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Simple passive low pass filter

Dougster

Nov 8, 2014
21
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Nov 8, 2014
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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
 

Gryd3

Jun 25, 2014
4,098
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Jun 25, 2014
Messages
4,098
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
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.
 

Dougster

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

LvW

Apr 12, 2014
604
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Apr 12, 2014
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"...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.
 

BobK

Jan 5, 2010
7,682
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Jan 5, 2010
Messages
7,682
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
 
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