RC time constant and filter equations
An ideal first-order RC network links timing and cutoff frequency through the same product RC.
τ = R × Cfc = 1/(2πRC)Vcharge = Vs(1 − e^(−t/τ))Vdischarge = V0e^(−t/τ)XC = 1/(2πfC)where:
- τ
- Time constant[s]
- fc
- First-order cutoff frequency[Hz]
- XC
- Capacitive reactance[Ω]
Real capacitors add tolerance, leakage, ESR and parasitic inductance.
How to use the RC Circuit and Time Constant Calculator
Calculate RC time constant, cutoff frequency, charging and discharging voltage, capacitive reactance, impedance, gain, and phase at one frequency. Start by choosing the operating mode, then enter values from nameplates, measurements, or project documents in the units shown. The result updates immediately and keeps intermediate quantities visible for review.
- Confirm the system and calculation mode.
- Enter measured or manufacturer-rated values.
- Review the primary result and intermediate metrics.
- Continue with the related protection or equipment-selection checks.
Calculation method and validation
The equations and symbol definitions above show the complete calculation basis. Inputs are checked before calculation so zero, negative, incompatible, or physically impossible combinations are flagged instead of silently producing a misleading result.
How to interpret the result
Use the main result as a starting value and read every supporting metric. Ideal linear first-order RC network Capacitance entered in microfarads and resistance in kilohms No ESR, ESL, leakage or dielectric absorption A passing or recommended result means the entered mathematical conditions are satisfied; it does not certify the complete installation or a particular product combination.
- Ideal linear first-order RC network
- Capacitance entered in microfarads and resistance in kilohms
- No ESR, ESL, leakage or dielectric absorption
Selection limits and next checks
Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions. Power snubbers and mains timing circuits require component pulse, voltage, insulation, creepage and manufacturer checks. Compare the output with the current applicable standard, local rules, exact VIOX product datasheet, environmental conditions, protection coordination, and the engineer's project requirements before procurement or installation.
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Assumptions
- Ideal linear first-order RC network
- Capacitance entered in microfarads and resistance in kilohms
- No ESR, ESL, leakage or dielectric absorption
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Power snubbers and mains timing circuits require component pulse, voltage, insulation, creepage and manufacturer checks.