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Found Out Something Else Today

Discussion in 'Electronic Components' started by Watson A.Name - Watt Sun, Dark Remover, Jan 26, 2004.

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  1. I bought a bag of 12 Amidon ferrite Beads, # FB-77-6301 so I could
    make some more inductors for my V boost circuits. I wound about ten
    turns of 24 gauge telephone wire on one, which is a toroidal sleeve
    ..375" OD, .194" ID, and .41" long. I measured the inductance at 405
    microhenrys, which is more than I expected.

    I was reading the literature that comes with the beads, and it said
    that some mixes are conductive. I thought that ferrite was mostly
    glass, and was an insulator. So I measured one with my DMM and I
    found that the resistance was down in the hundreds of ohms, much
    _lower_ than I had expected. I had thought that I really didn't need
    to wind them with pvc insulated wire, but now I'm thinking that it's
    not such a bad idea after all. I have wound the toroids before and
    seen the insulation scraped off the enameled wire by the sharp edges
    of the totoid but I didn't think much about it. So it's possible that
    it could short out with the low resistance material of this particular
    toroid. That's something I never expected.


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  2. Jim Thompson

    Jim Thompson Guest

    Beads are designed to be lossy. Seems a wee bit foolish to me to use
    them for inductor cores.

    ...Jim Thompson
     
  3. It would definitely lower the "Q". You can wrap a thin strip of
    paper around the core first, or spray paint the core first, to protect
    the wire from the edges.
     
  4. The high permeability ferrites are all quite conductive (semi
    conductive, actually), and this represents a resistive loss to any
    inductor would on them. Think of the core as a single turn load with
    a resistor. For one offs, I usually take the edges off the core with
    silicon carbide sandpaper. If I am going to be pulling a lot of turns
    through the hole, I may paint the ends of the core with a thin layer
    of epoxy to protect the wire.

    By the way, if you are making energy storage inductors, these ungapped
    cores will saturate a only a few amp turns. I like to use the two
    piece cores made to be put on power cords as RFI suppression. I
    sandwich a layer of paper between the halves to make an energy storage
    gap. It is also easy to wind wire around the individual halves than
    it is to pull it through the hole.
     
  5. DarkMatter

    DarkMatter Guest

    If you had, the inductance would have been LOWER than expected, not
    higher. It is very likely that you made no such shorts. Mag wire
    insulation is in the 1500 V range.

    Wind four or six of them at ten turns each to see what your
    tolerances are.

    Typically, ferrites are in the 20% range. That is a considerable
    number.
     
  6. DarkMatter

    DarkMatter Guest

    Good for snubbers, but not much else.
     
  7. Lossy at a few MHz and up. Much, much less lossy at a hundred kHz.

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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
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    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  8. Yeah, I got some 600 grit wet-or-dry sandpaper that i may use on it.
    I also started sanding down the other beads that I sawed in half with
    a ceramic tile saw. I used a round 60 grit sanding disk for that, but
    it's getting worn down on one side, where it's beginning to polish
    instead of sand. That side may make a good way to take the edges off
    the beads. Maybe I should save the dust and make an old fashioned
    coherer.. Yeah, right.


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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  9. Sounds interesting, but the two halves are kind of largish for my
    needs. That's why I picked these beads. I've been sawing the
    suppressor beads from keyboard cables in half, and they're not too
    big, about a half to 5/8 inches in diameter. But it's a hassle sawing
    them in half. As for the winding, I'm only able to get ten turns on
    them, and that only takes a minute or so to wind.

    The guys on Candlepower Forum use a bead they buy from Digi-Key or
    Mouser, and it has an even smaller center hole, so they can only wind
    a few turns on it. It works okay.

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    ###Got a Question about ELECTRONICS? Check HERE First:###
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    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  10. Boris Mohar

    Boris Mohar Guest

    You shouldn't need to take measures to prevent saturation if you are using
    them as RFI suppression common mode choke. The differential mode currents
    will cancel and the core will not saturate. That is why you often find high
    permeability toroids in CM choke applications.
     
  11. Mark J.

    Mark J. Guest


    Perhaps use them as Memory Cores? ;)
     
  12. Roger Gt

    Roger Gt Guest

    X-No-Archive: yes
    "Mark J." wrote in message
    Sometime ago I bought a "jar" of .08 dia memory
    cores, I can get 4 to 6 turns of #28 to #30
    enameled wire on them. They work great to
    suppress RFI and since I got about 60,000 of them
    for $3 I will likely never run out. I have strung
    up to ten on a piece of #22 wire for a Power input
    filter too. (Don't have the exact value of the
    inductance or loss, but it worked!
     
