Math help please...

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  1. - 20th June 2008, 09:09 #1
    Ioannis's Avatar
    Ioannis
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    Default Math help please...

    I am designing a project based on nano-Watt PIC like the 12F683. It is going to be powered by a supercap of 1Farad and I want to calculate the time it will work on the cap. The data are:

    Working voltage: 4.2 to 2.5 Volts
    Capacitor: 1 Farad
    Current: 208 micro amps

    As this needs Integration to calculate the time the cap lasts, I am little confused. Can anyone help me with this?

    I came up with the equation in the attached file. Is it correct?

    Thanks,
    Ioannis
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    Last edited by Ioannis; - 20th June 2008 at 09:18.
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  2. - 20th June 2008, 09:23 #2
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    Hi, Ioannis

    Q ( Coulombs ) = I (A ) * t ( s )= C ( F ) * U ( V )

    ...

    ( 4.2 - 2.5 ) * 1 = 208 E-6 x t

    >> t = 8173 s ...

    That's all !

    Nice general equation, indeed ... may I laugh a little ???

    If your current draw was constant ... but just have a look to the curves at the end of the datasheet ...

    You'll have some extra time !!!

    BUT you must take into account the Supercap self discharge current ....

    A look at the datasheet ... shows .... 315 µA ( Horror !!! )

    http://www1.produktinfo.conrad.com/d...n-Gold-Cap.pdf

    So, You'll have some minus time ...


    1.7 / ( 208 + 315 ) E-6 = 3250 Sec ...

    Just hope an hour !!!

    Alain
    Last edited by Acetronics2; - 20th June 2008 at 09:44.
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  3. - 20th June 2008, 10:18 #3
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    Hi Alain.

    Thanks for the reply. I was not too rusty after all!

    Be well my friend and thanks again.
    Ioannis
    www.signalcom.gr
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