# Couple of questions

#### fishguts

Joined Sep 3, 2007
22
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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#### BrianH

Joined Mar 21, 2007
43
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

#### BrianH

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

Brian

#### fishguts

Joined Sep 3, 2007
22
hint: you'll need to take natural logs of both sides in order to get rid of e.

Brian
Alright thanks for the help!

#### BrianH

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

Brian