  13. Tim Wescott

    Tim Wescott Guest

    That seems like a huge tradeoff, though -- you have a nice energy storage
    gap, but your inductance goes way down and now you need a ton of wire. I
    can certainly see doing this to get good linear inductance characteristics,
    though.

    Most of the switching supply designs I see don't have gaps in their
    inductors, at least not explicitly (but you do have to design around
    saturation). Is there something I'm missing?
     
  14. Is there something I'm missing, too? Like, there are millions of gaps
    between the magnetic particles in the ferrite and the binder, which I
    believe is ceramic, and is not magnetic. If so, why should there need
    to be an additional air gap added?
     
  15. The powdered metal cores have distributed gaps between high mu
    particles, but the ferrite is (ideally) a single crystal, solid
    material with no gaps. In order to get high permeability and low
    hysterisis the material must reach high flux with low magnetization
    (the definition of high mu). But ferrites do not have as high
    saturation flux as iron (2,000 to 3,000 gauss versus 10,000 to 20,000
    gauss). The energy stored in an inductance is 1/2 * L * I^2. So
    using the core ungapped gives a high inductance for a low number of
    turns but a low I^2. By adding a tiny gap, you lower the L by say a
    factor of, say 25 but also raise the saturation magnetization by 25
    (25 times as many amp turns to reach the same saturation flux). You
    increase the turns count by 5 to get the inductance back (since
    inductance goes as the square of the number of turns), but the
    saturation flux still supports 5 times the current. But now the stored
    energy has gone up by a factor of 25 (same L, 5 times the current,
    squared). All that extra energy is stored in that tiny gap. Of
    course, if you don't mind using a core volume with 25 times more
    volume, you can store it there.
     
  16. Well, the reason I wanted to use a higher mu core material in the first
    place was to reduce the number of turns. That's so that the wire
    resistance would not lower the peak current. When you have only a volt
    and a half at a couple hundred mA to work with, you need all the
    reduction in resistance you can get.

    I think one possible solution is to work with the lower inductance by
    raising the freq of oscillation. That allows the circuit to get by with
    less turns and less reasistance. I don't know how they do it, but the
    commercial V boost circuits use inductors smaller than the size of a
    pencil eraser, maybe about the size of a 5 mm LED but not as tall. And
    they're also surface mount, making them really squat, like the size of a
    3 mm screw head. And apparently to get around the internal resistance,
    the inductance is lower, maybe 10 uH or less. The freq may be as high
    as .5 to 2 MHz in some of the Zetex, Maxim and LT chips.

    For an example of a converter using a (not so) small ferrite bead with
    no air gap and just a few turns, see URL (thanks, Stepan)
    http://www.e-f-w.com/community/modules.php?name=News&file=article&sid=32
     
  17. And you can do that by just using more core volume than the absolute
    minimum. But it is hard to get low losses this way, because of the
    higher volts per turn applied to the core resistance. But for very
    low power operations, having a core several times the smallest
    possible may still be plenty small.
    (snip)
     
  18. You say "absolute minimum" and "smallest possible" but I have no way
    of knowing what these are. I have used some 180 uH chokes that are
    smallish, maybe 6 mm diameter by 7 mm long, and they worked, but their
    resistance was too high, maybe 1.5 ohms. So I got some 100 uH chokes
    from Mouser, # 580-22R104, which are about 8 mm diameter by 10 mm
    long. They have a resistance less than a half ohm, and can handle a
    lot of current, so I have no problem driving three white LEDs to super
    brightness and sucking 200 or more mA from a single AA cell.

    The guys on Candlepower Forum use a smallish core, and enough current
    to drive a 1 Watt Luxeon Star LED to full brightness. One of their
    small converters is called the "Ill Pill"
    http://flashlightreviews.home.att.net/mods/lambda_ill_pill.htm

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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  19. Mark J.

    Mark J. Guest

    In (Watson A.Name -
    Watt Sun, Dark Remover):

    How long do those AA batteries last at 200mA/cell???
     
  20. buck rojerz

    buck rojerz Guest

    I know it's OT, but since you brought up the thought about current per
    cell...

    I was given one of those little tiny digital cameras, about 2" square and
    about 3/4" thick. I forget the name. Anyway, the single AAA battery
    never would last long enough to even fill the memory with images, so I
    checked it out. It was drawing a whopping 850ma from a single AAA
    battery! No wonder it was given away. I will concede that there MAY
    have been something wrong with it, however it took quite good pictures
    and worked well, with the exception of battery life. It was supposedly
    the smallest digital camera in the world, at the time of its release and
    I don't mean a video camera, rather a digital, single image camera that
    you could download images from, to a computer thru USB. No flash. res
    was 1024x768(probably interpolated)

    buck
     
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