How this calculator works
Resistance sets how quickly charge can move into or out of a capacitor. Their product is the exponential time scale. The same product sets the cutoff of an ideal first-order RC filter. The transient mode assumes a step from zero for charging, or discharge from an initial voltage with no source. Fractions can be multiplied by the relevant voltage or charge.
Formula
RC = tau; fc = 1/(2 pi tau); charging fraction = 1-exp(-t/tau); discharge fraction = exp(-t/tau).
Worked example
At 1000 ohms and 0.001 F, tau = 1 s and fc = 0.15915494 Hz. A 2 s time constant with 0.001 F needs 2000 ohms.
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Frequently asked questions
Is time constant the same as charging time?
No. It is the characteristic exponential time scale. Choose a final charge fraction to obtain a particular charging time.
Which resistance should I use?
Use the Thevenin resistance seen by the capacitor with independent sources suppressed, for a circuit that reduces to one time constant.
Does every RC network have one time constant?
No. Multiple independent capacitors can produce several time constants. This tool models a single ideal first-order network.