How this calculator works
The selected topology is the half-wave zener-clamped supply analysed in Microchip AN954. Its approximate available-current calculation adds capacitive reactance and surge resistance in the denominator, as the application note does. This is an estimate for that rectifying circuit rather than the impedance magnitude of an ordinary series RC load. Regulation requires load current below available current. The model does not check surge withstand, capacitor safety class, bleeder discharge, zener heating or touch safety.
Formula
Xc = 1/(2 pi f C); Iin ≈ (sqrt(2) Vrms - Vz)/[2(Xc + R)]; Vout ≈ Vz - Vd.
Worked example
With 100 V RMS, 50 Hz, 1 microfarad, zero series resistance, a 10 V zener and a 1 V diode drop, the estimate gives 20.6436 mA available current and 9 V output.
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Frequently asked questions
Is this an isolated low-voltage supply?
No. Every output node can be hazardous relative to earth. This calculation is for circuit analysis, not an approved mains-powered construction design.
Why use the half-wave formula?
Rectification changes the waveform and available current. The AN954 approximation is tied to the stated half-wave topology; it should not be reused for a bridge dropper.
Can the load draw more current?
Once load current exceeds what the dropper can provide, zener regulation is lost and the output falls. Component heating must be checked separately.