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Capacitive Transformerless Power Supply Calculator

A capacitive transformerless power supply converts high-voltage AC mains (120 V or 230 V) to low DC output (3–12 V) using a voltage-dropping capacitor and Zener regulation, without a transformer. Electronics engineers and hobbyists use this topology for compact, lightweight power circuits where weight and cost matter more than efficiency. This calculator determines both the input AC current drawn and the regulated DC output voltage, enabling you to validate component selection before building.

Last updated:

Creators Dominik Czernia, PhD
7,066 people find this calculator helpful

Understanding Capacitive Transformerless Power Supplies

A transformerless power supply eliminates the bulky transformer entirely, replacing it with a capacitor that exploits capacitive reactance to drop AC voltage. Two main variants exist: capacitive and resistive designs. Capacitive circuits dissipate far less energy as heat and occupy a smaller footprint, making them ideal for low-current applications.

The capacitive design works as a voltage divider: the series capacitor C₁ presents impedance to the AC input, reducing voltage magnitude. A rectifier and smoothing capacitor then convert the result to DC. A Zener diode clamps and stabilizes the final output to a precise voltage—typically 5 V, 12 V, or 24 V depending on the diode rating.

Key advantages include:

  • Minimal size and weight compared to transformer-based supplies
  • No magnetic coupling losses
  • Lower component count in simple designs
  • Suitable for low-current loads (under 100 mA)

The main trade-off is poor efficiency when load current changes significantly; output voltage and input current both shift as impedance varies.

Capacitive Power Supply Equations

The input AC current depends on the voltage drop across the series capacitor and resistor network. The output voltage is determined by the Zener diode specification and the forward-bias voltage of the rectifier diode.

I_in = (√2 × V_RMS − V_z) ÷ (1 ÷ (π × f × C₁) + 2 × R₁)

V_out = V_z − V_d

  • V_RMS — RMS value of the input AC voltage (typically 110 V or 230 V depending on regional standard)
  • V_z — Zener diode voltage rating, which sets the nominal regulated DC output
  • V_d — Forward-bias voltage drop across the rectifier diode D₂ (usually 0.6–0.7 V for silicon)
  • f — AC mains frequency in Hz (50 Hz in Europe/Asia, 60 Hz in North America)
  • C₁ — Capacitance of the series voltage-dropping capacitor in farads
  • R₁ — Resistance of the series current-limiting resistor in ohms

Design Considerations and Load Current

Output current capacity in a capacitive transformerless supply is inherently limited. The circuit is not suitable for high-current applications; typical designs deliver only 10–100 mA before output voltage regulation fails.

To increase output current without changing the source voltage or Zener rating, connect capacitors in parallel to C₁, or use a larger capacitance value. A larger C₁ lowers the series impedance and allows more current to flow.

If you need to raise input current from the mains (which may seem counterintuitive), you can:

  • Increase input RMS voltage — choose a supply rated for higher mains if available
  • Lower the Zener voltage — a lower V_z increases the voltage drop across the impedance, pushing more current
  • Reduce series resistance — smaller R₁ decreases total impedance

Be cautious: higher input current means higher mains loading and greater heat dissipation in the dropping network.

From AC to DC: Capacitor Role and Rectification

A standalone capacitor cannot convert AC to DC; it blocks DC and passes AC. Conversion requires three stages:

  1. Voltage reduction: C₁ uses capacitive reactance (X_C = 1 ÷ (2πfC)) to attenuate the high AC input
  2. Rectification: A diode bridge or single diode allows current to flow in only one direction
  3. Smoothing: A second filter capacitor on the output charges and discharges, averaging the pulsating DC into a stable waveform

The Zener diode acts as a voltage regulator, maintaining a nearly constant output voltage across load variations by diverting excess current to ground. Without the Zener, output voltage would swing significantly with load changes.

