Capacitive Transformerless Power Supply Calculator

Not safe for beginners. This circuit is not isolated from the mains: every part of the circuit, including the DC output and its ground, sits at mains potential from the power grid and can give a lethal electric shock. Only build it inside a closed enclosure that cannot be opened without tools, with a fuse on the mains side, an X2-class capacitor, and a bleeder resistor. If in doubt, use an ordinary isolated adapter.

You must use an X2-class film capacitor rated for direct connection to AC mains (e.g. 275 VAC). Do not use electrolytic or ordinary ceramic capacitors as the series capacitor — they are not designed to withstand continuous mains voltage and can fail dangerously. The value used is this calculator result, rounded up to the nearest standard value.

The current through the series capacitor is nearly constant regardless of load. When the load draws full current, the zener only dumps the remainder; when the load is removed, the entire current (e.g. 53.58 mA in the 220 V / 12 V / 50 mA example) flows through the zener. No-load zener power = V_zener × I_total, e.g. 12 V × 53.58 mA = 0.643 W. Always rate the zener at least 2× the no-load power.

Practically below 100 mA. Larger currents need big, expensive, risky X2 capacitors, the inrush resistor dissipates significant power, and regulation worsens. For loads above ~100 mA, use an isolated switching supply or conventional transformer — safer and more efficient.

The bridge rectifier calculator covers the conventional AC-to-DC bridge rectifier: Vpeak = Vrms × √2, DC voltage after diode drops, ripple from the filter capacitor, and PIV — for supplies using a transformer or straight mains without capacitive dropping. This calculator is specific to the capacitive dropper topology: a series capacitor as reactive voltage dropper plus shunt zener, with no transformer and no isolation.

Because the current through the capacitor and the output voltage are not in phase: the voltage across the capacitor leads the current by 90°, so the correct vector sum uses Pythagoras, not scalar subtraction. For V_mains much larger than V_zener (e.g. 220 V vs 12 V), the error is small — the difference between 208 V and the exact result √(220² − 12²) = 219.67 V is only about 5%. The calculator uses the scalar approximation for simplicity, labels it (est.), and the result is conservative for capacitor selection.

The inrush resistor (series, 47–100 Ω fusible) limits the surge current when the plug is inserted exactly at the mains waveform peak — without it, the empty capacitor briefly behaves like a short circuit with peak current Vpeak/R, e.g. 311 V / 100 Ω = 3.11 A. The bleeder resistor (470 kΩ–1 MΩ across the capacitor) drains the capacitor charge after the plug is pulled so it cannot shock you when touched, dissipating a small power, e.g. 0.103 W at 220 V with 470 kΩ.

Poor — that is an inherent limitation of this topology. The output voltage is clamped by the zener only while current is available; if the mains drops 10% (grid tolerance) or the load rises, the zener starves and the voltage collapses. Capacitors also have ±10–20% tolerance. A healthy design keeps a zener current margin of at least a few mA at full load, and never rely on this topology for loads whose current varies drastically.

Technically yes for small currents (e.g. 50 mA in the example above), but strongly discouraged for devices that are touched, programmed via USB, or connected to a computer — the ground of this circuit is live relative to mains and will damage laptops and endanger the user. For microcontrollers, use a cheap, safe isolated power module.

The core formula Xc = 1/(2πfC) is exact for an ideal sine wave. Main error sources: mains voltage tolerance (±10%), film capacitor tolerance (±10–20%), and the scalar V_drop approximation (~5% at 220 V / 12 V). Always verify by measuring actual current, and pick a standard capacitor one size above the computed result if the zener current margin is too thin.