How to Use the Capacitive Transformerless Power Supply Calculator
A capacitive (transformerless) power supply drops AC mains voltage using the reactance of a series, non-polarized "X2"-rated capacitor instead of a bulky iron-core transformer. Because reactance — not resistance — does the work of limiting current, very little power is wasted as heat, which makes the circuit compact and efficient for small, low-current DC loads such as LED indicators, relays, or standby electronics. This calculator applies the standard reactance and Ohm's-law relationships engineers use to size the dropping capacitor and its companion safety resistor.
Reactance, impedance, and current
- Capacitive reactance: Xc = 1 / (2πfC), where f is the line frequency in hertz and C is the capacitor value in farads. This is the AC "resistance" the capacitor presents at mains frequency.
- Total series impedance: Z = √(R² + Xc²), combining the safety resistor R with the capacitor's reactance Xc.
- Available RMS current: I = V / Z, where V is the RMS supply voltage. This is the current the dropper can feed into the downstream bridge rectifier and DC load.
- Resistor dissipation: P = I² × R — this continuous power sets the wattage the safety resistor must be rated for.
Choosing the capacitor and safety resistor
- The dropping capacitor must be an X2-rated (or X1-rated) safety capacitor built for direct connection across the AC mains — ordinary ceramic or electrolytic capacitors are not rated for this and can fail dangerously.
- Typical values run from about 0.1 µF to 2.2 µF depending on the current the load needs; larger capacitance or higher line frequency both increase the available current.
- The series resistor (commonly 47 Ω–1 kΩ, flame-proof/fusible type) limits the inrush current at switch-on and acts as a fuse if the capacitor fails as a short.
- Always wire a bleeder resistor (roughly 330 kΩ–1 MΩ) directly across the capacitor so it discharges to a safe voltage within about a second after the supply is unplugged — without it, the capacitor can hold a lethal charge indefinitely.
Frequently Asked Questions
How does a capacitive transformerless power supply work?
It uses a non-polarized capacitor in series with the AC line to drop voltage by capacitive reactance (Xc = 1/(2πfC)) instead of resistively, so very little energy is lost as heat compared with a resistive dropper. The current that passes through, set mainly by the capacitor's reactance, is then rectified (typically a bridge rectifier) and regulated (often with a zener diode) to power the DC load.
What capacitor value do I need for a target output current?
Rearranging Xc = 1/(2πfC) with I ≈ V/Xc gives C ≈ I / (2πfV). For example, to draw about 30 mA from a 230V, 50Hz line, C ≈ 0.03 / (2π × 50 × 230) ≈ 0.415 µF, so a standard 0.47 µF X2 capacitor is a reasonable choice. Verify the resulting current with this calculator before building the circuit.
Is a transformerless capacitive power supply safe to build?
Only with care. The whole circuit is directly connected to AC mains, so every part, including the DC output, sits at line potential and can deliver a lethal shock. Use an X2-rated dropping capacitor, a fusible or flame-proof series resistor, and a bleeder resistor across the capacitor, and always treat the circuit as live. Use an isolation transformer when testing or probing it.
Why include a series resistor with the dropping capacitor?
The resistor limits the inrush current spike that occurs the instant the circuit is plugged in while the capacitor is uncharged, protecting the rectifier diodes from a current surge, and it acts as a fuse if the capacitor ever fails as a short circuit.