Capacitor Reactance & RC Circuit Calculator
Enter capacitance, frequency, and resistance to get capacitive reactance (Xc), RC time constant, and -3dB cutoff frequency. pF through mF units. Nothing uploaded.
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Learn more: capacitive reactance and RC circuits
What is capacitive reactance (Xc)
Capacitive reactance measures how much a capacitor opposes alternating current. Xc = 1/(2πfC) where f is frequency and C is capacitance. Unlike resistance, reactance decreases as frequency increases - at high frequencies, a capacitor passes current easily; at low frequencies it blocks it. This is why capacitors are used for AC coupling, high-pass filtering, and frequency-dependent signal conditioning.
The RC time constant and charging curves
The RC time constant (τ = RC) is the time for a capacitor to charge or discharge to approximately 63.2% of its final voltage. After 5τ, the capacitor is considered fully charged or discharged (99.3%). The time constant is used in timing circuits, filters, signal conditioning, and debouncing switches. A longer time constant provides slower rise/fall times; a shorter constant provides faster response.
Cutoff frequency and filter design
The cutoff frequency fc = 1/(2πRC) is the frequency at which the output power of an RC filter is reduced to half (-3dB) of its passband value. For a low-pass RC filter, signals below fc pass through; signals above fc are attenuated. It is the most fundamental parameter in filter design and roll-off calculations. Understanding your cutoff frequency is essential for audio filters, power supply ripple reduction, and sensor signal conditioning.
Common RC circuit applications
RC circuits are everywhere in electronics. High-pass RC filters remove DC bias and low-frequency noise from audio signals. Low-pass RC filters suppress high-frequency switching noise. RC delay circuits create soft power-on sequencing. Differentiator circuits use short time constants to detect edges. Integrator circuits use long time constants to smooth noise. The calculator handles all these calculations automatically.
FAQ
What is capacitive reactance?
Xc = 1/(2πfC). How much a capacitor opposes AC current. Decreases with frequency - capacitors pass high frequencies and block low frequencies.
What is the RC time constant?
τ = RC. Time for a capacitor to charge to 63.2% of supply voltage. After 5τ, fully charged. Used in timing circuits, debouncing, and analog signal processing.
What is the -3dB cutoff frequency?
fc = 1/(2πRC). The frequency where filter output power drops to half. Below fc for a low-pass filter - signals pass. Above fc - attenuated. The fundamental filter design parameter.