Capacitive Transformerless Power Supply Calculator

Formula

How this is calculated

A capacitive transformerless power supply drops mains voltage using a series capacitor's AC reactance (Xc = 1 / (2π f C)) rather than a transformer's turns ratio. Because the capacitor is a purely reactive element, it dissipates almost no power itself — unlike a resistive dropper — while it limits the current that reaches the rectifier and load. The resulting AC current I = Vs / Xc is then rectified (usually half-wave) and smoothed, giving an approximate DC load current of about 0.637 × I_ac. A zener diode or small linear regulator clamps the output to the desired voltage.

The capacitor value sets the available current, not the output voltage; to increase load current you increase C, and vice versa. A typical small circuit might use a 0.47 µF X2 safety capacitor on a 230 V / 50 Hz supply, which limits current to about 34 mA AC — sufficient for a handful of LEDs or a microcontroller. The zener must absorb the voltage difference between the supply and the load; under no-load conditions it carries the full current and dissipates maximum power, so choose a zener rated well above the calculated worst-case dissipation.

IMPORTANT: The supply capacitor MUST be a safety-rated type (class X2 for across-the-mains use; class Y if between live and earth). The circuit is mains-referenced — there is no isolation — making every part of the circuit potentially live. This design is only appropriate in sealed, non-touchable enclosures where no user contact is possible. The calculator gives design estimates only; always verify against measured current with a series ammeter and a current-limited bench supply before connecting to mains.