• Hand-wound coil in Colpitts oscillator

    From RodionGork@21:1/5 to All on Sat May 4 21:16:39 2024
    Friends and Colleagues, Hi again!

    Here is a Colpitts oscillator scheme I'm experimenting with:

    https://tinyurl.com/2yq9754k (simulation, NPN with common-base I suppose)

    I wound a coil from some length of wire I had at hand - and it has the following parameters:

    diameter - about 32mm,
    about 75 turns
    wire - 0.3mm

    It is wound "heap-style" - so it has only about 6-7 mm "length" and perhaps about 2mm "thickness".

    I measured frequency with oscilloscope (actually, with two different ones)
    and it is about 350 kHz, which means that inductance is about 400 uH.

    However, calculating by winding parameters over and over by certain
    formulas in books and internet I get result twice lower. For example this calculator https://www.66pacific.com/calculators/coil-inductance-calculator.aspx
    (formula matches with some old book I have at hand) - gives 250 uH.

    Questin 1: What may I be missing? perhaps, capacitance between turns? how
    can I add this in simulation - should it be capacitor in parallel with the coil?

    Question 2: If I move ferrite core into the coil, frequency reduces perhaps
    1.5 times (I tried few different pieces of ferrite found on my desk) -
    which at first amused me as I thought inductance is increased hundreds
    times. But most probably this doesn't work this way because coil has almost unidirectional current and the core is saturated by magnetic field?

    --
    to email me substitute github with gmail please

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  • From Bill Sloman@21:1/5 to RodionGork on Sun May 5 13:12:32 2024
    On 5/05/2024 7:16 am, RodionGork wrote:
    Friends and Colleagues, Hi again!

    Here is a Colpitts oscillator scheme I'm experimenting with:

    https://tinyurl.com/2yq9754k (simulation, NPN with common-base I suppose)

    I wound a coil from some length of wire I had at hand - and it has the following parameters:

    diameter - about 32mm,
    about 75 turns
    wire - 0.3mm

    It is wound "heap-style" - so it has only about 6-7 mm "length" and perhaps about 2mm "thickness".

    I measured frequency with oscilloscope (actually, with two different ones) and it is about 350 kHz, which means that inductance is about 400 uH.

    However, calculating by winding parameters over and over by certain
    formulas in books and internet I get result twice lower. For example this calculator https://www.66pacific.com/calculators/coil-inductance-calculator.aspx (formula matches with some old book I have at hand) - gives 250 uH.

    Question 1: What may I be missing? perhaps, capacitance between turns? how can I add this in simulation - should it be capacitor in parallel with the coil?

    There's going to be capacitance between the turns, but it will be
    picofarads rather than nanofarads. You can measure it by measuring
    resonant frequency of the coil in isolation - exciting it from a
    variable frequency generator through a judiciously chosen resistor to
    find the frequency where you get the highest voltage swing across the
    coil. Too small a resistor and you'll get a rather broad peak, too big
    and you won't have a detectable voltage swing across the coil


    Question 2: If I move ferrite core into the coil, frequency reduces perhaps 1.5 times (I tried few different pieces of ferrite found on my desk) -
    which at first amused me as I thought inductance is increased hundreds
    times. But most probably this doesn't work this way because coil has almost unidirectional current and the core is saturated by magnetic field?

    The inductance might increase a thousand times if you found an ungapped
    pot core pair that could clamp around the core.

    Just dropping in a chunk of ferrite will just slightly shorten the
    magnetic path length around the coil. You could clamp a U-core pair
    together around the coil which would do almost as well as pot core pair.

    The optimal solution is to wind your core around a toroid of magnetic
    material, but that get tedious (though there are machines that can do
    the job for you.

    Yours is a dumb newbie post - sci.electronics.design is normally
    populated by people who know a bit more about what they are doing.

    --
    Bill Sloman, Sydney

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  • From RodionGork@21:1/5 to All on Sun May 5 15:51:28 2024
    There's going to be capacitance between the turns, but it will be
    picofarads rather than nanofarads. You can measure it by measuring
    resonant frequency of the coil in isolation - exciting it from a
    variable frequency generator through a judiciously chosen resistor

    Thanks for the technique explanation. I guess it is going to be bit too
    high resonant frequency to be measurable with anything I have at hand, but perhaps I can improvise something.

    The inductance might increase a thousand times if you found an ungapped
    pot core pair that could clamp around the core.

    Oh, I completely forgotten core should be "closed" (I guess a ring would do also).
    Pieces I used are mainly sticks of old magnetic antennas etc, that's no
    good.

    Yours is a dumb newbie post - sci.electronics.design is normally
    populated by people who know a bit more about what they are doing.

    That's true, I sincerely apologize and promise to make better attempt next time, thanks for your patience!

    --
    to email me substitute github with gmail please

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)