Calculator KB-53
Capacitive dropper calculator
A simple power source for a low-power load: excess voltage is dropped across a capacitor. The circuit is not isolated from the mains, so treat the entire thing as dangerous.
Result
In brief: Calculate the capacitive dropper for a transformerless 230 V mains power supply: capacitance, nearest standard value, actual current, protection and discharge resistors. Example: E6-series capacitor: 2.2 µF.
Calculation example
| Mains voltage | 230 V |
|---|---|
| Mains frequency | 50 Hz |
| Output voltage | 12 V |
| Output current | 150 mA |
| E6-series capacitor | 2.2 µF |
|---|---|
| Exact calculation | 2.079 µF |
| Current with this value | 158.7 mA |
| Voltage across the capacitor | 229.69 V |
| Across the suppressor with no load | 1.9 W |
| R1 surge-current resistor | 22 Ω, 554.4 mW |
| Turn-on surge current | 14.78 A |
| R2 discharge resistor | 200 kΩ, 264.5 mW |
How it is calculated
If the output is less than 10% of the mains voltage: C ≈ 3200 · I / Uin
A capacitor blocks DC, but AC passes through it as it continuously charges and discharges, while its impedance at mains frequency limits the current. So the current depends on capacitance: to increase the current, you need a larger capacitor, and its size quickly becomes noticeable. The calculation uses the voltage across the capacitor itself—that is, the difference between the mains voltage and the output voltage.
What to look for
- Use a capacitor rated for this kind of operation: an X2 capacitor rated for 280 V AC is more reliable than a K73-17 or CL21 rated for 400–630 V DC.
- With no load, all the current flows through the regulating element. It’s best to use a TVS diode at the output: it’s rated for more power than a regular Zener diode.
- A fuse, an inrush current limiting resistor, and a bleeder resistor are essential: the capacitor retains a dangerous voltage long after the power is disconnected.
- It’s usually more sensible to use a ready-made compact switching power supply module like the HLK-PM01: it provides galvanic isolation and a stable output voltage.
Frequently asked questions
What dropper capacitor do I need for 150 mA from a 220 V mains supply?
If the output voltage is less than 10% of the mains voltage, C ≈ 3200 · I / U: 3200 · 0.15 / 220 = 2.18 µF, so the nearest standard value is 2.2 µF. At a higher output voltage, use the full formula: for 48 V and 20 mA, the result is 0.298 µF.
What voltage rating should the dropper capacitor have?
You can use a 400 V DC capacitor, but 630 V is more reliable. Better still, use a 280 V AC X2-rated capacitor: it is designed to operate directly from the mains.
What components do I need besides the capacitor?
A resistor in series with the capacitor to limit inrush current, a resistor in parallel with it to discharge it, a fuse of about 0.5 A, and a TVS diode at the output to prevent the voltage from rising when there is no load. The circuit is connected to the mains, so do not touch it while it is powered.
