The power line filters like commercial RFI or EMI filters usually composed of R L and C passive components and they don't usually work below under kHz frequencies. What makes it difficult or a challenge to design such filter with 200 Hz cut off for instance?

  • \$\begingroup\$ Hm, you deliver the keywords to this question yourself! So, can you add e.g. an LC low pass filter's circuit diagram, and give a formula for its capacity and inductance given said cutoff frequency? Can you see a problem? \$\endgroup\$ Commented Nov 6, 2018 at 18:30
  • \$\begingroup\$ My conclusion is we need much bigger values for L and C but I dont understand why cannot be constructed even though the filter might be bulky. Thats the only thing comes to my mind. It is similar to making an effective radio transmission for 50Hz requires very long antenna \$\endgroup\$
    – floppy380
    Commented Nov 6, 2018 at 18:33
  • 1
    \$\begingroup\$ You got it! So, that is the absolute main reason. Bulky, especially when it's the inductor made out of copper, is plain heavy, and expensive. It's certainly not impossible to build such a filter (but be careful what happens when you hit resonant frequency), but it consumes an undesireable amount of material if you need it to conduct much current. But you've figured that out yourself – congrats! If I were you: I'd take your exact comment, add it to the question (edit it), and ask whether there are other reasons! \$\endgroup\$ Commented Nov 6, 2018 at 18:37
  • \$\begingroup\$ Yes big but how big I dont know. Is the challenge marketing and selling or does it really require a filter as big as a camel for instance. Im not able to relate 10Henry to a size I mean. Or maybe when things get bigger power loss will be huge. But I dont know the main reason why on earth no one ever built it. \$\endgroup\$
    – floppy380
    Commented Nov 6, 2018 at 18:41
  • \$\begingroup\$ This is a 1H inductivity. But it's only designed for 300 mA – if you wanted to filter, say, 10 A of AC current, yours would have to be 33 times as big, probably even bigger due to thermal problems. But it has an ohmic resistance of up to 40 Ω – totally unusable as line filter, so, that has to go down by a factor of at least 100. Meaning you need 100 times the size for resistance reasons. This thing weighs 583g. I'll let you do the math. \$\endgroup\$ Commented Nov 6, 2018 at 18:45

1 Answer 1


I designed a 100 Hz power line filter comprising a 300 mH choke, an 8 uF capacitor and some series resistance. The design worked on 60 Hz AC at 230 volts for a Canadian oil platform - it was used to prevent massive voltage spikes on the AC connection to a small private telephone exchange. The voltage spikes were just resetting the phone exchange every ten seconds and only occured when the machine that drove pipes into the oil well was operating.

What makes it difficult or a challenge to design such filter with 200 Hz cut off for instance?

Well, I had to get an inductance of 300 mH and given that for a single turn the inductance was 10 uH, it meant I needed about 173 turns. This, stretched out end-to-end, was about 3 inches per turn or 43 foot of wire. So, to carry the bit of current to the phone exchange without too much volt drop I needed average sized connection wire of 7/0.2 (= 0.22 sq mm or about 24 AWG). If you do the math, the wire alone has the volume of a larger ferrite core so we are not talking suitable for a PCB because the core probably weighed around 300 grammes.

It soon mounts up but, the applications for this sort of filter are very few and far between unless you design HVDC switch yard reactor smoothing chokes: -

enter image description here

Note the choke is bottom left and compare this with the building middle right!


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