Cheap LED bulbs and timers often skip the transformer and drop mains voltage with a capacitor’s reactance. It is compact and efficient but not isolated from the mains – dangerous to touch.
How to use the Capacitive Transformerless Power Supply Calculator
- Mains RMS voltage
- Mains frequency
- Dropping capacitor C₁
- Series resistor R₁
- Zener voltage V_z
- Rectifier diode drop
Transformerless supplies are connected directly to mains voltage and can be lethal. This calculator is for understanding the circuit only. Everything is calculated in your browser.
How it works
Enter the mains RMS voltage and frequency, the dropping capacitor and series resistor, the Zener voltage and the rectifier drop.
Inputs explained
| Field | What to enter |
|---|---|
| Mains RMS voltage | Units: V, mV, kV, µV, MV |
| Mains frequency | Choose one: 50 Hz, 60 Hz |
| Dropping capacitor C₁ | Units: µF, nF |
| Series resistor R₁ | Units: Ω, kΩ, MΩ, mΩ |
| Zener voltage V_z | Units: V, mV, kV, µV, MV |
| Rectifier diode drop | Units: V, mV, kV, µV, MV |
What each result means
- Capacitor reactance = 1÷(2×pi×[Mains frequency]×[Dropping capacitor C₁])
- Available output current = ([Mains RMS voltage] – [Zener voltage V_z])÷sqrt((1÷(2×pi×[Mains frequency]×[Dropping capacitor C₁]))^2 + [Series resistor R₁]^2)
- Output voltage = [Zener voltage V_z] – [Rectifier diode drop]
- Power in R₁ = (([Mains RMS voltage] – [Zener voltage V_z])÷sqrt((1÷(2×pi×[Mains frequency]×[Dropping capacitor C₁]))^2 + [Series resistor R₁]^2))^2×[Series resistor R₁]
Example values
How the main result changes when Mains RMS voltage changes and everything else stays at the example values above:
| Mains RMS voltage (V) | Available output current (mA) |
|---|---|
| 115 | 15.2068 |
| 172.5 | 23.696 |
| 230 | 32.1853 |
| 287.5 | 40.6745 |
| 345 | 49.1637 |
| 460 | 66.1422 |
Formula
X_C = 1/(2πfC₁), I ≈ (V_rms − V_z) / √(X_C² + R₁²), V_out ≈ V_z − V_d
Worked example
Inputs — Mains RMS voltage: 230 V; Mains frequency: 50 Hz; Dropping capacitor C₁: 0.47 µF; Series resistor R₁: 100 Ω; Zener voltage V_z: 12 V; Rectifier diode drop: 0.7 V.
Results — Capacitor reactance = 6,772.55077 Ω; Available output current = 32.1853 mA; Output voltage = 11.3 V; Power in R₁ = 0.1036 W.
Key terms
- Capacitive dropper
- Using a capacitor's reactance to drop voltage.
Frequently asked questions
Is this kind of supply safe?
It is not isolated – every part can be at mains potential.
Why use an X-rated capacitor?
It is designed to fail safely across the mains.
Why the series resistor?
It limits inrush current when switched on.
How much current can it provide?
Usually only tens of milliamps.