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Time calculation problem

Discussion in 'Electronic Basics' started by Brian, Mar 5, 2006.

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  1. Brian

    Brian Guest

    In a pdf file from Rifa on "Electrolytic Capacitors Application Guide" it
    states the following:

    The ripple current from the main and from the load has to be known. First
    calculate the capacitor voltage charge time.

    tc = [arccos (Umin / UMax)] / [2 x 3.14 x fmain]

    Then it give an example:

    tc = [arccos (359 / 361)] / [2 x 3.14 x 50] = .000335

    I have to be missing something, because I can't get the same answer as they
    do in the example. I get tc = 0.0192, which is not right. Can someone tell
    be what I am doing wrong?


  2. Hi,

    Yes! Use radians NOT degrees.

    Cheers - Joe
  3. Brian

    Brian Guest

    Thanks for your reply Joe. Isn't [2 * 3.14 * 50] the number of radians in 1
    second at 50 Hz.?

  4. Hi again,
    Yes, but the arccos of a number gives you radians and the
    2*pi*f bit gives you radians per second. Dividing one by the
    other, the radians cancel and you're left with seconds.

    Cheers - Joe
  5. Brian

    Brian Guest

    Isn't that what I want -- charging time in seconds? That's what this formula
    is suppose to give me. Am I missing something here?

  6. Grab my equation tool at:
    and extract out the file EQ.EXE that's inside.

    At a DOS prompt, type in:
    EQ acos(359/361)/2/pi/50

    It will print out the figure you are getting from the PDF file.

    The others have pointed out that your ARCCOS() function is probably
    returning something other than radians -- in particular, degrees. For
    example, if you use my equation tool and enter this:
    EQ deg(acos(359/361))/2/pi/50

    you will then get your 0.0192 out.

    This whole thing is called "operator error." ;)

  7. Brian,

    As I said at the beginning, switch (your calculator) to radians
    and you will get the answer they do.

    ACOS (359/391) = 0.105 rads

    2*pi*50 = 314.159 rads/sec

    So Tc = 0.105/314.159 ~= 0.335 sec (allowing for rounding)

    It is simply rads over rads per second.

    Cheers - Joe

  8. Brian

    Brian Guest

    Thanks Joe, that is what I was missing (switching the calculator to
    radians). Sometimes, the simplest things, are the hardest to figure out
    where you went wrong. Thank you for your patience.

  9. Brian

    Brian Guest

    Thanks for your offer Jon. Since Joe told me what I was doing wrong, I won't
    need your offer.


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