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Help winding my own inductor?

Discussion in 'Electronic Basics' started by DaveC, Dec 19, 2003.

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  1. DaveC

    DaveC Guest

    20 millihenry, 1 amp, 25 KHz. Pot core, I presume. Don't know enough about
    these to know if I need a core with a gap or not. Willing to learn...

    I have a 20 mH inductor (off the shelf) but is getting hot with the current
    I'm passing through it.

    Where do I start? References and advice welcome.

  2. Robert Baer

    Robert Baer Guest

    If your 20mH inductor is getting hot, then (obviously) you are forcing
    more current thru it than it is rated for...and you are very likely
    saturating it, meaning the actual inductance may be 1uH or less.
    First, with a given core configuration (with or without a gap), you
    determine the effective pernmeability in amps per turns squared; then
    calculate the number of turns needed for the inductance you would like
    to have.
    The ampere turns gives you the flux density, and from the effective BH
    curve, determine if it is saturated.
    Increase the core area and/or the gap and re-calculate; repeat as
    Then use wire of a size as follows: cross-sectional in circular mils =
    milliamps of current rating for the inductor.
    That wire size, and number of turns can be used to calculate if it
    will fit in the available space.
    Tis not symple.....
  3. A practical inductor that you describe will need a gap to store the
    energy of the inductor without magnetic saturation (either pot or EE
    core) or a distributed gap (powdered metal toroid) and also the wire
    window to carry the wire size that will handle the current for those
    turns. The easy to wind solution (filling a bobbin) would be a pot
    core or some other variant of an EE core. 20 millihenries at 1 amp
    would need a substantial core size.

    The Fair-rite catalog has a design guide for their core types that is
    pretty applicable to any brand.

    Getting your hands on a selection of cores and bobbins about the right
    size is the hard part. I have been collecting them for 20 years (most
    recently shopping on ebay), just to have the parts to make prototypes.
  4. Genome

    Genome Guest

    That's a mother big inductor. Are you sure you meant 20mH

    Your starting sum is

    Aw.Ae = L.Irms.Ipk/Bpk.J.Kcu.Kw

    L is the value of your inductor in Humphreys
    Irms is the rms current you want it to carry in amps
    Ipk is the peak current you expect it to suffer from in amps
    Bpk is the maximum flux you want the core material to hack in Teslas
    J is the operating current density of your wire. 4E6 amps/meter^2 for a
    nominal 30C temp rise.
    Kcu is the utilisation factor of your wire.... 0.7 for round copper.
    Kw is how well it fills the available winding area. 1 for a single winding.

    Aw is the winding area
    Ae is the effective area of the core.

    In your case

    Aw.Ae = 20mH x 1 x 1/0.3 x 4E6 x 0.7 x 1
    Aw.Ae = 23.8E-9

    Square root it

    Aw = Ae = 154mm^2

    Ooooh maybe it's not so big.

    Find a pot core that sort of does that. And they won't...... Bugger

    Number of turns

    N = LIpk/BpkAe
    N = 20mH x 1/0.3 x 154E-6
    N = 433

    Just make sure that when you wind it you lock the door and take the phone
    off the hook. Begin 1.... 2..... 3.......30.... 80...... 333. 'How many
    sugars do you want in your tea?' Arghhhhhh, bastard.......

    Size of wire.... err

    154E-6/433 = 0.35565mm^2

    Square rooted.... 0.596mm diameter, enammelellmeleded heavy insulation...
    AWG 24 or 25 if you am an American.

    Al value is L/N^2 or 106nH per root turd. Now that's just got to be an
    almost standard set. N27 or 3C80 or some such.

  5. DaveC

    DaveC Guest

    Use maybe 78 material from this catalog? It is recommended for applications
    <200 KHz, but it says it's for power supply applications (meaning 50 - 60
    Hz?). Other materials seem appropriate, also, but don't come in pot cores.

    What is a good mail-order supplier for such cores and bobbins?

  6. DaveC

    DaveC Guest

    Yes, I'm sure. I measured it with reliable L meter, at 25 KHz.
    Don't understand what "And they won't" means... Can't use a pot core for this
    Hmm... the one I've got now is about that wire size. Is it overheating
    because it's core is saturating? Maybe I can just put it in another pot core?
    Or is the wire size the *only* thing that determines whether or not it gets

    (Yes, 'Merican.)
    Where would I find these "standard" pot core sets? A mail order supplier,
    maybe? Suggestions?

  7. I hope that anyone who thought Genome was an idiot is now firmly
    disabused. Although he still couldn't resist misspelling 'turn'.
  8. DaveC

    DaveC Guest

    Being a non-pro, I was wondering about that term :) Thought maybe I'd slept
    through some important topic in electronics lecture...
  9. Nah, our Genome merely has a mild case of whatever James
    Joyce suffered from. I say a "mild" case because one can
    usually figure out what Genome's on about in a single
    reading. ;>)

    Mark L. Fergerson
  10. Tim Wescott

    Tim Wescott Guest

    Oh, when you're halfway through winding a large toroid you start thinking of
    them in those terms.
  11. Tim Wescott

    Tim Wescott Guest

    Amidon ( has been the standard place to buy this
    stuff for amatuer radio operators, and they have a reasonable selection of
    cores. They seem to list the fair-rite numbers in their ferrite cores (they
    also have iron powder cores).

