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small variable capacitors (measured in uF)? measuring capacitance?

Discussion in 'Electronic Components' started by Mad Scientist Jr, Jun 5, 2007.

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  1. I would like to hook up a variable capacitor to some audio circuits I
    built for learning about synths and amps
    (schematics are here:
    http://www.geocities.com/usenet_daughter/tone_generators.htm )
    http://www.runoffgroove.com/littlegem.html
    http://www.runoffgroove.com/ruby.html
    http://makezine.com/09/crackerboxamp/
    )

    because in some of these the capacitors change the pitch, tone or
    other qualities.

    These circuits run off a 9V battery and use capacitors with ratings
    such as
    220 uF
    100uF,
    0.1 uF,
    0.01 uF
    0.047 uF
    so I would want variable caps in this range.
    Do these exist? Online all I am seeing is ones rated in pF.

    Also, assuming I find these, and get them working in the circuit, and
    find a desired setting for a capacitor, how do you measure the
    capacitance? I have a multimeter but have really only used it to
    measure ohms.

    Thanks...
     
  2. They'd be HUGE. Easily as big as a fridge or a car, even a truck in the
    case of 220 µF.

    What matters is the timing, as it's timing that matters in amps and
    synthesizers, except for power supplies or sample and hold circuits where
    capacity is more important.

    To get long timing with small capacitors, use large value resistors, and
    make sure you have low leakage capacitors.
     
  3. Also, use a fixed capacitance and vary the resistance. That's what is
    usually done, and is why variable capacitors are rare.
     
  4. They'd be HUGE. Easily as big as a fridge or a car, even a truck in the
    Are you serious? That's crazy! The fixed capacitors themselves are so
    tiny!
    Aha! Thanks for the advice.
     
  5. Well, I was slightly overdoing it, on the assumption that you'd be looking
    at something rigid enough to support the weight, and given that it would
    need air or solid dielectric separation.
     
  6. David

    David Guest

    \
    http://www.stormwise.com/page3.htm
     
  7. John A

    John A Guest

    You cannot buy variable capacitors with these values, but you could make up
    or purchase an arrangement of switches and fixed capacitors quite cheaply
    which a) would be the next best thing and b) would directly address your
    "how do you measure the capacitance" question. Commercially they are called
    Capacitance Substitution Boxes or Capacitance DecadeBoxes . They're not
    infinitely variable, of course, but are finitely practical!

    John A
     
  8. Nice. I'm not sure that's what the OP wanted, (more likely a single
    continuous control of something), but if this switched-cap box were built
    to 1 nF resolution, for $40 extra or so, you can add a variable capacitor
    shown on the page David linked to: http://www.stormwise.com/page3.htm

    That way you can have any infinitely variable value, just not in one sweep.
     
  9. John A

    John A Guest

    sweep.



    Well, I intentionally didn't suggest adding a variable as a) it would
    re-open the Pandora's Box of how to calibrate the thing, and, b) the
    combined tolerances of the fixed components would make a nonsense of such
    calibration anyway.

    As the OP sounds as though he may want several capacitors to be
    simultaneously variable his best homebrew move may be to build up
    a number of simple, two or three decade BCD-style, successive approximation
    systems. Each will need 4 switches and capacitor combos (15 caps of one
    value) per decade - a nice little homebrew project.

    John A via rec.radio.amateur.homebrew
     
  10. John A

    John A Guest

    sweep.



    Well, I intentionally didn't suggest adding a variable as a) it would
    re-open the Pandora's Box of how to calibrate the thing, and, b) the
    combined tolerances of the fixed components would make a nonsense of such
    calibration anyway.

    As the OP sounds as though he may want several capacitors to be
    simultaneously variable his best homebrew move may be to build up
    a number of simple, two or three decade BCD-style, successive approximation
    systems. Each will need 4 switches and capacitor combos (15 caps of one
    value) per decade - a nice little homebrew project.

    John A via rec.radio.amateur.homebrew
     
  11. Initially, I didn't assume the OP wanted a large variable capacitor at all,
    but was mistakenly trying to match fixed sizes with variable sizes without
    considering why such things are not common practise. Calibration aside, the
    variable I suggested adding IS a viable idea, assuming your switched-cap
    box is valid. After all, if you CAN get any value, then you set it as you
    need. If it's tuning frequency, you measure the frequency.

    But as I said WAy early in the thread, first reply, if you really want a
    sweep of possible timing values, just use a fixed cap and a variable pot,
    as standard. The OP was talking about synthesizers, after all, he mentioned
    them explicitly.
     
  12. Another thing: That variable cap on the page David (and I) linked to, is
    more than 1 nF. So long as the fixed caps had a tolerance tight enough, it
    should cover (1.398 nF allows for greater than 20% tolerance on nominal 1
    nF increments). Not that any of this matters, in practise.
     
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