[RFI] wall warts and shielding the DC side - theory?

[Dave (NK7Z)]

Sometime for fun, take a wall wart apart.  Most times, you will see 
where a low pass filter "should" go, but the parts were not 
installed...  I am sure they were installed when tested, but once they 
hit the assembly line, the parts mysteriously don't get stuffed.

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

My favorite definition of an intellectual: 'Someone who has been educated beyond his/her intelligence'.  Arthur C Clarke

On 9/25/24 10:09, David Eckhardt wrote:
> 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 at 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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>