RC Circuits: Calculating Voltage

[rickstar008]

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]

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

 

[rickstar008]

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]

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

 

[rickstar008]

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]

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.