Basics calculator

Capacitive divider

1 V from 10 V: 10 nF over 90 nF, 1 : 10

Two capacitors share AC the other way round from resistors: the small one drops most.

Opens in the app on 1 December, with Circuit Notebook 1.0 See it worked out

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The example, worked out

Given

I want: Vout from C1 and C2
Input voltage (AC): 10 V
C1, on top: 10 nF
C2, below: 90 nF
Frequency: 1 kHz

Working

the smaller capacitor drops the more
at 1 kHz: what the tap can drive

Results

Output voltage1 V

Ratio Vout / Vin1 : 10

Tap impedance at 1 kHz1.59 kΩ

Seen by the source9 nF

The formulas

Vout / Vin = C1 / (C1 + C2)the small capacitor drops the most
Zout = 1 / (2π f (C1 + C2))how stiff the tap is
fL = 1 / (2π R (C1 + C2))below it, a load R fades the output

The usual traps

It is the other way round from resistors: a bigger C2 gives a smaller output.
No DC passes: the tap floats until a resistor sets its level.
Class 2 ceramics (X7R, Y5V) lose capacitance with voltage: the ratio moves with the signal.

Questions

How does a capacitive voltage divider work?

Vout / Vin = C1 / (C1 + C2): the small capacitor drops the most. Zout = 1 / (2π f (C1 + C2)): how stiff the tap is. fL = 1 / (2π R (C1 + C2)): below it, a load R fades the output.

What does it give for typical values?

1 V from 10 V: 10 nF over 90 nF, 1 : 10. Output voltage: 1 V; Ratio Vout / Vin: 1 : 10.

What are the usual mistakes?

It is the other way round from resistors: a bigger C2 gives a smaller output. No DC passes: the tap floats until a resistor sets its level. Class 2 ceramics (X7R, Y5V) lose capacitance with voltage: the ratio moves with the signal.

In the app

Your circuit, drawn live

Change a value: the schematic redraws with your numbers on the parts.

The part you can buy

Results rounded to standard values (E12, E24, E96) and power ratings.

The working, shown

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Into your notebook

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Opens in the app on 1 December, with Circuit Notebook 1.0

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