Capacitive Dropper Calculator
Calculates capacitor values for transformerless power supplies. Estimates reactance, current limiting, and safety resistor power ratings.
About
A capacitive power supply uses the reactance of a capacitor to reduce mains voltage for low-current applications (e.g., LED drivers, IoT sensors). Unlike resistive droppers, capacitors do not dissipate active power as heat, making them efficient. However, the circuit is non-isolated, meaning the output is directly connected to the mains. This poses a severe shock hazard.
This calculator determines the required series capacitance based on the desired load current and output voltage. It uses the vector relationship between voltages, as the capacitor voltage is 90 degrees out of phase with the resistive load voltage.
Formulas
The capacitive reactance Xc depends on frequency f and capacitance C:
Since the capacitor voltage Vc and load voltage Vout are in quadrature:
The required capacitance is:
Reference Data
| Capacitor (X-Cap) | Value (μF) | Reactance @ 50Hz | Reactance @ 60Hz | Max Current (approx 230V) |
|---|---|---|---|---|
| 104 | 0.1 μF | 31.8 kΩ | 26.5 kΩ | ~7 mA |
| 224 | 0.22 μF | 14.5 kΩ | 12.0 kΩ | ~15 mA |
| 334 | 0.33 μF | 9.6 kΩ | 8.0 kΩ | ~23 mA |
| 474 | 0.47 μF | 6.8 kΩ | 5.6 kΩ | ~33 mA |
| 684 | 0.68 μF | 4.7 kΩ | 3.9 kΩ | ~48 mA |
| 105 | 1.0 μF | 3.2 kΩ | 2.6 kΩ | ~70 mA |