Capacitor Ripple Voltage Calculator

Equivalent Series Resistance became a critical spec in the 1990s with the rise of high-frequency SMPS. A reservoir capacitor at 50 Hz mains is dominated by the capacitive-reactance ripple term I/(f·C); at 500 kHz switcher frequency, that term vanishes and the resistive ESR term I·ESR dominates. Suddenly the capacitor's effective impedance floor (its ESR) set the output ripple, regardless of how many more microfarads you added.

The "capacitor plague" of 1999-2007 was an industry-wide failure cascade: conventional aluminum electrolytic capacitors made with a defective electrolyte formulation bulged, leaked, and failed prematurely on motherboards, graphics cards, and consumer electronics worldwide. The root cause was a stolen-and-botched formulation from Rubycon's aqueous electrolyte. The response accelerated adoption of conductive polymer capacitors (OS-CON, Poscap) which have 1/10th the ESR and ~10× the life of conventional aluminum.

Modern SMPS designs use a hybrid approach: bulk aluminum polymer (low ESR for ripple current) paralleled with ceramic multi-layer caps (extremely low ESR at > 1 MHz) to handle transient currents from fast load steps. The X7R/X5R ceramic capacitor revolution (1990s onward) pushed per-piece capacitance from 1 µF to 47 µF in standard 1206 packages — making polymer-aluminum bulk caps optional on point-of-load converters.

Enter the ripple current, operating frequency, capacitance, and equivalent series resistance (ESR). The tool computes capacitive ripple Vc = I/(f·C), resistive ripple Ve = I·ESR, total ripple Vt (worst-case sum), and the capacitor's reactance Xc = 1/(2πfC). Typical ESR values: aluminum electrolytic 50-200 mΩ, polymer 10-30 mΩ, ceramic 1-10 mΩ.

Rule of thumb: if Ve > Vc by 3× or more, adding more capacitance won't help — you need lower ESR, i.e., polymer or parallel ceramics. For typical buck converter output, place a 47-100 µF polymer in parallel with 2-4 × 10 µF ceramics to cover both low-frequency ripple and high-frequency transients. Everything runs client-side; no values leave your browser.