    Most of the posters are on track as far as I can tell, here's my summary and
    two bits:

    - Design for the inductance you need _at_ your maximum current. The
    effective inductance of your coil will go down as the core saturates, take
    that into account.

    - If you put a constant voltage on your core you can watch the current and
    see if it's saturating: a constant inductance will be indicated by a
    constant slope as the current increases. As the core starts to saturate the
    current will curve upward slightly. If the current _really_ curves upward
    before you've reached your design current you've got problems.

    - If you're making a core for a switching power supply you probably don't
    have to worry about gapping it -- gapping the core reduces the inductance
    variation with variations in current by limiting the overall reluctance of
    the core. If you need controlled inductance for some reason, run away.

    - The winding will exhibit loss due to current x voltage drop. The core
    itself will also show a loss due to hysteresis. The core loss is
    proportional to the coil voltage x frequency (actually the core flux x
    frequency). You need to size both winding and core, but if you use a
    material that's recommended for your frequency and application you'll
    probably get the right size core.

    - One final note: if you do this again for much higher frequencies
    (100-500kHz), make homeade litz wire to reduce skin effect: calculate the
    wire cross section you need, and make it up with three to five strands of
    smaller wire, loosely wound. This will bring the effective AC resistance of
    your wire down closer to the DC resistance.
  12. I read in that Tim Wescott
    Yes. I bought a demister antenna kit from Maplin that required a small
    ferrite toroid to be wound with 1.6 mm wire. In two words - im possible.
  13. They mean switching power supplies. Just what it sounds like you are
    I wish I knew. If you can't find any supplier, I may be able to
    scrounge 1 or two core sets and bobbins from my personal stash to get
    you started. This is the easiest component to make yourself that can
    compete with a commercial product in quality, but it is very hard to
    find the parts in low quantity. And that is a real shame.
  14. The first turn probably wasn't so bad.
  15. Tweetldee

    Tweetldee Guest

    Yeahhh I was laughing my ass off while reading his answer. That was a
    riot!!! I'll bet he's a good one to have at a party...
    Tweetldee at att dot net (Just subsitute the appropriate characters in the

    Never take a laxative and a sleeping pill at the same time!!
  16. Tweetldee

    Tweetldee Guest

    True enough, but the rest of those turds were just plain nasty!!
    Tweetldee at att dot net (Just subsitute the appropriate characters in the

    Never take a laxative and a sleeping pill at the same time!!
  17. Genome

    Genome Guest

    Well sort of. Humphreys are more cuddly.

    Pot cores have an alarmingly extreme mismatch between Aw and Ae. It's not
    their fault, it's just the way they are and it prob don't matter. Rooting
    things give a guess as to the bigness. Chances are a pot core will have a
    bigger Ae versus its Aw but if its Aw times its Ae does the biz then I
    wouldn't argue with it.

    I seriously hate bastards who *define* their *meaning* with arseholelicks.
    That would be Asterixs.

    Oh well,.......

    Yooz losses, like heating things cum from the core and the copper. Now,
    being Mr thick brick..... Oh bolicks its got something to do with exorcising
    the BH loop and prximity layer such like. Simple.

    And stop giving me Chritmas cards, I'll have to retaliate.
    Ahhhhhhhhhhhhhh. Manys the time I've fantasized about a bit of the anal with
    Hillery Clinton's. Licky licky.
    You phone up your local TDK rep and listen to an Indian rolling about
    laughing when you don't want to buy a million off.
    Not a problem.

    Do not rely on others to define your existance. Go work it out for yourself.

    They will hate you for it and you will be seriously fucked up.

    Nerrrr Nerrrrr nerrrrr
  18. Brian

    Brian Guest

    Using a program we have developed over the years (which will do coils and core
    selections, plus many other things as well). I plugged in your numbers to come
    up with a core size. Take a look at it at

    As you can see, we could have used a little bit more information. If you would
    like us to change something, let us know and we will enter in the new data (and
    display it for you). This program will also give you all the formulas used.

    The core used is "Magnetics" molypermalloy powder core (MPP core).
  19. DaveC

    DaveC Guest

    Sorry. How 's THIS instead? =-o
    Thanks (I think...)
  20. bushbadee

    bushbadee Guest

    Not impossible
    What you need is a shuttle.
    If the hole is real small you can use a needle as a shuttle.
    If it is larger then you can take a wire coat hanger and cut a section off
    and with a riffle file, file notches in a section of it and wrap your wire
    around the notches in the wire (Or a metal or wood rod) that will keep your
    wire neat and nice while you pass it around the core.
    I have wound many of them

    As an alternet use a pot core if applicable.
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