On Tue, 1 Apr 2008 05:58:32 -0600, Steve Flood wrote:
>Followup - when I rewire my shack next month for 230v, I am
>bringing in a 10awg, 4 wire cable (black-black-white-bare) from the
>service panel to a NEMA 14-30 wall socket. (My power supply will
>have a 120 circuit in it as well, so I followed the advice of a
>previous AMPS topic.) The 230v EMI filters I looked at (Qualtek)
>only have two connectors on the 'load' side. What happens to the
>neutral lead? Does it just 'go past' the EMI filter?
As I'm sure you're aware, a 240V load has no connection to neutral
-- it draws its power from +120 and - 120. The green wire (equipment
safety ground) is not permitted to carry current (except in the case
of a fault, to blow a fuse or trip a breaker). The filter you've
found would need to serve only the 240V load. If you want to filter
the 120V outlet, you'll need a 120V filter. The filters I've seen
work on both hot and neutral.
A typical filter has capacitors from each side of the line to
ground, inductance in series with each side of the line, and a
capacitor across the line. It is CRUCIAL that the "ground"
connection for those capacitors go to the filtered equipment by a
VERY short path (what my EE profs called "zero length" for
emphasis). The length of that wire has inductance, which essentially
defeats ability of the capacitors to short out the noise.
FWIW -- I would be FAR more concerned with filtering noise sources
in your home and shack than in filtering incoming power for your rig
(unless, of course, the power supplies for your rigs are noise
sources). The primary coupling mechanism is the power line acting as
an antenna and radiating trash that is picked up by our ham
antennas. While both antennas (the power line and the ham antennas)
do, indeed, work in the opposite direction, any piece of ham gear
that lacks its own RFI filter has no place in our shacks in the
first place! I'd place my filtering on the computers, little power
supplies for outboard gear, and battery chargers.
When adding a peripheral like an EMI filter, we should always ask,
"What problem are we trying to solve?" "Is the connected load a
noise source?" "Is the connected load sensitive to RF on the power
line?" "Does the connected load already have an RFI filter?"
Finding the right answer to those questions could save a lot of
money.
Another important point. Capacitance between the AC line and ground
creates leakage current in the green wire, which puts AC noise on
the chassis. When we make signal connections from one chassis to
another, that noise voltage is impressed on the shield, current
flows, and there's IR drop on the shield that is then added to the
signal. This noise, which usually takes the form of a "buzzy hum,"
is what we mistakenly call a "ground loop." It's really leakage
current, the path for which is capacitance between the AC line and
the chassis. Much of that capacitance is in the power transformer,
but some of it is line filters.
73,
Jim Brown K9YC
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