I needed to calculate the voltage on a capacitor after a given time, and also needed to calculate the time required to charge a capacitor to a given voltage.. maths being rusty (and losing my SC which had all the formulas programmed in ) I resorted to Excel.

The attached .xls (in .zip) may be of use to others.. it contains 2 simple routines..

1.) Given applied voltage, R, C, and time (t), it outputs the instantaneous voltage across the capacitor at t.

2.) Given applied voltage, R, C, and the voltage required across the capacitor, it outputs the time at which this voltage appears across the capacitor.

You will need the LN() function installed in Excel or (other spreadsheet) If you have an RC circuit, you can solve the following differential equation:

EMF = Vr + Vc (Kirchoff's Voltage Rule)
Vr = IR = (dq/dt)*R
Vc = Q/C
EMF = (dq/dt)*R + (Q/C)

After solving the 1st order differential equation, you get
q = C(EMF)(1-e^(-t/(RC)))
i = dq/dt = (EMF/R)e^(-t/(RC))
This is for a DC circuit where the capacitor and resistor are in series.
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