The typical wall wart from China (the vast majority of them) contain no
decoupling or choking on either the input or output. Sure, they carry all
the required regulatory markings, but they are bogus from China with no
testing or evaluation wrt RFI/EMC. Them's are the facts of today.
The RFI, as you may realize, is generated internally due to the switching
power conversion. The days of RFI quiet real transformers which contain
iron and copper are unfortunately gone.
One side connects to the grid and the other, DC or LV side, connects to
your equipment. Any appropriate ferrites clamped onto either side will do
some good. But realize, ferrites are a band aid. The Chinese engineers
and suppliers do not spend additional funds on the wall warts to alleviate
RFI. Again, them's 'r do facts of ta'day.
Dave - WØLEV
On Wed, Sep 25, 2024 at 3:49 PM Greg Troxel <gdt@lexort.com> wrote:
> I'm writing to ask a theory question about an aspect of RFI management
> that I have always been unclear on.
>
>
> Consider a device connected to L and N from a 120V outlet, and having a
> 0/+12 cable to some other small device. I understand that common mode
> current on either the DC cable or the AC wiring will radiate, and that
> choking impedance is the right answer.
>
> Thinking about building a transmitter, it seems tricky for common-mode
> current to be efficient if it isn't sort of balanced on the two wires.
> Otherwise, it's sort of an end-fed antenna. Therefore, having an
> effective choke on the DC cable, even with no choking impedance on the
> AC side, should greatly reduce the current on the AC wiring. This is
> pretty much all you can do if the device has a plug and not a power
> cable, unless you use a choked extension cord.
>
> My questions
>
> - Is the above analysis iscorrect or confused?
> - If confused, does choking the DC cable side mostly address the RFI
> issue, by some other mechanism?
> - What is the equivalent circuit that explains generation of
> common-mode current?
>
>
> A related question is that given a physically small device that plugs
> in, how are these able to generate common-mode current, given the lack
> of a counterpoise for an endfed (which is of course not quite endfed,
> but very off center)?
>
> 73 de n1dam
>
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>
--
*Dave - WØLEV*
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