RC Time Constant Calculator
Enter any two of resistance, capacitance and time constant, and this calculator finds the third along with the charge times and cutoff frequency. The time constant tau equals R times C and sets how fast the capacitor charges.
How to use the RC time constant calculator
- Choose which value you want to solve for.
- Enter the other two, for example resistance in ohms and capacitance in farads.
- Read the time constant, charge times and cutoff frequency.
Examples
1k ohm and 1 microfarad
R = 1000 ohm, C = 1e-6 F
tau = 0.001 s, cutoff = 159.15 Hz
Solve resistance
tau = 0.001 s, C = 1e-6 F
R = 1000 ohm
Frequently asked questions
What is the RC time constant?
The time constant tau is resistance times capacitance (tau = R x C), measured in seconds. It is the time for the capacitor to charge to about 63% of the supply voltage.
How long does an RC circuit take to fully charge?
Roughly five time constants. After 3 tau a capacitor reaches about 95% of the supply, and after 5 tau about 99%, which is usually treated as fully charged.
What is the cutoff frequency of an RC filter?
The corner frequency is f = 1 / (2 pi R C). At this frequency a first-order RC filter attenuates the signal by 3 dB. For 1k ohm and 1 microfarad that is 159.15 Hz.
Can I solve for resistance or capacitance?
Yes. Enter the time constant plus either resistance or capacitance and the calculator rearranges tau = R x C to find the missing component.
Does the time constant depend on voltage?
No. Tau depends only on R and C, not on the supply voltage. A higher voltage charges to a higher level but follows the same timing curve.
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