Design non-isolated AC-DC supplies with series capacitor voltage drop
2026-03-28T00:00:00Z
⚠️ LETHAL VOLTAGE WARNING: This calculator models a non-isolated mains-connected supply. The output is directly referenced to live mains voltage — there is NO galvanic isolation. Contact with any part of the circuit can be fatal. Use only in safety-certified designs with full understanding of the risks.
C = I / (2πf(V<sub>in</sub> – V<sub>out</sub>))
μF
Approximate values only — based on a simplified ideal-capacitor model. Real circuits require rectifier, load, and ripple analysis before use.
(1284.6%)
Capacitive transformerless power supplies represent a cost-effective but inherently risky approach to AC-DC conversion that eliminates bulky isolation transformers by relying on a series capacitor to drop mains voltage. The capacitor presents frequency-dependent reactance Xc = 1/(2πfC) that opposes the AC current flow, causing a voltage drop proportional to the load current. This design was popular in early television sets and inexpensive consumer electronics but presents multiple safety hazards: the lack of galvanic isolation between mains and load creates electric shock risk, capacitor failure (short circuit) can cause dangerous inrush currents exceeding 100 A at switching moments, and there is no protection against accidental contact with live circuits. Modern design practice severely restricts use to applications meeting strict safety standards, low-power requirements (<50 W typical), and where isolation is not critical. The calculation methodology accounts for charge/discharge cycles: during the positive half-cycle, the capacitor charges through a rectifier bridge; during the negative half-cycle, the capacitor discharges to supply load current. The output ripple voltage is determined by the capacitance value and load current: larger capacitance reduces ripple, while higher load current increases ripple. Thermal stress on the capacitor is significant due to high AC current through the series element; ceramic or film capacitors rated for AC operation (not electrolytic) are mandatory. The voltage across the series capacitor under fault conditions (short-circuited load) can exceed 2000 V peak, causing catastrophic failure and potential hazard.
Real-world implementation of capacitive PSUs demands rigorous safety engineering and thorough understanding of failure modes. The inrush current at power-on can reach several amperes, requiring current-limiting components (series resistor or NTC thermistor) to protect the rectifier diodes and capacitor from shock stress. Ripple voltage must be specified carefully: audio applications typically tolerate 5–10% ripple, logic circuits require <2%, and high-precision analog circuits demand <0.5% ripple. The temperature coefficient of the AC operating capacitor directly affects output voltage stability: ceramic capacitors may exhibit ±20% capacitance change over the operating temperature range, while film capacitors provide better stability (±5%). Advanced designs incorporate active regulation using linear or switching regulators after the capacitive dropper stage, ensuring stable output despite mains voltage and load variations. Leakage current through the capacitor contributes to failure over time; capacitors rated for continuous mains exposure typically survive 5000–10000 hours before dielectric breakdown. Cost advantage of the capacitive approach must be weighed against reduced reliability and higher field failure rates compared to transformer-based or isolated switching supplies. Professional designers increasingly avoid this topology except in extremely cost-constrained, low-power, non-critical applications where isolation hazard is documented and accepted. Modern standards (IEC 61010, CE marking) impose strict constraints on transformerless designs, pushing industry toward safer isolated topologies even when cost increases slightly.
⚠️ SAFETY WARNING: Capacitive power supplies present high electric shock and fire hazards. They are NOT galvanically isolated and can be lethal. Use only where isolation is not required, in safety-certified designs, and with proper understanding of risks. Always include foldback current limiting, thermal management, and fault protection.
Before proceeding, clearly document that galvanic isolation is not required for your application and that transformerless operation is acceptable per relevant standards (IEC 61010, local regulations). Implement electrical safety training for all personnel servicing the equipment. Mark all stages post-rectifier as high-voltage danger zones. For any application where isolation might be desired in the future, consider transformer-based or isolated switching supplies instead; retrofitting from transformerless to isolated topology is expensive and error-prone.
Use the formula C = I<sub>out</sub> / (2πf(V<sub>in</sub> – V<sub>out</sub>)) to determine required capacitance. Select the AC line voltage: 120 V RMS (North America) or 230/240 V RMS (Europe/Asia). Calculate for minimum expected mains voltage (typically 10% low tolerance) to ensure adequate output voltage. Use high-quality film or ceramic capacitors rated for continuous AC operation; do NOT use electrolytic capacitors in the series position. Include a 1–2 μF safety margin above calculated value to handle manufacturing tolerances.
At power-on, inrush current limited by only Xc can exceed 100 A for milliseconds. Add either a 10–50 Ω series resistor or a 10–100 mA NTC thermistor (negative temperature coefficient) that reduces resistance as it heats up, providing soft-start characteristics. Use fast-recovery or schottky rectifier diodes rated for at least 5× peak inrush current (typically 1N4007 or equivalent for protection). Include bleeder resistor across the output smoothing capacitor (often 1–10 kΩ) to discharge high voltage when powered off, improving operator safety.
