QUOTE (Dave, NK7Z): I am sure they were installed when tested, but once
they
hit the assembly line, the parts mysteriously don't get stuffed.
Ever wonder why? Once a product is handed off to a contract manufacturer,
anything goes. These contract manufacturers make their $$ by cheaping down
the product. The first target, of course, are the RFI/EMC components as
they add nothing to the function of the product.
Another Dave - WØLEV
On Wed, Sep 25, 2024 at 10:15 PM Dave (NK7Z) <dave@nk7z.net> wrote:
> 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@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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> >> http://lists.contesting.com/mailman/listinfo/rfi
> >>
> >
>
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--
*Dave - WØLEV*
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