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Capacitive Dropper Circuit Principle

2025-09-20

Capacitive dropper circuits are often found in low-cost appliances because they are much cheaper than transformer-based or switching power supply solutions.

Cost-effectiveness is essentially the only advantage of this voltage reduction method. Its disadvantages include:

Small output current
Significant voltage fluctuation with load changes
Safety hazards due to direct connection to mains electricity

Therefore, its application range is limited to situations where the current requirement is very small and the circuit is not easily accessible to humans.

Working Principle

A capacitor's capacitive reactance at a specific AC frequency limits the maximum operating current. After current limiting, the load lowers the output voltage, with the capacitor controlling current flow and dynamically distributing voltage between itself and the load.

For example, with China's 220V/50Hz mains supply, a 1μF capacitor has a reactance of:Xc = 1/(2πfC) = 1/(2×3.14×50×1×10^-6) ≈ 3180Ω

The maximum current through the capacitor would be:Ic = U/Xc = 220/3180 ≈ 70mA

This calculation method allows selecting the appropriate capacitor value based on the device's current requirements.

Safety Precautions

Use only non-polar capacitors with a voltage rating above 400V
Not suitable for dynamic, capacitive, or inductive loads
For DC operation, use half-wave rectification with an appropriate zener diode
Maintain isolation from mains to prevent electric shock
Do not change capacitor values during repairs

Application in DC Power Supplies

In a typical DC power supply: mains → step-down → rectify → filter → regulate → DC output

For simple circuits like sound/light control switches, a transformer would be overkill. A capacitor can replace it, simplifying the circuit and reducing costs.

Circuit Structure

In the basic configuration:

C1 drops the mains voltage
D2 performs half-wave rectification
C2 filters the pulsating DC
D3 provides voltage regulation
R1 discharges C1 after power-off
D1 provides a discharge path during the negative half-cycle

While it can be understood as a simple capacitive-resistive voltage divider, the capacitor's primary role is current limiting, which in turn determines the voltage across the load.

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