Capacitor Impedance Calculator

A capacitor’s impedance drops with frequency: Xc = 1/(2πfC). At DC it’s an open circuit (infinite impedance), while at high frequencies it approaches a short circuit. This frequency-dependent behavior is what makes capacitors essential for filtering, coupling, and decoupling in electronic circuits.

Real capacitors have ESR (Equivalent Series Resistance) from their leads, electrodes, and electrolyte. ESR sets a floor on impedance — no matter how high the frequency, the impedance can never drop below ESR. The total impedance is |Z| = √(Xc² + ESR²). At the frequency where Xc equals ESR, the capacitor transitions from capacitive to resistive behavior.

For bypass (decoupling) capacitors, low ESR is critical. Ceramic capacitors (MLCC) have ESR in the milliohm range, making them ideal for high-frequency decoupling near IC power pins. Aluminum electrolytics offer large capacitance but higher ESR (0.01–1Ω), so they handle low-frequency bulk filtering. A common design practice places both in parallel: a 10µF–47µF electrolytic for bulk and a 100nF ceramic close to the IC.

Key Points

• Xc = 1/(2πfC) — reactance decreases with frequency
• ESR sets the minimum impedance floor at high frequencies
• |Z| = √(Xc² + ESR²) — total impedance with ESR
• Ceramic capacitors: low ESR (mΩ), ideal for HF decoupling
• Electrolytic capacitors: high C but higher ESR, for bulk filtering
• Place bypass capacitors as close to IC power pins as possible

Applications

• Power supply decoupling and bypass filtering
• Signal coupling and DC blocking
• EMI filtering and noise suppression
• Capacitor selection for switching regulators
• Audio crossover network design
• Impedance matching in RF circuits