Capacitive Transformerless Power Supply Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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A capacitive transformerless power supply result can look convincing even when one unit or assumption is off. Power-electronics formulas are useful first-pass design tools, but ideal duty-cycle and component equations do not capture every switching loss or device limit. This page keeps the calculation narrow enough to trace the answer back to the values you enter.

What this calculator does

The Capacitive Transformerless Power Supply Calculator connects RMS voltage (VRMS), Zener voltage (Vz), Forward bias voltage (Vd) to the page’s approximate output current. Supporting values are included only when they follow from the same relationship, so you can compare the headline with the quantities behind it.

How to use it

Enter RMS voltage (VRMS), Zener voltage (Vz), Forward bias voltage (Vd), Frequency (f), Capacitance (C1) using the units shown beside each field. Keep all values from the same physical case, then check the headline result and any supporting values before changing one input at a time for comparison.

How the calculation works

Capacitive reactance is XC = 1/(2πfC). The page estimates available RMS voltage as max(0, Vsupply−Vzener−2Vdiode) and output current as I = Vavailable/√(R²+XC²).

Example

With the default setup (RMS voltage (VRMS) = 230 V; Zener voltage (Vz) = 12 V; Forward bias voltage (Vd) = 0.7 V; Frequency (f) = 50 Hz), the page reports approximate output current of 0.068013 A. This is a useful baseline: change one input and confirm the new value follows the proportionality in the formula.

How to interpret the result

Treat the approximate output current as an ideal circuit-design quantity for the entered topology and values. It is most useful for first-pass component sizing or comparison, followed by checks against ratings, tolerances, ripple, losses, and thermal behavior.

Limitations and notes

Real circuits include switch and diode losses, ESR/ESL, ripple, tolerances, thermal limits, control-loop behavior, transients, electromagnetic interference, and manufacturer derating that ideal equations may not capture. Recheck RMS voltage (VRMS), Zener voltage (Vz) first if the result looks surprising, because an incorrect unit or definition there can dominate rounding error. Safety-critical or standards-based work still needs the applicable design rules and independent verification.