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curve fitting a charging capacitor type curve

  1. #1
    Registered User
    Join Date
    11-14-2005
    Posts
    2

    curve fitting a charging capacitor type curve

    Hi all,

    Newbie to this forum, but need some excel help. I have collected a bunch of data from an object as it heats up. The curve is very similar to a capacitor charging current curve. In my case, manually trying to approximate this curve, I come up with a formula of y= (rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve like this or do I need an aftermarket solution? I need something that will use the data points and perform calculations to determine the curve.

    Thanks,

    Sean

  2. #2
    Bill Martin
    Guest

    Re: curve fitting a charging capacitor type curve

    mcgradys wrote:
    > Hi all,
    >
    > Newbie to this forum, but need some excel help. I have collected a
    > bunch of data from an object as it heats up. The curve is very similar
    > to a capacitor charging current curve. In my case, manually trying to
    > approximate this curve, I come up with a formula of y=
    > (rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve
    > like this or do I need an aftermarket solution? I need something that
    > will use the data points and perform calculations to determine the
    > curve.
    >
    > Thanks,
    >
    > Sean
    >
    >



    Have you tried to use the Solver? Set up a table with a column of empirical
    data, a column of calculated data using your equation, and a column with the
    square of the error between the two with a sum at the bottom. The calculated
    data needs to be calculated using spreadsheet cells to store each of the
    parameters you're trying to fit.

    Then ask Tools/Solver to minimize the sum of the column of squared errors by
    varying the paramater value cells.

    I suspect Solver will work pretty well with this kind of problem.

    Good luck...

    Bill

  3. #3
    tmharvey
    Guest

    RE: curve fitting a charging capacitor type curve

    Sean, check out Chartool & Smooth_Curve_Bezier the smart guys at XLRotor have
    developed, it leverages Excel's native chart smoothing capability to fit the
    curve.
    It's free too.
    http://xlrotor.com/resources/files.shtml
    Cheers...Terry

    "mcgradys" wrote:

    >
    > Hi all,
    >
    > Newbie to this forum, but need some excel help. I have collected a
    > bunch of data from an object as it heats up. The curve is very similar
    > to a capacitor charging current curve. In my case, manually trying to
    > approximate this curve, I come up with a formula of y=
    > (rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve
    > like this or do I need an aftermarket solution? I need something that
    > will use the data points and perform calculations to determine the
    > curve.
    >
    > Thanks,
    >
    > Sean
    >
    >
    > --
    > mcgradys
    > ------------------------------------------------------------------------
    > mcgradys's Profile: http://www.excelforum.com/member.php...o&userid=28806
    > View this thread: http://www.excelforum.com/showthread...hreadid=484942
    >
    >


  4. #4
    Herbert Seidenberg
    Guest

    Re: curve fitting a charging capacitor type curve

    Assuming your data is in xvalu and yvalu
    xvalu yvalu yvalu2 yvalu3 k_ast k_diff k_rc k_amp
    0 0.0104 2.5896 0.0000 2.6000 -0.0273 -0.2057 2.6273
    1 0.4459 2.1541 0.4885
    2 0.8011 1.7989 0.8862
    3 1.1477 1.4523 1.2100
    4 1.3927 1.2073 1.4735
    5 1.6954 0.9046 1.6881
    6 1.9086 0.6914 1.8627
    7 1.9831 0.6169 2.0049
    8 2.0567 0.5433 2.1206
    9 2.2272 0.3728 2.2148
    10 2.2512 0.3488 2.2915
    11 2.3140 0.2860 2.3540

    Name all columns.
    Guess what your asymptote is and enter into k_ast.
    Enter into the yvalu2 column
    =k_ast-yvalu
    Enter into k_rc, k_amp, k_diff respectively
    =LN(INDEX(LOGEST(yvalu2,xvalu),1))
    =INDEX(LOGEST(yvalu2,xvalu),2)
    =k_ast-k_amp
    Do a Goal Seek to set k_diff to zero by changing k_ast
    Your best fit curve is in yvalu3
    =k_amp*(1-EXP(k_rc*xvalu))


  5. #5
    Jerry W. Lewis
    Guest

    Re: curve fitting a charging capacitor type curve

    if you know time and are trying to fit rise and ambient, then the
    function is linear in the unknowns and you can simply use SLOPE and
    INTERCEPT (or LINEST, if you prefer). In that case, your X column for
    fitting would be the calculated value =1-exp(-time)

    Jerry

    mcgradys wrote:

    > Hi all,
    >
    > Newbie to this forum, but need some excel help. I have collected a
    > bunch of data from an object as it heats up. The curve is very similar
    > to a capacitor charging current curve. In my case, manually trying to
    > approximate this curve, I come up with a formula of y=
    > (rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve
    > like this or do I need an aftermarket solution? I need something that
    > will use the data points and perform calculations to determine the
    > curve.
    >
    > Thanks,
    >
    > Sean



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