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Variable capacitor

Discussion in 'Electronic Design' started by Ignoramus21085, Oct 25, 2005.

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  1. I need to make a variable capacitor from 1000 pF to 19,000 pF
    (roughly). Using something like a 11 position rotary switch is fine. I
    can build something homegrown, but would like to know if there is
    something already available, so that I am not missing anything
    obvious. If I should build it, I will appreciate some design
    tips so that I do not miss anything obvious. Thanks.

  2. search for capacitor decade box, they are quite expensive, so a couple
    of rotary switches will do what you want.

  3. Thanks. I need an assortment of caps, from 1000 pF to 10,000 pF. It
    would be best if they were of axial type, makes them easy to parallel
    and solder together. What kinds of caps would be best? Polyethylene
    film or ceramic or what? I do not need much from them in terms of
    accuracy or voltage.

  4. Tam/WB2TT

    Tam/WB2TT Guest

    The best I have seen for a variable capacitor, and these would probably be
    iompossible to find new, is a 3 x 365 PF. With 11 steps, your rotary switch
    would have a resolution of (19000 - 1000)/11 = 1636 PF. If you only change
    the setting infrequently, you could use 8 SPST switches, BCD coded, and you
    would have a resolution of 19000/256 = 74 PF. You would need capacitors of
    74PF, 148PF, 296PF, 592PF, 1184PF, 2368PF, 4736PF, and 9472PF. Since you
    never need less than 1000PF, you could actually do a little better than this
    by adding a non switched capacitor.

  5. Yes. There will be another, parallel non-switched 1,000 pF cap. This
    is for frequency regulation. With 1 megohm pot for duty cycle
    adjustment, and permanent 1,000 pF cap, I would have the following 11
    values for frequency:

    pF Hz
    1000 500
    2000 333
    3000 250
    4000 200
    5000 166
    6000 142
    9000 100
    10000 90
    12000 76
    13000 71
    16000 58
    19000 50

    (that's for a low frequency switching circuit, hence very low frequency).

    The exact values do not matter terribly too much. What I want is to
    find a cheap assortment of radial caps that would let me implement
    these capacitances easily.

  6. Think "1-2-4-8-etc, binary" and a small array of
    miniature toggle switches.... BTDT, very useful.
  7. Use a BCD or binary coded switch ( or make one from multiple poles). Thay way you can get 16 values
    from 4 capacitor values - see also :
  8. That's interesting, but in my case, I want to select frequency with a
    rotary switch, not with binary bit toggle switches. Maybe one day I
    will sell this welding machine, and no one will understand binary bit
    logic except for those literate in copmputer logic and such, so no one
    will want to buy this welder.

    Regarding selling it, it is not at all my current purpose. It would be
    uneconomical to spend so much time to make a few hundred bucks. I want
    to have a super nice modern welder for myself, made out of this cyber
    tig, hence my current project.

    That's my plan. Since I do not know what the future will hold, I have
    to make sure that I end up with something sellable.

  9. No problem, you can get rotary switches with a binary output. Google
    "rotary hex switch".
  10. Don Foreman

    Don Foreman Guest

    Given this low frequency, you could make a synthetic variable
    capacitor using 1 cap, a few resistors, a pot and a dual opamp.
    You'd need both + and - bias supplies for the opamp. Varying the
    pot varies the effective capacitance to ground.
  11. Chris Jones

    Chris Jones Guest

    If this is for varying the frequency of your weld pulse thing, then I would
    suggest going for caps in the ratio 1,2,5,10,20,50,100,200,500 etc. or even
    just 1,3,10,30,100,300 etc. since that way you cover a large range in few
    steps. I doubt you'll see much difference between 180Hz and 200Hz or
    whatever it is in your welder, but it might be more interesting to explore
    a wider range.

    Also forget about selling it, I reckon. If someone buys it then it will be
    more pain than it's worth, they'll keep calling you to fix it or they'll
    break it and accuse you of causing everything that ever went wrong in their
    life. Just build it to work the way you want, maybe you'll even feel like
    keeping it.

    The only concession I would make with regard to selling it, is to make the
    modifications to the welder itself basically reversible so you can get back
    to what you started with.

  12. Yes, that's my plan indeed, it will be approximately exponential
    scale, from 50 to 500 Hz.
    I do intend to keep it. I would not undertake this project to make a
    few hundred bucks. In the time that I spend on this, I could have made
    thousands of $$s by computer programming.

    I agree. There will be some drilled holes in the front panel, but not
    many. I could cover them with a picture of a naked woman or some such.

  13. that's interesting, can you elaborate?

  14. What he said, but look also for thumbwheel switches with BCD output.
  15. Tam/WB2TT

    Tam/WB2TT Guest

    Since this is not a sine wave oscillator, why not tune it with a variable
    resistor; like a nice 5 or 10 turn pot? You would only need 2 or 3 capacitor
    values to cover the range. A 74HC4046 should work well.

    Another approach that I would look at is to cover the top octave, and use
    some binary counters to get the lower frequencies. Use the rotary switch to
    pick the octave.

    There are also a number of ways that you could do it with a single tuning
    knob, and no band switch.


  16. Check out datasheet for XR2206:

    the pot is used for duty cycle adjustment
    I am not quite up to date on all the terms that you use, sorry. I am
    new to electronics.

  17. Ken Smith

    Ken Smith Guest

    If you can't get a binary encoded switch, you can use one with multiple
    decks to do the same job. Fir that matter, you can get the same range of
    values with less capacitors if you want.
  18. Mike Young

    Mike Young Guest

    I don't suppose I could interest you in a single chip solution, no weird
    stuff to glue together into a messy bundle? 14 pins, only Vcc, gnd, and all
    12 tick-tocks running concurrently on their own pins. (Trade ya one or a
    dozen for that messy 'zuki motor in your garage.)
  19. Mike Young

    Mike Young Guest

    Better still: single chip, 8 pins; 1024 frequency steps between 50 and 500
    Hz; duty cycle variable in 0.10% steps.
  20. Yea, I am interested... Let me know what the solution is. Would it be
    controlled by a pot? I feel that I am indeed missing something clever.

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