# Pulse response of rc circuits

Discussion in 'Homework Help' started by newtoelectronics, Jan 11, 2009.

1. ### newtoelectronics Thread Starter New Member

Jan 11, 2009
2
0
Hello,

I am new to electronics and am trying to learn on my own from a textbook. A lot of times I get stuck and have no one to ask. Hopefully the online community can help me when needed. I appreciate all of your help in advance!

Here are the questions:

1. Derive a formula for finding time at any point on an increasing exponential voltage curve. Use this formula to find the time at which the voltage is at 6 V after switch is close.

The applied voltage is 12 and the time constant is .02 ms.

I know the formula for an increasing exponential curve is Vf +(Vi-Vf e^-t/rc), but am unsure what to do here.

2. For a circuit with 15V applied and time constant = .01 ms, how long does it take for the cap to charge to 8V.

I know the formula for finding the time it takes for the cap to discharge is -rc*in(v/vi), but am unsure how to solve for the time it takes to charge.

Thanks again!

2. ### vvkannan Active Member

Aug 9, 2008
138
11
hello newtoelectronics,
The formula for the voltage across the capacitor is Vc = V(1-e^(-t/rc)) as there is no initial voltage given here.
rearranging this equation Vc /V = 1 - e^(-t/rc).
e^(-t/rc) = 1 -(Vc/V)
-t/rc = ln (1- (Vc/V))

t = -rc x ln (1 - ( Vc/V))
substituting the supply voltage for V and capacitor voltage at any point for Vc and the time constant we can get the time.
hope this helps