I have a RC circuit that has R=1MΩ and C=1μF. I need to figure out how long will it take for Vc to reach 90V. I'm using the formula t=2.3*RC*log V/v. The answer I get is 1.2s but my book says 356.7ms. What am I doing wrong?

 

You did not mention the value of the input voltage?

hgmjr

 

Input voltage is 300V.

 

The book is correct - you are using the wrong formula.

 

What formula am I supposed to use?

 

Vc -> Condensator voltage
V -> Input voltage
t -> time

source: http://en.wikipedia.org/wiki/RC_circuit
hope that help

 

Could it be (Vf-Vi)(1-e^-t/RC)+Vi?

 

Could it be (Vf-Vi)(1-e^-t/RC)+Vi?

No - look at TO2's post.

Rearrange the equation such that the time t is the subject.

 

Hmm
T=-RC*In(Vin-Vc)/Vin=1s*In(210/300)=-1s*In0.7=-1s*-0.356.67m=356.67mS
http://www.allaboutcircuits.com/vol_1/chpt_16/8.html

 

No - look at TO2's post.

Rearrange the equation such that the time t is the subject.

Got it. Figured it out...Thanks