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Winding up a coil ?

Discussion in 'Electronic Basics' started by phil, Apr 29, 2004.

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  1. phil

    phil Guest

    I would like to build the fm transmitter kit from
    http://www.uoguelph.ca/~antoon/circ/fmt1.htm

    The instruction that L is 8-10 turns of a gauge 22 wire.
    My g.22 wire is insulated, do I need do remove the
    insulation before doing the winding?

    Thanks.
     
  2. Dbowey

    Dbowey Guest

    Phil posted:
    << I would like to build the fm transmitter kit from
    http://www.uoguelph.ca/~antoon/circ/fmt1.htm

    The instruction that L is 8-10 turns of a gauge 22 wire.
    My g.22 wire is insulated, do I need do remove the
    insulation before doing the wi >>

    The instruction calls for the coil to be "close wound." The recommended wire
    for this is enamel insulated. If that is what you are using, there is no need
    to remove the insulation. If the insulation of your wire adds to the diameter
    of the wire significantly, don't use it. The spacing between the *wire* of
    adjacent turns will affect the reactance of the inductor, therefore, the
    frequency.

    Don
     
  3. Yes, but a minor effect, in addition the spec says 8-10 turns, therefore
    an accurate inductor has not been specified. The only reason for using
    enamelled wire is to fit more turns in a smaller space. If you have lots
    of space there, is no electrical reason not to use pretty much any bit
    of wire, plastic insulated or not. There is just a convention mind set
    that cores use enamelled wire, but for a few turns, space is usually
    irrelevant so there is no need to follow that convention. In fact, I
    have found multistranded wire to have slightly better performance.

    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.
     
  4. Dbowey

    Dbowey Guest

    Kevin posted:
    ---
    1. How can you presume to know the effect of the insulation will be minor,
    when you do not know the type and thickness of the insulation?

    2. At VHF and above, the *conventional mindset* methods will always save you
    a lot of trouble.

    3. In what manner did you find multi-stranded wire to have better performance,
    recalling that we are discussing 88 MHz and above?

    Don
     
  5. Ahhmmm... Its trivial. I am reasonably familiar with typical hook up
    wire and have made measurements on such wire that agree with the
    elementary theory. That is, the approximate inductance of a coil doesn't
    depend on wire thickness, or wire spacing. i.e. its N^2.uo.ur.A/L.
    Arguable, the capacitance is less as there is more space. Only second
    order effects modify this.

    Again, packing wires close together is usually done for space reasons,
    not performance reasons.

    Just what do you expect the insulation to do to the inductance and self
    capacitance?
    Not always.
    It resulted in less phase shift error. I made quite a lot of
    measurements on a transformer used to couple to the mains power in an
    effort to measure power line impedance. Stranded wire gave better
    results. Its that simple. You should actually try it sometime.

    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.
     
  6. You don't have to remove the insulation, but the conductors will be
    farther apart than if they were insulated with enamel. So the
    inductance will be different. The instructions say to wind the turns
    close wound, which means each turn touches the next. The heavy
    insulation won't allow this.

    Also, it says the coil should be 1/4" diameter, and a pencil is larger
    than this. And also, the number of turns should be less than 8, more
    like 6 or 4. I would start with 6, and then trim off some wire to bring
    the frequency up. If you see herringbone pattern on the TV screen when
    it's tuned to ch 5 or 6, then you know that the coil has too many turns,
    and the signal is well below 88 MHz. With 8 turns, the freq might be as
    low as ch 4.
     
  7. Two other things I forgot to mention. You should put a .01 uF, or .05
    or .1 uF ceramic capacitor from +9V to ground. And you should keep
    leads short, expecially around the RF oscillator Q2.

    Also, increase R5 from 100 to 750 or until the voltage at the collector
    of Q1 is about 4.5V.
     
  8. phil

    phil Guest

    What's is the connection between an fm oscillator and tv frequencies,
    these start from 200mhz at the very least and go up to 800mhz?
     
  9. Not in the U.S., they don't. Yet. The low VHF TV channels are from 54
    to 88 MHz, so if you have too many turns or too much capacitance for the
    tuning cap, it will oscillate down in the TV channels.

    Sometimes they oscillate so nastily that the second harmonic is right in
    the middle of a channel in the high VHF TV freqs from 174 to 216 MHz,
    usually ch 7 or 8. The little transmitters can cause Ch 7 to become
    all wavy and herringbone.

    Did I tell you about the one wireless mic I had running that was making
    a 9th harmonic that was causing the local cell phone site to drop off
    customers on one channel? Yeah, really.
     
  10. Rich Grise

    Rich Grise Guest

    Can you please point out where it says enamel insulated? The page
    I see when I click that link says "hookup wire," which is definitely
    not enamel insulated. They're depending on the wire insulation (typically
    plastic) to give the correct winding-to-winding spacing.

    Good Luck!
    Rich
     
  11. Rich Grise

    Rich Grise Guest

    Well, if N is number of turns, and L is length, then clearly they're
    related, i.e. N/L = turns per length = 1/spacing.

    It makes a difference.

    Hope This Helps!
    Rich
     
  12. what the instructions say is to closely wind the coil using hookup
    wire...Im sure the circuit designer knows that the insulation will
    space the conductors apart......he or she has probably specified suing
    the closely wound hookup wire because it will be easier for a beginner
    to wind an inductor like that as opposed to spacing the windings a
    wire diameter or so apart.

    Its an fm transmitter with many cousins on the net and in books, most
    circuits of this type call for a little spacing between turns and this
    particular designer is using the insulation to make that
    space....might sacrafice some Q but its a low power fm transmitter.
     
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