Capacitive Transformerless Power Supply Calculator

Capacitive Transformerless Power Supplies

A capacitive dropper limits AC mains current using a series capacitor's reactance. After rectification and regulation (Zener or LED load), the circuit delivers low DC current without a transformer—common in low-power applications like LED indicators.

Dropping Capacitor Formula

$$X_C = \frac{1}{2\pi f C}$$ $$I = \frac{\Delta V}{X_C}$$ $$C = \frac{I}{2\pi f (V_{\text{in}} - V_{\text{out}})}$$

$f$ is AC line frequency (50 or 60 Hz), $I$ is load current, and $\Delta V$ is the voltage across the dropping capacitor. Example: 20 mA at 230 V input, 5 V output, and 50 Hz gives about 283 nF.

Frequently Asked Questions

What is a capacitive dropper?

A series capacitor that limits AC current by its reactance. It replaces a transformer in very low-power circuits where isolation is not required.

Why must V_out be less than V_in?

The capacitor must drop voltage across itself to limit current. If output voltage equals or exceeds input, no voltage remains across the capacitor and the formula breaks down.

Is this circuit safe for mains?

Capacitive droppers are not isolated from mains voltage. Use only in enclosed, properly insulated products and add a discharge resistor across the capacitor.

What frequency should I use?

Use your local AC line frequency: 50 Hz in most of Europe and Asia, 60 Hz in North America. The reactance depends directly on frequency.

What capacitor type is required?

Use a mains-rated X2 safety capacitor, not a standard electrolytic. X2 capacitors are designed for continuous AC line duty.

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Capacitive Transformerless Power Supply Calculator

Capacitive Transformerless Power Supplies

Dropping Capacitor Formula

Frequently Asked Questions

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