magnetic path. The magnetic resistance of a piece of
    material is length/(mu * cross section). The coupling between the cores is
    the fraction of the primary flux which gets into the secondary. Part of the
    primary flux goes through the air missing the secondary, the other part
    crosses the gap to the secondary. The ratio of those parts depends on
    geometry, and it is approximately 2*gap width/distance between the poles (if
    distance is much higher then the gap). Thus the coupling is ~ (1 -

    I can derive more accurate estimates if mu and the geometry is known.

    Vladimir Vassilevsky
    DSP and Mixed Signal Consultant
  9. Hi Vladimir

    That great info :)

    Do you by any chance have a reference for where you got this info (or
    was it derived from general textbook material?)


  10. CWatters

    CWatters Guest

    Take apart an electric toothbrush charger?
  11. Tina

    Tina Guest

    Don't overlook thinking about the plastic as part of a parallel plate
    capactor if fringing fields are an issue. You'd be doing electric
    field stuff rather than magnetic.
  12. [/QUOTE]
    The electric toothbrush chargers I have seen rely on one part being
    *inside* the other.

    -- Richard
  13. The electric toothbrush chargers I have seen rely on one part being
    *inside* the other.

    -- Richard[/QUOTE]

    See "Power with no Strings Attached", page 31 of the July 5 (#14)
    issue of EDN

    Best regards,
    Spehro Pefhany

  14. Do you by any chance have a reference for where you got this info (or
    was it derived from general textbook material?)

    The main idea is from the general EE textbook. There is an artificial trick
    to calculate the static magnetic fields: consider the magnetic path as an
    electric circuit, where:

    mu = conductivity
    B = voltage
    Flux = current

    And apply Ohm and Kirchoff laws to it just like you do with electric
    Of course, you have to account for the "magnetic currents" in the air and
    the complex geometry of the "magnetic conductors". However it is pretty
    simple to derive the estimates which are good enough for the practical

    Vladimir Vassilevsky
    DSP and Mixed Signal Consultant
  15. Joerg

    Joerg Guest

    What's the cost of the milling for the circuit boards and potting or
    riveting in the core halves over there? Since you only want to transfer
    100mW you may be able to use this solution. I had a case with a core of
    about 10mm and three turns. The coupling was quite good but only in the
    tens of MHz, of course.

    What is the voltage you need on the secondary side?

    The winding and potting into the core halves would have to be contracted
    out. If that is not an option I think it would have to be 13.56MHz or
    27.12MHz ISM in this case, with just trace inductors. At 27.12MHz you
    may even be able to get away without a core but with more (free...)
    turns on the board. However, in that case the regulatory folks would
    have to check whether such ISM band usage is legal in all the countries
    where the product is going to be marketed.

    ISM requires a narrow tolerance clock source, either crystal driven or
    resonator driven. Personally I prefer resonators for anything that might
    get banged around a bit.

    Should be in line and same orientation. But this would be a custom part.

    Precise calculations would require a software like EESOF (Agilent) but
    my experience is that simulations are of limited value with air-coupled
    inductors. Too many variables. It may be better to measure and slap on a
    huge margin. For calculating a single coil scripts like this help:

    If all this has to be small, cheap and transfer 100mW or more I'd look
    at higher frequencies. In the range below 100kHz things become large.
    For example, the cores for charging electric toothbrushes and stuff like
    that are almost a cubic inch in volume. Many of those operate around
  16. Joerg

    Joerg Guest

    The electric toothbrush chargers I have seen rely on one part being
    *inside* the other.

    Not necessarily. On the Philips Sonicare they a butting up against each
    other, with plastic in between. The two ridges in the base are only
    there to prevent the toothbrush from falling over.
  17. John Larkin

    John Larkin Guest

    Does that run at line frequency, or is there electronics?

  18. Joerg

    Joerg Guest

    I held the analyzer to it and AFAIR it was running around 60kHz. At line
    frequency you wouldn't be able to achieve enough coupling.

    Those toothbrushes work great, BTW.
  19. I need to transfer some energy accross a platic barrier. I will be
    1) Curve both solenoids into half-circles, then you have an
    air-core toroid with the plastic sheet separating the two halves.
    Better yet, take a toroid and a glass-cutter and score it and
    break it into two halves and use those as the cores. (ferrites
    will break like glass, use glass "cutting" techniques)

    2) Don't cross-post unless absolutely necessary. It was not
    necessary for this question.

    3) I hope you are using a full wave bridge on the output side
    and not some [email protected]#$%^&* half-wave rectification scheme. Resonating
    the secondary is not necessary with a ferrite core toroid arrangement.
    I'm not sure it would be necessary with highly sub-optimal windings,
    but it is likely to involve you in lily-painting. Optimize the
    winding geometry and core, then paint your lilies if you absolutely
    feel you must.

    4) You could also do this with capacitive transfer. (I should
    probably keep my mouth shut and not mention this, but ...)

    5) Two right-cylindrical solenoids is an absolutely atrocious
    way to transfer energy. The coupling coefficient is negligible.

    6) Calculate NOTHING -- build, measure, adjust. The calculations
    needed to get decent accuracy are just NOT owrth the time and effort.

  20. CWatters

    CWatters Guest

    The electric toothbrush chargers I have seen rely on one part being
    *inside* the other.[/QUOTE]

    Well yes. But I'm a great believer in changing the problem to make the
    solution easier :)

    They so work across a plastic barrier - just not a flat one. So deform the
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