What does one time constant mean?

After one time constant, a charging capacitor reaches about 63.2% of its final voltage; a discharging capacitor falls to about 36.8%.

When is a capacitor practically charged?

A common engineering estimate is about five time constants, which reaches roughly 99.3% of the final value.

Can this design a real mains snubber?

No. Use it for first-order intuition only. Mains and power snubbers need safety-rated parts and qualified design.

RC Circuit Calculator

Calculate RC time constant, capacitor charge/discharge voltage, cutoff frequency, impedance and simple low-pass or high-pass filter behavior.

RC circuit parameters

Resistance R

Unit

Capacitance C

Unit

Common uses:

Time constant

10 ms

Scale for circuit charge and discharge.

Filter cutoff frequency

15.9155 Hz

Boundary of the LPF or HPF passband.

Source voltage V0 (V)

63% (1tau)

10 ms

~ 3.161 V

95% (3tau)

30 ms

~ 4.751 V

99.3% (5 tau) - effectively settled

50 ms

~ 4.967 V

The chart shows an ideal exponential charge and discharge model.

For real circuits, check tolerances, leakage, ESR, source impedance, and load.

RC time constant and capacitor voltage

RC circuit search results expect the first-order resistor-capacitor timing model: time constant, charge/discharge voltage and practical 3 tau or 5 tau settling estimates.

τ = R C

tau is the time constant, R resistance and C capacitance.

V_{C} (t) = V_{0} (1 - e^{- t / τ})

VC(t) is capacitor voltage during charging.

V_{C} (t) = V_{0} e^{- t / τ}

Discharge voltage decays exponentially.

f_{c} = \frac{1}{2 π R C}

fc is the first-order filter cutoff frequency.

Design limits

Ideal first-order model

Real circuits can include source resistance, load impedance, capacitor ESR/leakage, tolerance, dielectric effects and safety constraints in mains or power circuits.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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