RC Charge Curve Equation

att: Roff

A few years ago Ron H. emailed an RC charge time equation back when Poptronics was still operating. Does anyone have this? It comes in handy when calculating relaxation oscillator frequencies or period. It should be handy for all of us to have.

I lost the info in a computer failure. Roff (ak RonH), could you please post it?

Bob

Google Images has lots of graphs of the voltage across a charging and discharging capacitor. But most don't have grid lines.

I found Ron's old message from 2001!

For a low pass type RC circuit:

V = Vf+(Vi-Vf)e^(-t/RC)

or

t = -RC*ln[(V-Vf)/Vi-Vf)]

where

V = instantaneous voltage
Vi = initial voltage
Vf = final voltage (the voltage that your cap would approach after an infinite amount of time has passed.)
e = 2.71828
t = time in seconds
R = resistance
C = capacitance

Thanks again Ron / Roff.

Bob