Line Filters1 min read
Bay note 09 — Line filter stages without guesswork
X caps, Y caps and CM/DM chokes as a sequence — how to specify a line filter so the bay builds a stage, not a lucky LC.
A line filter is a small system. Specifying only "10 A filter" gets you a shape, not a spectrum fix.
The cast
| Part | Role |
|---|---|
| X capacitor | Across L–N, differential mode |
| Y capacitors | L/N to earth, common mode (leakage current limited) |
| CM choke | Common-mode impedance |
| DM choke / leakage | Differential-mode impedance |
Order of attack
- Read which mode fails on the LISN (CM vs DM dominant)
- Set Y-cap budget from touch-current limits
- Size CM choke (see Bay note 04)
- Add X and DM impedance for the differential hump
What to put on the print
- Line voltage, Iload, frequency
- Class I or II (earth present or not)
- Max earth leakage
- Target standard / curve
- Footprint and terminal style
Browse line filters or send a LISN plot with the enquiry.