Need help

Discussion in 'Homework Help' started by ///m3dave, Feb 25, 2010.

  1. ///m3dave

    Thread Starter New Member

    Dec 8, 2009
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    I have a RC circuit that has R=1MΩ and C=1μF. I need to figure out how long will it take for Vc to reach 90V. I'm using the formula t=2.3*RC*log V/v. The answer I get is 1.2s but my book says 356.7ms. What am I doing wrong?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    You did not mention the value of the input voltage?

    hgmjr
     
  3. ///m3dave

    Thread Starter New Member

    Dec 8, 2009
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    Input voltage is 300V.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The book is correct - you are using the wrong formula.
     
  5. ///m3dave

    Thread Starter New Member

    Dec 8, 2009
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    What formula am I supposed to use?
     
  6. TO2

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    Jun 19, 2009
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  7. ///m3dave

    Thread Starter New Member

    Dec 8, 2009
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    Could it be (Vf-Vi)(1-e^-t/RC)+Vi?
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    No - look at TO2's post.

    Rearrange the equation such that the time t is the subject.
     
  9. Jony130

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    Feb 17, 2009
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  10. ///m3dave

    Thread Starter New Member

    Dec 8, 2009
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    Got it. Figured it out...Thanks
     
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