🔧 Capacitive Transformerless Power Supply Calculator
Estimate the dropper capacitor value needed for a capacitive (transformerless) power supply that limits current using a capacitor’s reactance instead of a transformer.
How a capacitive (transformerless) power supply works
Instead of a bulky transformer, a capacitive power supply uses a capacitor in series with the AC mains to limit current, relying on the capacitor’s reactance rather than resistive dropping (which would waste power as heat). A small series resistor is normally added to limit inrush current and surge damage. The current is then rectified and regulated for the low-voltage DC load.
How this calculator works
The calculator finds the total impedance needed to limit current to your target value: Z = V / I. Since impedance combines the resistor and the capacitor’s reactance as Z² = Xc² + R², it solves for the required reactance Xc, then computes the capacitor value from Xc = 1 / (2π·f·C), rearranged to C = 1 / (2π·f·Xc).
Safety note: capacitive power supplies connect directly to mains voltage with no isolation, so the capacitor must be an X-rated safety capacitor, a bleeder resistor should be placed across it to discharge stored charge, and this design should only be built or used by someone experienced with mains-voltage safety practices. This calculator is for general educational and design-estimation purposes only.