# 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

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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.

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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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