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Q&A Power Line Filter. Location of Cy and common mode choke.

A circuit under question: Why are CY capacitors on the source side, and not on the load side? Henry Ott describes this with: Because filter attenuation is a function of impedance mismatch, t...

3 answers  ·  posted 8mo ago by 2kind‭  ·  edited 7mo ago by Nick Alexeev‭

Question capacitor filter common-mode-choke
#3: Post edited by user avatar Nick Alexeev‭ · 2024-05-27T14:56:52Z (7 months ago)
Fixed spelling in the title.
  • Power Line Filter - Loaction of Cy and CMC
  • Power Line Filter. Location of Cy and common mode choke.
  • A circuit under question:
  • ![cmc filter](https://electrical.codidact.com/uploads/z382g8qsy81mos28y50wcwq7cxtv)
  • Why are CY capacitors on the source side, and not on the load side?
  • Henry Ott describes this with:
  • “Because filter attenuation is a function of impedance mismatch, the role of the power line filter is to maximize the mismatch between the source and the load impedance”
  • Can someone explain the reasoning behind this?
  • Why for maximum attenuation the high impedance filter element shall face low impedance LISN, and the high impedance noise source shall face low impedance Cy?
  • A circuit under question:
  • ![filter with common mode choke](https://electrical.codidact.com/uploads/z382g8qsy81mos28y50wcwq7cxtv)
  • Why are CY capacitors on the source side, and not on the load side?
  • Henry Ott describes this with:
  • > Because filter attenuation is a function of impedance mismatch, the role of the power line filter is to maximize the mismatch between the source and the load impedance
  • Can someone explain the reasoning behind this?
  • Why for maximum attenuation the high impedance filter element shall face low impedance LISN, and the high impedance noise source shall face low impedance Cy?
#2: Post edited by user avatar Lundin‭ · 2024-05-16T13:03:12Z (8 months ago)
capacitor filter cmc
capacitor filter common-mode-choke
#1: Initial revision by user avatar 2kind‭ · 2024-05-03T19:14:06Z (8 months ago) 
Power Line Filter - Loaction of Cy and CMC
A circuit under question:
![cmc filter](https://electrical.codidact.com/uploads/z382g8qsy81mos28y50wcwq7cxtv)

Why are CY capacitors on the source side, and not on the load side?

Henry Ott describes this with:
“Because filter attenuation is a function of impedance mismatch, the role of the power line filter is to maximize the mismatch between the source and the load impedance”

Can someone explain the reasoning behind this?

Why for maximum attenuation the high impedance filter element shall face low impedance LISN, and the high impedance noise source shall face low impedance Cy?
capacitor filter cmc