# Couple of questions

Discussion in 'Homework Help' started by fishguts, Sep 30, 2007.

1. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
I have two question that I am having problems with. My first question is how do I find RMS voltage? In our notes in class our teacher did not go over that. MY second question is how do I find the time constant if I am given something like at t=37.5ms the resistor voltage is 1.115v. If it help I attached the assignment. I would appriciate any tips to get me started in solving these problems. Thanks

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2. ### BrianH Active Member

Mar 21, 2007
43
0
For your first question, you can find the approximate R.M.S. value by multiplying peak amplitude by 0.707. (or divide by 1.414).
For your second question, you need to use the capacitor charge formula;

Vc = Vs . 1- e^-t/rc

t is 37.5ms, Vs is 5V, Vc is (5 - 1.115)

Make rc the subject of the equation, and solve for your answer.

Hope this helps,

Brian

3. ### BrianH Active Member

Mar 21, 2007
43
0
hint: you'll need to take natural logs of both sides in order to get rid of e.

Brian

4. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
Alright thanks for the help!

5. ### BrianH Active Member

Mar 21, 2007
43
0
Did you manage to come up with an answer?

Brian

6. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
Yeah I came up with an answer. Do you know any good websites where I can learn more about the sin waves and such? I had some trouble figuring out some of the last problems.