Capacitor ESR & Ripple Current Calculator

ESR — Equivalent Series Resistance — lumps every loss mechanism inside a capacitor into one number: lead resistance, plate resistance, dielectric loss and skin effect. Every rippling current through ESR dissipates power as heat (P = I² × ESR), which raises capacitor temperature, dries electrolytic electrolyte, further raises ESR, and accelerates death. A 2 A RMS current through 100 mΩ ESR dissipates 400 mW — enough to warm an aluminum electrolytic 8–10 °C above ambient. Switching supply engineers budget ESR before picking parts.

Most datasheets publish Dissipation Factor (DF) or tanδ at 1 kHz. Convert it: ESR = tanδ / (2πfC). For a 100 µF electrolytic at 100 kHz with tanδ = 0.08, ESR ≈ 0.08 / (2π × 100 000 × 100×10⁻⁶) ≈ 1.27 Ω — which is what the calculator above outputs. Some datasheets now list ESR directly in mΩ at the rated switching frequency; prefer that over the tanδ-derived figure.

Q = Xc / ESR = 1 / tanδ at any single frequency. Higher Q means lower loss. Film and C0G ceramic capacitors have Q = 200–2000 (tanδ 0.0005–0.005); aluminum electrolytics have Q = 3–15 (tanδ 0.07–0.33). Q matters for resonant circuits (LC filters, tank circuits) where loss broadens bandwidth and reduces peak impedance.

Check the datasheet's ripple current rating at the operating frequency. The rating drops at lower frequencies (large current swings heat the capacitor more) and rises at high frequencies (skin effect means less plate area participates). Derate to 80% of the datasheet value for long life. Two capacitors in parallel roughly double the rated ripple current and halve the effective ESR.

Every real capacitor has tiny series inductance (ESL, typically 2–20 nH). SRF is where Xc = Xl, so impedance bottoms out at ESR only. Above SRF, the capacitor behaves like an inductor and its impedance rises with frequency. A 100 µF bulk capacitor with 10 nH ESL self-resonates near 5 kHz — well below the switching frequency of most buck converters. That is why decoupling networks use multiple values: 100 µF for low frequencies, 10 µF for mid, 100 nF for high, each covering its own band.

Conduction loss P = I² × ESR — current flowing through the parasitic resistance of leads, plates and connections. This is what the ripple current calculator outputs. Dielectric loss P = ω × C × V² × tanδ — molecular dipoles flipping back and forth in the dielectric at AC frequencies. It dominates at high voltages and high frequencies (why high-voltage capacitors run warm even with zero external current). Leakage loss P = V × I_leak is usually negligible except for supercapacitors and high-voltage ceramic parts.

Aluminum electrolytic life roughly halves for every 10 °C rise above rated temperature. So 2 W dissipation in a 100 µF 105 °C part running at 40 °C ambient and 60 °C capacitor case means the part is operating 45 °C below rating, giving roughly 2⁴.⁵ ≈ 22× the rated life. Let that same part run at 95 °C (only 10 °C below rating) and life drops to 1/2 of rating.