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Capacitor Problem — Parallax Forums

Capacitor Problem

bennettdanbennettdan Posts: 614
edited 2006-05-08 18:49 in General Discussion
· If I combine a .022uf cap and a .o1uf cap in series would I get a .0145uf cap reading? I think I am doing the math right.

Comments

  • BullwinkleBullwinkle Posts: 101
    edited 2006-05-06 07:05
    Nope.

    You should get 0.006875 uF

    The formula is Ct = (C1 x C2)/(C1 + C2). Identical to the formula for resistors in parallel.
  • BullwinkleBullwinkle Posts: 101
    edited 2006-05-06 07:09
    Or more correctly 6.875 nF
  • bennettdanbennettdan Posts: 614
    edited 2006-05-06 16:24
    I thought the formula for series Caps is 1/ct = 1/.022 + 1/.01 ?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-06 16:26
    It is 1 / [noparse][[/noparse] (1/C1) + (1/C2)].· It is the reciprocal of the sum of the reciprocals.
  • BullwinkleBullwinkle Posts: 101
    edited 2006-05-07 01:48
    1/Ct = 1/C1 + 1/C2

    equals

    Ct = 1/(1/C1 + 1/C2)

    which works out to

    Ct = (C1 x C2)/(C1 + C2)

    Either way the answer is 6.875 nF
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-07 02:00
    You should solve·for CT -- not the reciprocal of it (1 / CT).· Nobody does nanofarads; capacitance is expressed as millifarads [noparse][[/noparse]mF], microfarads [noparse][[/noparse]uF], or picofarads [noparse][[/noparse]pF].
  • bennettdanbennettdan Posts: 614
    edited 2006-05-07 03:05
    I am looking for a way to make a .016 uf cap for a RC timer curcuit
  • bennettdanbennettdan Posts: 614
    edited 2006-05-07 03:22
    Hey bullwinkle I played with the formula and I understand how to plug the values in now thanks for your help. one more question for future reference what is the parallel formula for caps?
  • Mike GreenMike Green Posts: 23,101
    edited 2006-05-07 03:48
    Capacitors in parallel are additive. In other words, Ctotal = C1 + C2 + ... + Cn
  • BullwinkleBullwinkle Posts: 101
    edited 2006-05-08 03:17
    What he said... smilewinkgrin.gif
  • bennettdanbennettdan Posts: 614
    edited 2006-05-08 17:07
    Thanks guys the reason I am asking I have chip that is controlled by my stamp that needs a RC frequency of around 15khz with a 10k pot so I need about a .06uf cap by my figures, but does caps in series cause problems with the RC curcuit or does it really matter to have more than one cap to make the curcuit you need?
  • Mike GreenMike Green Posts: 23,101
    edited 2006-05-08 17:20
    I would try to select the nearest standard value capacitor that gets you close to the frequency you want with a standard value resistor, pick the next lowest standard value resistor and add a series pot to adjust the frequency to what you want. You can always measure the actual pot value and substitute a second fixed resistor once you've got the frequency where you want it.
  • bennettdanbennettdan Posts: 614
    edited 2006-05-08 17:53
    So muti caps are not used in a RC curcuit ?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-08 18:49
    Typically, in an RC-circuit, one uses a potentiometer instead of searching for the perfect capacitance.· It's OK to parallel for value (or series, for that matter), if your "junk-box" isn't brimming with goodies.· Are lives at stake?· Just run with it, man.
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