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Simplified Capacitive-Dropper Current Model

Explore the captured average-current approximation and diode voltage subtraction for a nonisolated capacitive-dropper circuit.

Formula & worked example
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How to use this calculator

Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

At 120 V RMS, 60 Hz, 1 µF, 100 Ω and a 12 V zener, this simplified model gives about 28.65 mA and 11.3 V after a 0.7 V diode drop.

I ≈ (√2 Vrms − Vz)/[1/(πfC)+2R]; Vout = Vz−Vd.

Worked example

Enter these known values and leave the other values blank.

RMS voltage (V==RMS==): 120 V
Zener voltage (V==z==): 12 V
Forward bias voltage (V==d==): .7 V
Frequency (f): 60 Hz
Capacitance (C==1==): 1 μF
Resistance (R==1==): 100 Ω

Output voltage (V==out==): 11.3 V
Approximate average input current: 28.647 mA

Assumptions and limitations

This is a simplified average-current approximation, not an exact AC impedance or transient simulation. It omits load behavior, ripple, surge current and component dissipation.
A transformerless dropper provides no galvanic isolation from its supply. Its nominal low output voltage does not imply a touch-safe output. This calculation cannot establish component ratings or safe construction.
Use positive voltage, frequency, capacitance, resistance and current. The zener voltage must exceed the forward diode drop; the numerator must be positive.
The square-root-two factor assumes a sinusoidal RMS supply. Rectifier topology and conduction intervals must match the approximation.
Enter known quantities and leave the unknown blank. Editable quantities support inverse solutions and locks; inconsistent locked quantities are reported. Unit selectors convert the quantity independently of its display.
Reviewed hash-pinned model correction: This is a simplified average-current approximation, not an exact AC impedance or transient simulation. It omits load behavior, ripple, surge current and component dissipation. A transformerless dropper provides no galvanic isolation from its supply. Its nominal low output voltage does not imply a touch-safe output. This calculation cannot establish component ratings or safe construction. Use positive voltage, frequency, capacitance, resistance and current. The zener voltage must exceed the forward diode drop; the numerator must be positive. The square-root-two factor assumes a sinusoidal RMS supply. Rectifier topology and conduction intervals must match the approximation. Enter known quantities and leave the unknown blank. Editable quantities support inverse solutions and locks; inconsistent locked quantities are reported. Unit selectors convert the quantity independently of its display.

Common questions

Does a low output voltage mean the output is isolated?

No. This topology has no galvanic isolation from its supply. Output voltage alone does not establish a touch-safe circuit.

Does the result include surge or ripple?

No. It is a simplified average-current model that omits transient surge, ripple and component power ratings.

How do I use the inverse calculation?

Clear the quantity you want to find and enter the remaining known values in their selected units. The model can only solve branches with enough independent information.

References

Normalize the captured mixed-case Pi symbol to the mathematical constant PI used by all variants.

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