What is the formula for Capacitive Dropper Supply Current?

The Capacitive Dropper Supply Current formula is I = 2π × f × C × Vrms = Vrms / Xc, where I—Available RMS current (A); f—Line frequency (Hz); C—Dropper capacitor (F); Vrms—Line RMS voltage (V).

What does I mean in the Capacitive Dropper Supply Current formula?

I is Available RMS current (A).

Capacitive Dropper Supply Current

Q: How do you calculate Capacitive Dropper Supply Current?

Use the formula I = 2π × f × C × Vrms = Vrms / Xc. A capacitive dropper (X-rated capacitor) supplies a small DC current from the mains by using the capacitor’s reactance to limit current without dissipating heat. Enter the values you know in the calculator above and it solves for the remaining variable.

I = 2π · f · C · Vrms

Calculator

Supply Current

Line Frequency

Dropper Capacitor

Line Voltage

Result

—

Formula

I = 2π × f × C × Vrms = Vrms / Xc

Description

A capacitive dropper (X-rated capacitor) supplies a small DC current from the mains by using the capacitor’s reactance to limit current without dissipating heat. The available current equals the line voltage divided by the capacitor’s reactance. It is a cheap, transformerless way to power low-current loads such as LED night-lights and timer ICs—but the circuit is not isolated from the mains and is dangerous to touch.

Variables

I — Available RMS current (A)
f — Line frequency (Hz)
C — Dropper capacitor (F)
Vrms — Line RMS voltage (V)

Practical Notes

Use an X2-rated capacitor and a bleeder resistor. The output is NOT mains-isolated—never use where a user can contact the low-voltage side. Roughly 1 µF gives ~70 mA at 230 V/50 Hz.

Related Concepts

All Power & Efficiency formulas →

Linear Regulator Dissipation Efficiency Buck Converter Boost Converter Peukert Effect Energy After Losses Solar Charge Time LDO Minimum Output

Need more features?

Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.

Try the App