Where C = capacitance in farads, I = dc load current in amperes, ΔE = peak-to-peak ripple voltage, f = ripple frequency (generally 120 Hz for full-wave or 60 Hz for half-wave), and 0.7 is the complement of the rectifier-current duty cycle, which is assumed to be 0.3 ( see the figure, b). I have used it countless times and would like to share it with the readers: C = 0.7(I)/ΔE(f) When I was in school, I derived this simple relationship for input-capacitor size that has been with me throughout my career. Other attempts sometimes calculate percent ripple, which I consider a largely useless term because we tend to visualize the waveform as an oscilloscope sees it-a sawtooth waveform with the limit being the minimum capacitor voltage. ![]() ![]() Therefore, common practice is to pick a large size, and if the ripple voltage is low enough, all is okay ( see the figure, a). A rectifier input capacitor's size is often considered nebulous.
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