Follow the rectifier bridge with an output smoothing capacitor (electrolytic is acceptable here) to reduce ripple ripple to acceptable levels. For audio: ≤10% ripple. For logic: ≤2% ripple. For precision analog: ≤0.5% ripple. Design ripple calculations assuming worst-case: minimum mains voltage and maximum load current. If load current varies significantly, add a linear regulator (7805, LM317) or switching post-regulator to maintain tight output voltage regulation. Operating point should be 50–70% of maximum load current for acceptable ripple and regulation performance; optimizing for lighter loads makes heavy-load performance worse.
Calculate heat dissipation in the series AC capacitor: P = I²<sub>rms</sub> × ESR (equivalent series resistance), where ESR typically ranges 0.5–2 Ω for film capacitors. Volume derating due to heat is critical; select capacitors rated for higher voltage and temperature to ensure long life in continuous operation. Specify output cooling (heatsink for regulators), ventilation (if enclosed), and worst-case thermal scenarios (hot climates, summer ambient). Test the prototype over full temperature range (–10 to +60°C typical) to verify output voltage stability. High-failure-mode devices likely involved in field returns: electrolytic output capacitors (dry-out after years), series AC capacitor (insulation degradation), and rectifier diodes (thermal overstress). Plan for documented product lifetime and replacement interval consistent with component rated life.
Scenario: Design a capacitive power supply for a 12 V, 100 mA LED driver circuit operating from 230 V mains (50 Hz). Standard filament-style LED lamp eliminates isolation requirement; cost target mandates elimination of transformer.
Interpretation: A 1.5 μF 400V film capacitor drops approximately 218 V at 100 mA continuous current, leaving roughly 12 V available for the load. Inrush current of ~154 mA is manageable with a 10 Ω limiting resistor and appropriate rectifier. For 5% output ripple at 100 mA, calculate output filter capacitor as C<sub>out</sub> = I / (2π f Ω V<sub>ripple</sub>) = 0.1 / (2π × 50 × 0.05 × 12) ≈ 106 μF; select 220 μF electrolytic for margin. Heat dissipation (0.01 W) is negligible. This design is practical for non-isolated LED drivers, but NOT suitable for applications requiring electrical safety isolation.
Cost reduction. Transformers are heavy, bulky, and expensive. For extremely cost-sensitive, low-power, non-critical applications where isolation is NOT required, eliminating the transformer can reduce BOM (bill of materials) by 30–50%. However, modern industry increasingly avoids this topology due to field safety incidents and regulatory pressure. Isolated switching supplies have become cheaper and are strongly preferred.
Yes, emphatic. Without current limiting, inrush current can exceed 100 A for milliseconds, instantly destroying rectifier diodes rated for 1–5 A. Add an NTC thermistor (10–100 mA rated) or series resistor (10–50 Ω, 1 W) to limit inrush. Some designs also add a relay that shorts the limiting resistor after ~1 second to improve efficiency. Testing at power-on must verify that rectifier diodes do not exceed rated peak current or junction temperature.
YES. Unlike transformer-isolated supplies, there is NO galvanic isolation between mains and output. Certain fault conditions (e.g., capacitor short or filter capacitor failure) can place mains voltage on the output. Never assume the output is <50 V until explicitly verified with a multimeter. Use this topology ONLY where isolation is irrelevant (e.g., decorative LEDs) or clearly documented as NOT isolated and accepted by users/installers.
Catastrophic mains voltage appears directly on the output, typically causing immediate failure of downstream circuits and posing fire/shock hazard. Output smoothing capacitor can also fail from overvoltage. Some designs add fast-blow fuses (100–500 mA) in series with the capacitor to interrupt fault current, but fuse must clear within milliseconds. In modern designs, this single-point failure mode has driven adoption of isolated topologies despite higher cost.
Film capacitors rated for AC operation typically survive 5,000–10,000 hours continuous, equivalent to 0.6–1.2 years of 24/7 operation. Temperature significantly affects lifetime: operating 10°C above rated temperature roughly halves lifetime. High-quality polypropylene capacitors last longer than polyester. Electrolytic capacitors (NOT suitable for series AC position) last 2,000–5,000 hours. Plan for periodic replacement or incorporate into warranty cost if product is expected to operate for many years.
Poor load regulation is intrinsic to capacitive dropper design. As load current increases, voltage drop across the series capacitor increases, reducing output voltage proportionally. Typical voltage variation is 10–30% from no-load to full-load. For applications requiring tight regulation (<5% variation), add a post-stage linear or switching regulator after the capacitive stage. This increases complexity and cost, often negating the transformer cost savings, which is why modern designs use isolated switching supplies instead.
Efficiency is typically poor: 40–60% overall. The series capacitor dissipates significant power due to equivalent series resistance (ESR); at 100 mA average current and 1 Ω ESR, dissipation is 10 mW continuous. Output filter resistors and rectifier diode losses add more. High-efficiency isolated switching supplies achieve 85–95% efficiency, making them preferred even for cost-sensitive applications where higher BOM cost is offset by reduced thermal management and product lifetime.
YES. IEC 61010 (safety in measurement, control, lab equipment) severely restricts transformerless supplies. CE marking (EU) and UL (USA) impose strict documentation and isolation requirements. New designs rarely pass certification with transformerless topology. If you’re designing for commercial sale, consult notified body testing lab early to confirm feasibility. Many jurisdictions now effectively prohibit transformerless supplies in consumer products due to safety liability, making this topology increasingly obsolete in regulated markets.
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