Capacitive Power Supply Calculator

Calculate the dropping capacitor value needed for a capacitive (transformerless) AC power supply design.

AC Line Voltage

V RMS

Line Frequency

Desired Output Current

What a capacitive power supply is

A capacitive (transformerless) power supply uses a series capacitor to drop AC line voltage down to a lower current-limited level, commonly used in small, low-cost, low-power circuits.

The formula

The capacitor’s reactance needed is X_C = V_line / I_out, and from reactance the capacitance is C = 1 / (2πfX_C), where f is the AC line frequency. This gives the capacitor value that limits current to the desired level at the given line voltage and frequency.

Capacitive supplies provide no isolation from the AC mains and must be designed and used with proper safety precautions; this calculator is for general educational and reference purposes only, not a substitute for a qualified electrical design review.

Last reviewed August 2026