ESR

Every capacitor, no matter how ideal it appears on the schematic, contains a small amount of series resistance. This is equivalent series resistance, or ESR: the combined resistance of the capacitor's internal connections, plate material, electrolyte, and leads. In a real component, current must flow through all these materials to reach the capacitor's charge storage mechanism, and each path introduces resistance. ESR is measured in milliohms and acts as a true series element with the capacitance, not a parallel leakage path.

ESR varies sharply by capacitor type. Ceramic capacitors typically have ESR values between 10 and 100 milliohms; film capacitors range from 50 to 500 milliohms; aluminum electrolytic capacitors run 50 to 1000 milliohms depending on age and condition; and supercapacitors can be as low as 1 to 10 milliohms. Temperature, frequency, and applied voltage all shift ESR during operation. Aging electrolytic capacitors see ESR climb as the electrolyte dries out, degrading performance long before capacitance itself drops below tolerance.

ESR matters most in high-frequency or high-current circuits. In switching power supplies, ESR determines the ripple voltage and energy dissipation in the output filter. A 1-ohm ESR capacitor with 10 amperes of ripple current dissipates 100 milliwatts and contributes 10 millivolts of noise; a 100-milliohm capacitor dissipates only 1 milliwatt but adds 1 millivolt. In audio equipment, low ESR is audible: high ESR electrolytic capacitors in signal paths introduce distortion and phase shift that competent designers avoid.

Measurement and selection

ESR is not measured with a simple ohmmeter. Specialized impedance analyzers sweep frequency from 10 hertz to 1 megahertz and plot impedance versus frequency; the minimum impedance value is the ESR. Many bench multimeters now include an ESR mode that estimates this value by injecting a known AC signal and computing the resistive component. At higher frequencies, capacitor impedance can rise again due to inductance, creating a second minimum that engineers must account for when choosing components for specific applications.

Component selection requires matching ESR to the load. Laptop chargers and server power supplies specify ESR limits explicitly because ripple rejection and thermal stability depend on it. A capacitor rated 10 microfarads at 16 volts with 500 milliohms ESR will not perform the same role as one with 50 milliohms ESR, even though both have identical nominal capacitance. Designers working with tight margins measure or test actual parts from the production lot, since manufacturer tolerances on ESR can be loose.