ALLPCB
Introduction
This article reviews the principle and practical implementation of capacitive dropper circuits. Although their use has declined, they remain relevant for some low-cost, low-power applications and are worth documenting for reference.
1. Overview of Capacitive Droppers
1.1 What is a capacitive dropper
A capacitive dropper is a power conversion topology that uses a capacitor (and often a series resistor) to reduce AC mains voltage before rectification and regulation. The capacitor presents a reactance at AC that limits the maximum current. In practice a discharge resistor is usually placed across the capacitor, so the arrangement is commonly called a resistive-capacitive (RC) dropper rather than just a capacitive dropper.
1.2 Typical block diagram
Typical capacitive dropper supplies include: the dropping network, rectifier, regulator (usually a zener), and output filter. The dropping network is the core topic here. Rectification is implemented with a bridge (full-wave) or a single diode pair (half-wave). Regulation is typically provided by a zener diode sized for the required output voltage. Output filtering is a capacitor to smooth the DC.
1.3 Operating principle
The capacitor limits AC current via its reactance Xc, given by:
Xc = 1/(2*pi*f*C)
For an AC voltage U, the AC current through the capacitor is approximately:
Ic = U / Xc = U * 2 * pi * f * C
Using full-wave rectification, the effective rectified voltage coefficient is about 0.89, so the rectified voltage can be approximated as 0.89*220 V on a 220 V mains. Substituting values yields approximate average currents proportional to the capacitance. For example, when mains is 220 V and f = 50 Hz, each 1 uF of capacitance yields about 60 mA average with full-wave rectification and about 30 mA with half-wave rectification.
The capacitor thus acts as a current limiter and, together with the load resistance, forms a voltage divider in a dynamic sense. Because the capacitor limits current but does not provide isolation or a fixed drop, a zener or other regulator is required to define the output voltage. The long-term life of the supply depends heavily on the dropper capacitor; specialized X2 or CBB capacitors designed for this application exhibit slower degradation and longer life.
1.4 Advantages and disadvantages
Advantages: low cost and small size for appropriate low-power applications. Also, the capacitor does not dissipate active power in the same way a series resistor does; it limits current by reactance.
Disadvantages: no safety isolation, posing a shock hazard; limited to low-power designs; not suitable for inductive or capacitive loads (works for resistive loads); poor transient and surge performance relative to other topologies; and output voltage is not inherently stable under varying loads without regulation. These limitations explain why other solutions have become more common as component costs fell.
2. Practical Use
2.1 Example practical circuit
A practical implementation typically adds input surge and overcurrent protection ahead of the dropper. Common front-end components include a fuse and an MOV for surge suppression. The dropper capacitor should be a safety-rated X2 or specially rated CBB type. A bleed resistor across the capacitor provides a discharge path when input is removed, meeting safety requirements for residual voltage decay.
A series flame-retardant wirewound resistor is often placed before the rectifier to limit inrush and protect the rectifier from surge stress. The rectifier is commonly a bridge diode. A zener diode provides voltage regulation, and a smoothing capacitor filters the DC output. If the output feeds downstream DC-DC converters, add a 0.1 uF ceramic capacitor close to the load to remove high-frequency noise.
2.2 Usage notes
Select the dropper capacitor value based on required load current and mains frequency using the formulas above. The dropper capacitor must be non-polar and safety-rated (X2/CBB types are typical). The dropper capacitor should be connected in the live conductor. This topology is not suitable for dynamic or high-power loads; if DC operation is required, half-wave rectification is sometimes used for simplicity, but half-wave delivers half the power of full-wave rectification.
Because the capacitor sets a relatively fixed current, the output voltage depends on the load resistance and therefore must be regulated with a zener or similar device. The zener's maximum regulation current must exceed the dropper current because when the load is open, the zener path must absorb the full capacitor current.
2.3 Key component selection
Fuse: choose an AC-rated fuse (for example 250 V AC) with current rating matched to the load.
MOV: used for surge suppression. Select based on mains voltage and expected surge energy. Typical families use different energy ratings (example: 7D, 10D, 14D) and voltage codes (e.g., 471 for 470 V clamping). Choose a part appropriate to the mains level and expected transient energy.
Series wirewound resistor: a flame-retardant wirewound resistor is recommended for inrush and surge limiting. Typical values are in the 10-50 ohm range; 33 ohm is a common choice, but select based on inrush and steady-state power considerations.
Dropper capacitor: voltage rating should exceed the expected peak mains voltage by a safe margin. For 110 V mains use capacitors rated at 275 V AC or higher; for 220 V mains use 600 V DC (or appropriate X2 rated AC) types. Typical capacitances for dropper applications: 0.1, 0.22, 0.33, 0.47, 0.68 uF. Use non-polar safety capacitors designed for mains use.
Bleeder resistor: the voltage rating must exceed the dropper capacitor voltage. The resistor provides a discharge path so that the capacitor voltage decays to a safe level after power is removed. The time constant requirement for the voltage to drop to 37% is one time constant: R*C < 1 s is a common safety target. Typical resistance values are 470 kΩ to 1 MΩ; the example circuit used 750 kΩ, 1 W.
Rectifier: if using discrete diodes, ensure the average forward current and peak inverse voltage ratings are adequate. A dedicated bridge is often simpler.
Zener regulator: select the zener voltage per the required output and ensure its power and current ratings exceed the maximum dropper current. The zener must absorb the full dropper current when the load is absent.
Dummy load: a resistor placed in parallel with the zener acts as a minimum load to prevent excessive voltage when no real load is connected. Choose a resistor with sufficient voltage rating and power rating (commonly >1/4 W). Typical resistance values range from about 10 kΩ to 1 MΩ depending on design.
Conclusion
This article analyzed the principle and practical considerations for capacitive dropper power supplies. Although the topology has important limitations and safety concerns, it remains a viable low-cost solution for certain low-power, resistive-load applications when designed correctly with appropriate safety-rated components.
