Capacitive Transformerless Power Supply Calculator

About the Capacitive Transformerless Power Supply Calculator

A capacitive transformerless power supply (capacitive dropper) uses the reactance of a capacitor to limit current from mains voltage instead of a transformer. By choosing the right X-rated capacitor, you can step down 120V or 230V mains to a low-voltage DC output for small loads — LEDs, microcontrollers, relays, and sensors.

The key component is an X2-rated film capacitor connected in series with the mains. Its reactance Xc = 1/(2πfC) limits the current to I = V/Xc, where V is the mains voltage minus the output voltage. A zener diode regulates the output, a bridge rectifier converts AC to DC, and a filter capacitor smooths the ripple.

This calculator sizes all components: the dropping capacitor, surge-limiting resistor, bleeder resistor, zener diode, and filter capacitor. It includes safety warnings and voltage rating requirements, because these circuits are NOT isolated from mains — they must only be used in enclosed, non-touchable applications.

When This Page Helps

Capacitive dropper circuits are cheap, compact alternatives to transformers for low-power applications inside sealed enclosures. However, incorrect component selection can be dangerous — undersized capacitor voltage ratings, missing surge protection, or inadequate zener power dissipation can cause fires or electric shock.

This calculator automates the design process, selecting the correct X2 capacitor value and voltage rating, surge resistor, bleeder resistor, zener diode, and filter capacitor. It includes safety warnings and a comparison table of standard X2 capacitor values.

How to Use the Inputs

1. Enter the mains voltage (120V for US, 230V for EU).
2. Enter the desired output voltage and current.
3. Enter the line frequency (60 Hz US, 50 Hz EU).
4. Set the zener voltage slightly above the output voltage.
5. Choose a voltage safety margin for the capacitor rating.
6. Review the component values and selection table.

Example Calculation

Tips & Best Practices

• Always use X2-rated film capacitors — never ceramic or electrolytic capacitors across mains.
• The output shares a ground with mains — use an isolation transformer for the oscilloscope when debugging!
• Add a 1MΩ bleeder resistor across the X2 capacitor to discharge it when power is removed.
• Use a fusible resistor or proper fuse as the first component after mains entry.
• For 50/60 Hz dual-frequency operation, design for 50 Hz (lower frequency = higher reactance = less current).

How Capacitive Dropper Circuits Work

The capacitor in series with the mains acts as a current limiter. Unlike a resistor, which converts excess energy to heat, a capacitor stores energy during one part of the cycle and returns it during the next. The voltage across the capacitor shifts 90° out of phase with the current, meaning power dissipation is ideally zero.

The maximum current available is I = V_rms / Xc, where Xc = 1/(2πfC). By choosing C, you set the current limit. A zener diode clamps the output voltage, and any excess current is diverted through the zener. This is why zener power rating is critical — it must handle the full output current at the zener voltage.

Component Selection Guide

**Dropping Capacitor (C1):** Must be X2-rated for across-the-line use. Voltage rating must exceed V_peak × safety margin. Use polypropylene film type — they self-heal from minor breakdowns and fail safely open-circuit.

**Surge Resistor (R1):** A 47-150Ω wirewound resistor limits inrush current. It must handle the peak inrush for several cycles. Some designs use an NTC thermistor instead, which has low resistance when warm (normal operation) and high resistance when cold (startup).

**Bleeder Resistor (R2):** Required by safety standards to discharge C1 when mains is disconnected. Typical value 1MΩ, which discharges a 1µF capacitor with time constant τ = 1 second.

Safety Considerations

Capacitive dropper supplies are used in billions of devices (LED bulbs, IoT sensors, appliance controls) but are inherently dangerous due to the lack of isolation. Key safety requirements: double-insulated enclosure, proper fusing, X-rated capacitor, bleeder resistor, and creepage/clearance distances meeting IEC 60950 or equivalent standards.

Frequently Asked Questions

• Is a capacitive power supply safe?

  The output is NOT isolated from mains — there is a direct electrical connection. These supplies must only be used inside sealed enclosures where no one can touch the circuit. They are common inside appliances but never for external/user-accessible circuits.

• What is an X2-rated capacitor?

  X2 capacitors are safety-rated for connection across mains. They are designed to fail open (not short) and withstand transient overvoltages. Never use a regular film or ceramic capacitor across mains — only X-rated capacitors.

• Why not use a resistor instead of a capacitor?

  A resistor would dissipate enormous power as heat: P = I × V_drop. A capacitor stores and returns energy each half cycle, so it dissipates almost no power (ideally zero). This makes the capacitive dropper much more efficient.

• What is the maximum current I can draw?

  Practically, capacitive droppers are limited to about 100-200 mA. Above that, the required capacitor becomes large and expensive, and the lack of isolation becomes more dangerous. For higher currents, use a proper switch-mode power supply.

• Why is a surge resistor needed?

  When the mains is switched on at peak voltage, the uncharged capacitor allows a very large inrush current for a fraction of a cycle. The surge resistor (typically 47-150Ω) limits this current to protect the bridge rectifier and capacitor.

• Why is a bleeder resistor needed?

  After power is disconnected, the X2 capacitor retains a dangerous charge. The bleeder resistor (typically 1MΩ) discharges it within a few seconds. Safety standards require X-cap discharge below 60V within 1 second.