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Parasitic capacitance in inductors

Discussion in 'Electronic Design' started by Paul Burridge, Nov 17, 2003.

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  1. Hi,

    Having read Chris Bowick's explanation of how these tiny capacitances
    arise between the turns of a coil, I'm left wondering if it's the
    whole story. Bowick states that they arise due to the resistance of
    the inductor's wire. Albeit tiny, this resistance leads to minute
    potential differences between turns, thereby giving rise to tiny
    capacitances. Is that really the whole story? Do not phase differences
    within the coil of the ac signal voltage make for far more potential
    difference between turns? In particular as signal frequencies get
    *very* high?

    p.
     
  2. Oh dear....

    Albeit tiny, this resistance leads to minute
    It isn't any story at all. Seems like darkness to me. The potential
    difference between coil loops is because loops are *inductors*!!!


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    http://www.anasoft.co.uk/replicators/index.html

    Understanding, is itself an emotion, i.e. a feeling.
    Emotions or feelings can only be "understood" by
    consciousness. "Understanding" consciousness can
    therefore only be understood by consciousness itself,
    therefore the "hard problem" of consciousness, is
    intrinsically unsolvable.

    Physics is proven incomplete, that is, no
    understanding of the parts of a system can
    explain all aspects of the whole of such system.
     
  3. Reg Edwards

    Reg Edwards Guest

    **** OLD WIVES' TALES ****

    The self-capacitance of a multi-turn solenoid has little to do with the
    capacitance between adjacent turns. All the little capacitances are in
    series across the coil. Their resultant becomes very small as the number of
    turns increases. And smaller still as the turns spacing increases and wire
    diameter decreases.


    Take a cylindrical coil former and cut it in half to form two smaller-length
    cylinders. The effective self-capacitance across the winding is closely
    related to the capacitance between the two cylinders.


    Download in a few seconds coil-design program SOLNOID3 from website below
    and run immediately.


    Amongst other useful things SOLNOID3 calculates self-capacitance and
    self-resonant frequency of an isolated coil.

    ----
    ............................................................
    Regards from Reg, G4FGQ
    For Free Radio Design Software go to
    http://www.btinternet.com/~g4fgq.regp
    ............................................................
     
  4. [snip]

    Reg, let's assume for a moment that you know what you're talking
    about. ;-)
    Why did you post this info under a separate thread? It clearly relates
    to the question I posed earlier about parasitic capacitances in
    inductors, so why not just follow-up? "old wives tales" tells no one
    anything about the content of your post. Fortunately I spotted it and
    thanks for the info, anyway.
     
  5. What newsreader are you using? Mine (Outlook Express) shows Reg's post as
    being part of the same thread, with the subject line changed.

    I believe it is the "References" header that permits a newsreader to support
    this. (It's been 13 years since I wrote a newsreader program, so I don't
    remember for sure.) "Thread" and subject line.
     
  6. Reg Edwards

    Reg Edwards Guest

    "old wives tales" tells no one
    ...................................................

    It tells you it was an old wife ready for ducking who palmed you off with a
    load of rubbish in the first place. And you accepted it without question as
    being gospel truth.

    Doing things this way is more educational. It sharpens your wits. Clears
    out the old-wives' nonsense. The circumstances under which the new
    information has been knocked in will greatly assist your memory to retain
    it. I'm sure you'll agree.

    Havn't you downloaded the program yet? Get a move on!
     
  7. In a coil of any reasonable Q (inductive reactance dominating
    resistance) the main voltage source that moves charge around in the
    inter turn capacitance is the inductive voltage drop, not the
    resistance. Even super conducting coils have a self resonant
    frequency.
     
  8. I agree that the adjacent turn capacitance is a small part of the
    total effective capacitance across a multi layer coil. The
    capacitance between the first turn of one layer and the last turn of
    the next layer has lots more voltage across it, and the capacitance
    between the start and end leads has the most voltage across it.
    Crossing back to the same start end for each layer cuts the inter
    layer capacitance by about half, and keeping the start and end leads
    short and as far as possible from each other also helps. A bit if
    extra insulation between layers (especially low permitivity material
    like teflon or air) also helps.
     
  9. Can you define "inductive voltage drop"?
     
  10. I beg your pardon, Reg??? On the contrary I posted a question on
    Bowick's explanation immediately after reading it. It didn't sound
    right to me. Tomorrow I'll dig up the exact quote from his book. It's
    odd he should screw up on something as simple as this as most of what
    he writes is very accurate and informative.
    Already got it, thanks.

    But it doesn't answer my original question re Bowick's explanation...
     
