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Simple passive low pass filter

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

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  1. Dougster

    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

    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

    luca_zzz

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

    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

    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

    BobK

    7,682
    1,688
    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
     
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