Capacitive Power Supply Basics
A capacitive transformerless power supply uses a capacitor instead of a transformer to drop the AC mains voltage to a lower level for rectification. The capacitor limits the current by its reactance at the mains frequency, similar to how a resistor limits current but without dissipating significant power. This makes the design extremely compact and lightweight, ideal for very low-power applications like LED indicators, standby circuits, and simple IoT sensors.
The circuit typically consists of a dropper capacitor, a safety resistor, a bridge rectifier, a filter capacitor, and a zener diode regulator. The X-rated dropper capacitor must be specifically designed for mains connection and self-healing in case of voltage spikes. Despite its simplicity, this circuit requires careful design because it is directly connected to the mains with no galvanic isolation, making it inherently dangerous.
Design Equations
The current output is limited by the capacitor's reactance. Since an ideal capacitor dissipates no power, the efficiency can be quite high. However, the circuit can only provide limited current (typically under 100mA) and has poor load regulation.
Safety Considerations
- No isolation: The output is directly connected to the mains. NEVER touch any part of the circuit while powered. Not suitable for user-accessible devices.
- X-rated capacitor required: Only use capacitors rated for continuous mains connection (X1 or X2 class). Standard film capacitors may fail dangerously.
- Bleeder resistor: A high-value resistor across the capacitor is essential to discharge it when unplugged, preventing electric shock.
- Surge protection: A series resistor (47-100 ohm) limits inrush current and protects against voltage spikes.
- Fuse: Always include a mains fuse for overcurrent protection.
Frequently Asked Questions
Why not just use a resistor to drop voltage?
A resistor dropping 230V to 5V at 30mA would dissipate about 6.75 watts as heat, which is wasteful and requires a large, hot resistor. A capacitor drops the voltage through reactance, which stores and returns energy each half cycle rather than converting it to heat. The capacitor itself dissipates essentially zero power (real capacitors have tiny losses). This makes capacitive power supplies much more efficient for low-current applications.
Why is the current limited?
The maximum output current is set by the capacitive reactance: I = Vac / Xc. A larger capacitor provides more current but increases the energy stored (and danger level). Practical limits are about 100-200 mA due to capacitor size, cost, and safety concerns. For currents above about 50 mA, a conventional transformer-based or switch-mode power supply is usually more practical and safer.
Can this circuit be used safely in consumer products?
Only with proper isolation and safety design. Many LED bulbs and small appliances use capacitive droppers internally, but all exposed parts must be insulated and the entire circuit must be enclosed. The design must meet IEC 60335 or similar safety standards. The output must never be accessible to the user. For any product where someone might touch the output, a transformer or isolated switch-mode supply is mandatory.