Practical Pitfalls and Design Tips

When designing or troubleshooting a capacitive transformerless supply, watch for these common issues:

  1. Capacitor voltage rating undersizing — C₁ must withstand the full input RMS voltage (often with a safety margin of 150–200%). A 230 V mains supply demands a capacitor rated for at least 400 V. Undersized capacitors fail suddenly and may rupture, creating a safety hazard.
  2. No-load instability — Without a minimum load, the output voltage rises well above the Zener rating. Always include a bleeder resistor across the output to draw a few milliamps continuously, stabilizing voltage regulation even when the main load is disconnected.
  3. Thermal runaway in the Zener diode — If input current is high and output current is low, most current flows through the Zener instead of the load. This heat dissipation can destroy the diode. Calculate expected Zener power (I_z × V_z) and choose a device with adequate thermal headroom or add a series resistor.
  4. Safety and shock hazard — The input and parts of the circuit remain at mains potential even with no transformer. Never touch the circuit during operation, and always use proper isolation and fusing to protect against short circuits and accidental contact.

Frequently Asked Questions

What is the main advantage of a capacitive transformerless design over a traditional transformer-based supply?

Capacitive designs are significantly smaller, lighter, and cheaper because they eliminate the heavy iron-core transformer. They also avoid transformer losses and magnetic coupling issues. However, they are best suited to low-current applications (under 100 mA) and provide poorer regulation under variable loads. For applications where size and cost matter more than perfect regulation and safety isolation, capacitive transformerless supplies are an attractive choice.

How do I select the correct Zener diode voltage for my application?

The Zener voltage directly sets your regulated output voltage. If you need 12 V DC at the output, select a 12 V Zener diode. Remember that the actual DC output will be slightly lower due to the forward-bias voltage of the rectifier diode (typically 0.6 V for silicon). So to achieve 12 V out, choose a 12.6 V Zener. Always verify the current-handling capacity of the Zener matches your expected load and account for heat dissipation at the worst-case operating point.

Why does my capacitive power supply voltage drop when I connect a load?

In capacitive transformerless supplies, regulation depends on the balance between series impedance and load resistance. As load current increases, the voltage drop across C₁ and R₁ becomes larger, so less voltage reaches the Zener and output. Unlike transformer-based or switched supplies, these circuits have no feedback loop to compensate. To improve regulation, use a larger capacitance value (lower reactance) and minimize series resistance, but expect some voltage sag as an inherent trade-off.

Can I use a capacitive transformerless supply to power a microcontroller or sensitive IC?

Only with careful design. Most modern microcontrollers need tightly regulated voltage (often ±5% tolerance). A basic capacitive supply without additional filtering and regulation typically exhibits 10–20% voltage variation across reasonable load ranges. Add an LDO (low-dropout linear regulator) after the Zener stage to achieve the tight regulation ICs demand. This provides both voltage stabilization and galvanic isolation benefits not inherent to the raw capacitive circuit.

Why is the capacitor's voltage rating so critical in transformerless supplies?

The series capacitor C₁ experiences the full RMS voltage of the AC input. On a 230 V mains supply, this can be 230 V peak or higher during transients. A capacitor rated for only 250 V may fail within months due to dielectric stress. Industry practice recommends selecting a capacitor rated at least 1.5× the peak input voltage, so for 230 V mains, use a 400 V or 600 V capacitor. Failure is not gradual; undersized capacitors often rupture suddenly, posing a fire and explosion hazard.

How do I increase the output current of my capacitive power supply?

The most effective method is to increase the capacitance of C₁, which lowers the series impedance and allows more current to flow from the source. You can parallel multiple capacitors or select a single device with higher capacitance. Alternatively, reduce the series resistance R₁ if your circuit design permits. Be aware that increasing current also increases heat dissipation in both C₁ and the Zener diode, so thermal management becomes critical. For loads requiring more than 100–150 mA, a capacitive transformerless supply becomes impractical; consider a switched-mode or small transformer-based supply instead.

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