# RC Circuits: Calculating Voltage

I've been trying to answer the following equation but I just can't get my head around it!

'calculate the time taken 't' for the voltage over capacitor array (Vc) to reach 80% of Vo, in terms of Vo, R, C1 and C2 (caps are in parallel)'

I know I need these two equations but I just don't know what to do with them:
Vc = Vo (1 - e^[-(t/tau)])
tau = R.C

I have tried solving it but I just doesn't make sense and there isn't an RC circuit calculator that I can check my answer with :(, anyways help would be fantastic, thanks

gneill
Mentor
What is Vc/Vo when Vc reaches 80% of Vo?

Hi there, in this part of the question, the values of Vc and Vo aren't given so I'm not too sure where to go from there

gneill
Mentor
No matter what the actual value of Vc is, the ratio Vc/Vo is...?

oooops sorry, the exact question I have is:

1) Consider the circuit in fig. 1
a) Derive the expression for the time taken for the p.d., Vc, across the capacitor array to reach 80% of Vo in terms of C1, C2, R and Vo

gneill
Mentor
oooops sorry, the exact question I have is:

1) Consider the circuit in fig. 1
a) Derive the expression for the time taken for the p.d., Vc, across the capacitor array to reach 80% of Vo in terms of C1, C2, R and Vo

And again I ask you to consider what the statement "Vc across the capacitor array to reach 80% of Vo" implies for the ratio Vc/Vo.