Capacitive Transformerless Power Supply Calculator (C1, current and R1, AN954)

Understanding the capacitive power supply calculator — concept and principle

A capacitive (transformerless) power supply uses a capacitor's reactance to drop mains voltage to a few volts for small electronics. This calculator follows Microchip application note AN954 to find the available output current or the capacitor needed, plus the resistor's power and other stresses.

The capacitor limits current by its reactance, giving about 25 mA at 4.5 V; the circuit is never isolated from the mains.

How it is calculated

Capacitor reactance XC = 1 ÷ (2πfC).
Available current (half-wave) I = (√2 × VRMS − VZ) ÷ [2 × (XC + R1)].
Required capacitor C = 1 ÷ (2πf × [(√2 × VRMS − VZ) ÷ (2I) − R1]).
Output voltage = VZ − VD.
Line current through C1 ≈ VRMS × 2πfC; power in R1 = (line current)² × R1, doubled for margin.

The principle

A capacitor on AC behaves like a resistance that dissipates almost no power, so it can limit current without the heat of a series resistor. The zener diode clamps the voltage, and the capacitor's reactance sets how much current can flow. Because current is set by the capacitor, the design must use the lowest line voltage and frequency to guarantee enough current, and the highest to check component stress.

Worked example

Default inputs: 220 V RMS, 60 Hz, C1 = 0.47 µF, R1 = 470 Ω, zener 5.1 V, diode drop 0.6 V.

XC = 1 ÷ (2π × 60 × 0.47 × 10⁻⁶) ≈ 5,644 Ω.
I = (311.1 − 5.1) ÷ [2 × (5,644 + 470)] ≈ 25 mA available.
Output voltage = 5.1 − 0.6 = 4.5 V; smallest load resistance = 4.5 ÷ 0.025 = 180 Ω.
Line current ≈ 220 ÷ 5,644 ≈ 39 mA; power in R1 ≈ 0.039² × 470 ≈ 0.714 W; suggested rating 1.43 W.
Apparent power drawn ≈ 220 × 0.039 ≈ 8.58 VA.

Each point is the calculator re-run with only “Lowest mains voltage (V RMS)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Lowest mains voltage (V RMS)	Output current available (mA)
55	5.9
110	12.3
165	18.7
220	25
275	31.4
330	37.7
440	50.5

How to read the result

These supplies suit very small, steady loads such as indicator LEDs or microcontrollers in sealed appliances. The current is fixed by the capacitor, so the zener must absorb whatever the load does not use, and efficiency is low. For anything a person can touch, use an isolated supply.

Common mistakes

Designing at nominal voltage only, leaving no margin at low line voltage.
Using a capacitor not rated for mains (an X2 safety capacitor is needed).
Under-rating R1, which also limits inrush current.
Measuring the circuit with a grounded oscilloscope without an isolation transformer.

Keep in mind: This circuit is not isolated from the mains; every node can be at line potential and can kill. Only qualified people should build it, and only for sealed products, with a fuse, a varistor and an X2 capacitor.

Frequently asked questions

How much current can a capacitive power supply provide?

Typically tens of milliamps; the capacitor's reactance sets the limit.

Why use an X2 capacitor?

X2 capacitors are safety-rated for connection across the mains and fail safely.

Is a capacitive power supply isolated?

No. Its output is connected to the mains, so it must not be touchable.

Sources: Microchip Technology, Application Note AN954, Transformerless Power Supplies: Resistive and Capacitive · IEC 60384-14, Fixed capacitors for use in electronic equipment — Part 14: Capacitors for electromagnetic interference suppression and connection to the supply mains

Checked 9 Nov 2026. R. Condit, Microchip Technology, Application Note AN954 “Transformerless Power Supplies: Resistive and Capacitive” (DS00954A, 2004), capacitive supply Equations 1–6 and design example.

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