larrymoencurly wrote:

V W Wall wrote in message ...

Is a ground needed for the input line filter to be effective even
if the filter doesn't connect to ground, as is the case with many
Antecs, including my 300W SmartPower?

The input line filter in the PS connects to "chassis ground", i.e.
the metal PS enclosure.

How can my PSU's filter connect to ground when it has no any low-value
Y-rated disk capacitors from line-ground, just X2 capacitors across
the lines? It looks a lot like the filter in this 380W Antec
TruePower (lower left):

http://terasan.okiraku-pc.net/dengen/no48/true/open.jpg

and has a .47 uF yellow boxy capacitor across the AC receptacle, a
choke in series with one of the AC lines, followed by a .22 uF
capacitor across the lines, then a choke in series with each AC line
(both wrapped around the same core), and finally another .22 uF
capacitor across the lines.

It's hard to tell from the picture. See:

http://www.pavouk.comp.cz/hw/Alim_PC_ATX_200W_fr.pdf (page 1)

for the kind of circuit I was refering to. Note the two 47 mmfd capacitors
with the mid point connected to the ground (shield). In an actual PS,
some heat sinks are connected to the case ground, and some are floating.
Note the center point of the two main filter capacators connected to
ground thru a varister.

The actual enclosure is bolted to the computer case so it is always at the
line ground wire potential.

Another whose filter that doesn't connect to ground is on page 12 of
this example 90W ATX PSU (C1, C2, LF01):

http://us.st.com/stonline/books/pdf/docs/6412.pdf

There doesn't seem to be any ground (shield) here. You're correct, the
filter capacitors are between the hot line and neutral. There are three
different ground symbols in the diagram. It's hard to figure which is which.
I assume the "three pronged" ones donate case ground.

RFI prevention is an emperical kind of thing. Each situation is unique.
I have seen several examples of the building wiring acting as a good
antenna, especially when not properly grounded.

Virg Wall