Tolerance Calculation for Timer 555 (help-plz)?

Discussion in 'General Electronics Chat' started by Shayan22h, Dec 22, 2012.

  1. Shayan22h

    Thread Starter Member

    Oct 24, 2012
    30
    3
    hi guysi i have a timer 555 circuit and I would like to calculate the tolerance, using differentiation method, my supervisor told me based on the formula for the output frequency of ur circuit
    f = (1.44/((Ra+2Rb)C))
    u can calculate the tolerance of ur circuit using
    √dRa^2 +dRb^2+dC^2 = b
    i can calculate df/dRa or df/dC but my problem is when i want to replace dRa or dC with numbers i still end up having df, i have one equation one unknown.
    i have attached my circuit and my calculation paper, any help would be much appreciated and i know there are some other ways of calculating tolerances but i kinda have to stick to this one .
    Best Regards,
    shayan
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    The components of a derivative like df/dR are generally not treated like ordinary algebraic quantities for the purpose of doing calculations. In cases like this, a derivative such as df/dC is generally not a constant.

    If you know the functional form of a derivative you can answer a question like, "How much of a change in capacitance will produce a 1% change in frequency.
     
    Shayan22h likes this.
  3. Shayan22h

    Thread Starter Member

    Oct 24, 2012
    30
    3
    Thank you so much for ur answer,
    the thing is i dont treat the component that is differentiated constant, i treat other component as the constant for example in df/dc i treat other 2 resistors as the constants.
    so does this question of "How much of a change in capacitance will produce a 1% change in frequency." help me to find out what the tolerance of the component is in the system ?
    Thanks for ur time,
    Regards,
    Shayan
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    From the expressions in your post it sure looks like you are trying to do calculations with infinitesimal differentials.

    I can't help you with you question. The tolerance of a component is specified by the manufacturer, not by any experiment you might perform. What you are interested in is how the tolerance on a component will affect the output frequency. Since you have three components you are in partial derivative land.
     
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