Need Formula Concerning RC Timing

Discussion in 'Math' started by Mathew Lewis McGill, Nov 5, 2015.

  1. Mathew Lewis McGill

    Thread Starter New Member

    Nov 5, 2015
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    I have a circuit that generates a pulse width. What I need is a formula to determine the time it will take for the output to switch given a voltage at the noninverting input to the comparator.

    upload_2015-11-5_16-16-12.png
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,147
    1,791
    Let's see a +5V source
    R=1.62 MΩ
    C=10nF

    The response of an RC circuit is

    v=5(1-e^{\frac{-t}{RC}})<br />
    You should be able to answer your own question with a bit of algebra
     
  3. Mathew Lewis McGill

    Thread Starter New Member

    Nov 5, 2015
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    Yippy, a quiz.
    OK, was you hoping that I'd show you? Cause I found that equation; It's all over the place.
    I was kind'a hoping to get an equation to solve "t"
     
  4. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Not even that "bit of algebra" available Mathew?
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    \frac{v}{5} = 1-e^{\frac{-t}{RC}}<br />
\frac{v}{5}-1=-e^{\frac{-t}{RC}}<br />
1-\frac{v}{5}=e^{\frac{-t}{RC}}<br />
    Take the natural logarithm of both sides
    ln(1-\frac{v}{5})=ln(e^{\frac{-t}{RC}})<br />
    Can you take it from there?
     
  6. Mathew Lewis McGill

    Thread Starter New Member

    Nov 5, 2015
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    for a voltage of 125mv, the delay is 410.2ms

    You're good with algebra, but seriously struggle with providing a simple straight answer.

    By the way, I plugged the original equation in Excel and massaged the numbers.

    If it's not too painful, I'd appreciate that equation now.

    Thanks
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    Guess not.
    ln(1-\frac{v}{5})=\frac{-t}{RC}<br />
-RCln(1-\frac{v}{5})=t<br />
    Are you really sure you could not have done that?
    Remember ln is the natural logarithm, or log to the base e (≈2.718281828...), and log is logarithm to the base 10. Seriously struggle with providing a straight answer? If I show you, but you don't understand how it is done, you'll just keep having to ask other people to solve your problems for you. Struggle a bit on you own and you'll remember how to do it next time it comes up. We're here to help not spoon feed answers to you.

    Also -- try not to bite the hand that feeds you next time. Petulance is unbecoming.
     
    Last edited: Nov 5, 2015
  8. Mathew Lewis McGill

    Thread Starter New Member

    Nov 5, 2015
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    Was just looking for the equation and didn't want to have to spell out my limitations. Sorry for coming off badly.

    Thank you.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    Congratulations on tricking someone into doing your homework for you. Doesn't happen too often around here, but it does happen from time to time.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    Doesn't matter. He only has enough fish for a day -- not enough to feed himself long enough to get a degree.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    Too true, my friend, all too true.
     
    Papabravo likes this.
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