Transformerless Power Supply

Design a capacitive dropper power supply (non-isolated).

DANGER: HIGH VOLTAGE

Transformerless power supplies are directly connected to the mains. They are extremely dangerous and lack electrical isolation. Never touch the circuit while it is connected to the outlet.

Design Parameters

Input Voltage (Vin) (Vrms)

Mains Frequency (f) (Hz)

Output Voltage (Vout) (Vdc approx)

Capacitance (C) (μF)

Calculation Results

Enter parameters to see max current

About this calculator

Overview

A capacitive transformerless power supply uses a capacitor's reactance to drop the mains voltage to a lower level. It is small and cheap but NOT isolated from the mains.

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Pro Tips

CRITICAL: This circuit is NOT isolated. Touching any part of it while plugged in is FATAL.
The capacitor (C1) must be an X2-rated safety capacitor.
A bleeder resistor (e.g., 1MΩ) should be placed in parallel with C1 to discharge it.
Output current is typically limited to less than 100mA.

Fun Facts

"Commonly used in LED bulbs, PIR sensors, and small smart home gadgets."
"Unlike a resistive dropper, the capacitive dropper does not waste much energy as heat."
"The circuit essentially acts as a constant-current source for small loads."

Formula: I ≈ (Vin - Vout) / Xc