Bifilar Coil Inductance?

Discussion in 'General Electronics Chat' started by wes, Oct 8, 2010.

  1. wes

    wes Thread Starter Active Member

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    Hello, I have a quick question that maybe someone could answer here.

    I know that if you have a Bifilar coil with the winding in a way so that the current goes in the opposite directions then your Inductance of that coil is close to zero (I know it will never reach 0, Impossible )

    Now the part I am not sure about has to do with the bifilar coil when wound in series con-fig (I think it is series )

    Here is a picture:
    http://merlib.org/node/5516

    it is the second one.

    Would the this configuration have a lower inductance then a standard coil.

    My reasoning for this is that since the coils are furthur apart, they would not be affected as much by the current flowing through the coils.

    If you follow the flow of the current then you see that it travels withing the coil then exits back out and then goes back into the coil through the second winding?


    So would the inductance be smaller or would it be the same as a standard coil or would it be bigger?
  2. marshallf3

    marshallf3 Well-Known Member

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    Good question due to the mutual inductance.

    You can compute the inductance of each coil given spacing, etc but two in perfect parallel?
  3. The Electrician

    The Electrician Senior Member

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    When you have a bifilar pair like this, you can measure the inductance of one of the coils, ignoring the other; call that value of inductance L.

    Now, if the two coils are connected in series aiding configuration, the inductance will be 4*L.

    If they are connected the other way, series opposing, the inductance of the combination will be very small.
    observativetiger likes this.
  4. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Which is why it is used to make resistors as opposed to conductors. All wires have some inductance, and by running next to each other, some capacitance. They also have resistance. The method of winding coils determines what characteristic is the predominant one used.

    I'm willing to bet this is off a HHO site, because this quote...

    is utter BS!. A bifilar coil with opposing windings stores no energy, the two opposing coils cancel the ability to store energy out. This is why it is predominately used for resistance. Do resistors store energy?

    We have heard the same tired false arguments by all the HHO fans out there before Beenthere put his foot down.

    http://www.allaboutcircuits.com/vol_1/chpt_8/12.html
    Last edited: Oct 8, 2010
  5. kubeek

    kubeek AAC Fanatic!

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    The two coils are not opposing, the current is still circulating clockwise, from A to B. Still I don´t see any point in this configuration apart from some different capacity.
  6. bertus

    bertus Administrator Staff Member

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  7. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Look again at the second illustration, one coil is clockwise, the other counterclockwise, thereby canceling the inductance. I'm not sure what the capacitance would be. Overall I think it would have a very complex frequency response.

    I thought beenthere would be interested in this thread (there is nothing wrong with it and it violates no rules), so I sent an email pointing it out.
  8. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    You got it backwards, the first illustration is a true coil/capacitor circuit, the second has two coils in opposition to each other. I have seen it many times from HHO nuts. Tesla actually invented the first as a super simple way to build a resonant circuit. Components have gotten cheaper, so it is pretty well obsolete.
  9. timrobbins

    timrobbins Active Member

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    Have another close look at the winding config guys, and the comment from Tesla - and think about what Tesla was up to!

    His bifilar arrangement is configured to purposefully give the opposite 'low inductance' effect that you guys are thinking of.

    Imagine applying a step voltage across one of the wires only, and what is generated across the other wire. Analogous to an automotive ignition coil.

    Ciao, Tim
  10. kubeek

    kubeek AAC Fanatic!

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    I think you should look again, both coils are clockwise.

    Attached Files:

  11. timrobbins

    timrobbins Active Member

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    I agree Kubeek.
  12. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    OK, I was wrong. Point taken.

    [​IMG]

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  13. wes

    wes Thread Starter Active Member

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    WOW, I did not think this would get this many replies this quick, lol

    First off, no it is not for some HHO project. It was a question out of curiosity.


    So after reading the replies, I am still not sure. So would the total inductance be the same as a standard coil or would it be higher?

    According to "The Electrician "


    Now, if the two coils are connected in series aiding configuration, the inductance will be 4*L

    I don't get how this could be. If you look at how the current will flow then how could that be possible?
  14. timrobbins

    timrobbins Active Member

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    Inductance is (L1 + L2)*M where L is inductance of each coil (say N turns per coil), and M is a mutual inductance factor (ranging between 1 and 2). If the winding was made from one wire with 2N turns, then inductance would be 4L (with same geometry). Its same as having two inductors of L and bringing them close together, their fields interact and you get a mutual and a stray coupling component. Its also like a transformer, where you start decoupling the windings such that stray field starts to become substantial.

    I think that is a somewhat correct description to mull over, off the top of my head.

    Ciao, Tim
  15. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    You have to hand it to Dr. Tesla, he was one smart feller. I wonder what he would make of the current crop of pseudo scientists. I suspect he would be highly amused, except when they start using his name to support their work.
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