Use this RC calculator to find the time constant of a resistor and a capacitor, how long the capacitor takes to charge, and the cutoff frequency of an RC filter.
The time constant, τ (tau), is the resistance times the capacitance. After one time constant, the capacitor has charged to 63% of the voltage. After five, it’s practically full:
- Time constant (τ = RC)
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- Time to charge to 63%
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- Time to charge to 99% (5τ)
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- Cutoff frequency (fC)
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Where You Use It
The time constant is useful for delays and timing, like a delay circuit that turns something on a moment after power-up.
The same resistor and capacitor also make a low-pass filter. Signals below the cutoff frequency pass through, and higher frequencies get weaker. Swap the resistor and the capacitor, and you get a high-pass filter with the same cutoff frequency.
Want to learn more? Check out how a capacitor works.