# 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
9,030
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You did not mention the value of the input voltage?

hgmjr

3. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
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Input voltage is 300V.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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784
The book is correct - you are using the wrong formula.

5. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
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What formula am I supposed to use?

Jun 19, 2009
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7. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
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Could it be (Vf-Vi)(1-e^-t/RC)+Vi?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
No - look at TO2's post.

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

Feb 17, 2009
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10. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
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Got it. Figured it out...Thanks