# Simple passive low pass filter

Discussion in 'General Electronics Discussion' started by Dougster, Mar 10, 2015.

1. ### Dougster

21
1
Nov 8, 2014
I'm wondering if one of you math wizards might help me. I want to make a simple, basic low pass filter (1 cap and 1 resister) to block any frequencies above about 300 cps. I found some formulas, but I get lost in the capacitive reactance and impedance stuff. I would like to use a low value resistor if possible. Input is about 210 volts @ 200 cps. Any ideas? Even approximate values would help. Thanks for any help you might give me.
Regards,
Doug

2. ### Gryd3

4,098
875
Jun 25, 2014
Is this a low power application?
There are calculators all over the net for audio nuts to calculate values for High-pass, Low-pass and Band-pass filters.

3. ### luca_zzz

10
0
Mar 10, 2015
R*C = 10 * (1/300) sec

4. ### Dougster

21
1
Nov 8, 2014
No, it's not low power. Thanks luca_zzz. That looks simple enough! I'll try it!!!

5. ### LvW

604
146
Apr 12, 2014
"...to block any frequencies above about 300 cps."

I suppose you know that it is not possible "to block" frequencies above the so-called "cut-off" frequency.
A first order RC lowpass - as envisaged by you - attenuates unwanted frequencies only with an attenuation rate of 20dB/decade.
For better attenuation you need a higher order filter.

6. ### BobK

7,682
1,689
Jan 5, 2010
10 Ohms, 47uF for a cutoff of 338 Hz.

Of course, cutoff frequency is the frequency where the response will be down by 3dB, then 6dB per octave after that.

Edit:

or 1 Ohm 470uF
or 100 Ohm 4.7uF
etc

Bob