Power Supply Filter Capacitor Calculator

Reservoir capacitor filtering emerged alongside vacuum-tube rectifiers in the 1920s, replacing the earlier "choke-input" filter (large iron-core inductor before the capacitor) with simpler capacitor-input topology. Early electrolytic capacitors had enormous size-to-capacitance ratios and short lives — a 50 µF / 500 V cap could be a 6-inch can that dried out in five years. Postwar development of aluminum foil etching and solvent-based electrolytes pushed capacitance density up 100× by 1970.

The peak diode current problem was famously analyzed by O.H. Schade at RCA in 1943: reservoir-filter conduction angles are surprisingly narrow (20-30° of the line cycle), so peak diode current is 5-15× the DC load current. This governs diode surge ratings, transformer RMS current (which exceeds DC current by the same factor), and why linear supplies have noticeably higher audible buzz from the transformer than SMPS.

Modern linear supplies are rare in new designs — SMPS efficiency (90%+) vs linear (50-70%) and size advantages drove the changeover after the late 1990s. But linear supplies still dominate low-noise analog applications (lab instruments, audiophile DACs, medical EEG) where the switching noise of an SMPS would corrupt measurements. A low-dropout regulator fed from a well-filtered linear DC rail remains the gold standard for ultra-low-noise analog power.

Enter DC load current, target peak-to-peak ripple voltage, line frequency (60 Hz US, 50 Hz EU), and rectifier type. The tool computes the required reservoir capacitance C = I × T / ΔV, where T is the ripple period (1/f for half-wave, 1/2f for full-wave). Also returns ripple frequency (60/120 Hz typical) and estimated peak diode current (~5× DC load as a rule of thumb).

Practical notes: round capacitance up to the next standard value (E6 series: 100, 220, 470, 1000, 2200, 4700, 10000 µF), add 20% margin for capacitor tolerance, and check voltage rating (should be at least 1.4× peak DC for long life). Use low-ESR types (105°C long-life aluminum or polymer) for SMPS; standard 85°C aluminum is fine for 60/120 Hz linear supplies. Everything runs client-side; no values leave your browser.