## A simple RC low pass filter

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Hi, I'm not an expert in constructing a simple passive low pass filter and I need help from someone who has an experience with this.
I need to make a simple low pass filter with R and C, like on the picutre, for filtering AC power voltage, with fc=500 Hz.
In theory fc = 1/(2*pi*R*C), and I can get the approximately right fc with using R=33ohm and C=10uF or R=68ohm and C=4.7uF or R=680ohm and C=470nF or R=3k3 and C=100nF ..., but which configuration will I use in project?
Signal Vin is a max 5V AC (reduced from 230V AC power network) and signal Vout is then connected to the a/d of atmega48.

## Attachment(s): A simple RC low pass filter

Choose any combination for which R is large compared to the source impedance/resistance.

Jim

Until Black Lives Matter, we do not have "All Lives Matter"!

Ok, my input circuit is shown on the picture below. Is it better to put the filter at pins Uin or Uout, where Uin is from a transformer (I don't have data about impedance/resistance). If I connect a filter at Uout pins does it mean that R in low pass filter need to be greater than R2 and how many?

## Attachment(s): divider.PNG

Note that the effective resistance of the divider as seen by the output (Thevenin equivalent) is the parallel combination of R1 and R2, i.e. R1*R2/(R1+R2). So you need to subtract this value from the R used in the RC network. If you select the values for the divider and C you can eliminate R altogether.

If you think education is expensive, try ignorance.

Ok so Uin is a direct connectionto 230V mains?

Yes, it's connected directly to 230V mains and it works ok :D , but since I'm using algorithm with Fourier series I must have a simple anti-aliasing filter.

@emuler
Excellent, if I understand correct, then I have a simple solution, I just need to compute adequate capacitor with respect to the voltage divider, that is

R= (R1*R2)/(R1+R2)

C = 1/(2*pi*R*fc)

Beware:

If the input signal is AC, then the output signal will go negative. A Mega48 (or any AtMega processor) ADC CANNOT handle negative inputs!

Jim

Until Black Lives Matter, we do not have "All Lives Matter"!