  11. V=L*(di/dt)
     
  12. Tim Auton

    Tim Auton Guest

    My newsreader (Agent, Paul's using Free Agent) can tell what
    newsreader somebody is using by displaying the headers ;)

    It is a known (to me, anyway) problem that Agent does this, it's
    rather irritating. Oooh, I just looked at the preferences and you can
    turn it off.

    Options - General Preferences - Message List, untick "Enable Threading
    by Subject" and "Start a new thread when follow-up subject changes".
    Then to force it to re-calculate its threads, right click on the group
    and change it to sort messages by something other than thread, then
    change it back - et voila! Perhaps restarting or getting new message
    headers would also do the re-threading, I don't know.
    It is the references header, which Agent otherwise deals with properly
    (preserving it, using it to spot cross-posts etc).


    Tim
     
  13. The actual quote from Bowick's book (at page 15) is:
    "Real-World Inductors
    "As we have discovered in preverous section of this chaptre, there is
    not perfect component and inductors are certainly no exception. As a
    matter of fact, of the components we have discussed, the inductor is
    probably the component most prone to very drastic changes over
    frequency.... As previously dicussed, whenever we bring two conductors
    into close proximity but separated by a dielectric and place a voltage
    differential between the two, we form a capactitor. Thus if any wire
    resistance at all exists, a voltage drop (even though very minute)
    will occcur between the widings, and small capacitors will be formed.
    This effect is called distributed capacitance."

    So this explanation is definitely wrong, then?
     
  14. Yep, as much as it refers to resistance. The resistance is usually
    insignificant. Its inductance that matters. That's were the volts drop
    is V=2.pi.f.L.I

    It seems to be the rule, that people who write books don't know what
    their talking about. So, you can see why I haven't wrote a book yet.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    http://www.anasoft.co.uk/replicators/index.html

    Understanding, is itself an emotion, i.e. a feeling.
    Emotions or feelings can only be "understood" by
    consciousness. "Understanding" consciousness can
    therefore only be understood by consciousness itself,
    therefore the "hard problem" of consciousness, is
    intrinsically unsolvable.

    Physics is proven incomplete, that is, no
    understanding of the parts of a system can
    explain all aspects of the whole of such system.
     
  15. Ban

    Ban Guest

    Kevin Aylward wrote:
    ||
    || It seems to be the rule, that people who write books don't know what
    || their talking about. So, you can see why I haven't wrote a book yet.
    ||
    || Kevin Aylward

    Kevin,
    with your awful English you will definetly need John Woodgate as a
    proofreader then.

    ciao Ban
     
  16. I read in sci.electronics.design that Paul Burridge
    >) about 'Parasitic capacitance in inductors', on Tue, 18 Nov
    2003:
    No, it's true, but it's not the whole truth. As has been explained, the
    inductive reactance of each turn contributes more than the resistance in
    the frequency range for which the inductor is suitable, and capacitances
    exist in real inductors, other than single-layer, that are larger than
    the inter-turn capacitances. These add to the *self-capacitance* but are
    not part of the uniformly *distributed capacitance*.
     
  17. I read in sci.electronics.design that Kevin Aylward <kevindotaylwardEXTR
    > wrote (in <Bwoub.287$
    ..net>) about 'Parasitic capacitance in inductors', on Tue, 18 Nov 2003:
    Tell us when you've writ it. (;-)
     
  18. Its no where near the truth. Its utter gibberish. For the resistance of
    the coil to matter, it must have a Q around or less than 1. As I said,
    any capacitance effects can only occur because of the difference in
    potential due to inductive impedance, not resistance.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    http://www.anasoft.co.uk/replicators/index.html

    Understanding, is itself an emotion, i.e. a feeling.
    Emotions or feelings can only be "understood" by
    consciousness. "Understanding" consciousness can
    therefore only be understood by consciousness itself,
    therefore the "hard problem" of consciousness, is
    intrinsically unsolvable.

    Physics is proven incomplete, that is, no
    understanding of the parts of a system can
    explain all aspects of the whole of such system.
     
  19. Get real, you have no f'ing idea what your talking about. There is f'all
    wrong with my English.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    http://www.anasoft.co.uk/replicators/index.html

    Understanding, is itself an emotion, i.e. a feeling.
    Emotions or feelings can only be "understood" by
    consciousness. "Understanding" consciousness can
    therefore only be understood by consciousness itself,
    therefore the "hard problem" of consciousness, is
    intrinsically unsolvable.

    Physics is proven incomplete, that is, no
    understanding of the parts of a system can
    explain all aspects of the whole of such system.
     
  20. It is a perfect explanation of the frequency limit of wire wound
    resistors. :)

    I have almost never found a technical reference book that contained no
    errors. I am just glad I have not been given the responsibility to
    create a zero error book on anything.
     
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