[RFI] Filtering 220VAC

[Jim Brown]

On 10/19/2023 5:46 AM, nlsa at nlsa.com wrote:
> When applying a common-mode filter (such as toroids) to a North American
> 220VAC line, only the two "hot" conductors should be included, correct?  Not
> the neutral.

If it is ONLY 240V, load current is on the two "hot" conductors, but 
noise is often present on the ground, so all three should be wound 
through the toroid core. Fair-Rite #31 is the weapon of choice for HF 
and VHF through 6M.

If that 240 branch circuit feeds both 240V and 120V outlets, all four 
conductors should go through the core.

The best ways to wind these chokes are 1) use a suitable length of NM 
(Romex) or 2) use three or four THHN solid conductors taped together to 
go through the core, splicing to whatever conductors are used for the 
circuit. In general, the choke should be as close as possible to the 
noise source.

In general, twisted pair/triplet/quad resists radiation of noise, and 
should be used when practical. I build twisted/triplet by cutting equal 
lengths of three colors of THHN stranded, putting one end in a bench 
vise, the other in a drill motor, pulling them taught, and running the 
drill motor slowly to produce the twist. Let the twisted cable sit at 
least overnight to develop memory. When released, it will untwist a bit, 
and twist ratio will not be equal from one end to the other, but that's OK.

73, Jim K9YC