[Lowfer] Sheilding power cords with aluminum foil ??
Andy - KU4XR
ku4xr at yahoo.com
Tue Jan 26 00:17:17 EST 2010
Clint:
This is some very good, detailed information. Thank you for
taking the time to explain it like you did. The power supply
in mu PC dosen't seem to be generating very much hash. The
problem I am having appears to be coming from the video
generating section of the computer.IE: move the mouse across
the screen, and hear rapid fire sounds, open the browser, or
a file on the hard drive, and hear all kinds of noise as the
screen changes. When nothing is changing , the noise is steady,
but refresh a page, or a grabber page refreshes, and noise
galore. Here's the kicker ::: unplug the monitor, and
dis-connect the SVGA cable, NO MONITOR connected, the noise
is still there !! I have dis-connected every cable to the PC
while it's on, mouse, keyboard, sound cables, ethernet, etc.
this frazzlin noise just won't go away. I can't help but think
it's something to do with the motherboard video.
I can " Beef Up " the powwer supply as one fella posted, and as
you mentioned by adding a dual choke with 3 MilliHenries Imp.
on each side, but I'm just a little leary of the " KABOOOMM "
that can happen with these SMPS. I've had numerous PC's but
have never had what I would describe as a " Quiet one "
I may try the extra Inductance, and hope it don't go Kaboom.
73 ti you and thanks for your info. :::
Andy - KU4XR - EM75xr - Friendsville, TN.
LOWfer Beacon " XR " @ 185.29866 KHz ( QRSS-60 )
Coordinates: N: 35* 43' 54" - W: 84* 3' 16"
http://www.myspace.com/beaconxr
http://webpages.charter.net/ku4xr/
--- On Mon, 1/25/10, C. Turner <turner at ussc.com> wrote:
> From: C. Turner <turner at ussc.com>
> Subject: Re: [Lowfer] Sheilding power cords with aluminum foil ??
> To: "Discussion of the Lowfer (US, European, & UK) and MedFer bands" <lowfer at mailman.qth.net>
> Date: Monday, January 25, 2010, 5:44 PM
> Since the vast majority of the QRM is
> being conducted into the power
> line rather than being radiated by the cord, shielding it
> will have no
> real effect at LF/MF frequencies.
>
> Remember also that at LF/MF frequencies, clamp-on ferrite
> cores are
> utterly useless as they simply cannot add enough reactance
> to the line
> on which they are installed to impede signal flow! If
> you look at the
> specs for such devices, you'll see that they are even of
> dubious utility
> even at HF, where they may offer only a few 10's of ohms of
> impedance.
>
> In general, at HF frequencies, snap-on ferrites are often
> more useful to
> prevent ingress rather than egress. Most susceptible
> devices do usually
> have *some* amount of intrinsic RF immunity, in many cases
> a few extra
> 10's of ohms of impedance at the interfering frequency may
> be all that's
> needed to reduce the RF coming in by several-fold and keep
> them happy.
> If you are trying to keep some noise from being emitted
> from a device,
> however, those extra 10's of ohms may not really knock
> things down too
> much - especially (as is often the case) you need to reduce
> their
> emissions by 10's of dB!
>
> If you wish to add filtering that is effective at LF/MF
> frequencies, you
> are talking about needing to add 100's of uH (at least) to
> whatever line
> is conducting these out. Taking the example of a
> switching power
> supply, you also need to shunt any RF on that line to the
> chassis/shield
> ground of that power supply - and at LF/MF frequencies,
> this will take
> many 1000's of pF to effectively do so.
>
> The practice of a "Pi" or "L" circuit is a good one:
> From the power
> supply, you first throw inductance in the line to add
> (preferably) 100's
> of ohms of reactance at the frequency of interest and then
> you shunt
> whatever remains to ground.
>
> ***
>
> Take an example of a typical computer supply line
> choke/filter
> combination - about 100 uH and 2200pF.
>
> At 15 MHz, we could see that the 100uH has a reactance of
> about 9.4k and
> the 2200pF has a reactance of about 5 ohms: A 9k
> resistor shunted to
> ground by a 5 ohm pretty much shorts out the signal so if
> everything is
> working properly, this is pretty effective at HF.
>
> At 150 kHz, that same 100 uH has a reactance of about 94
> ohms and the
> 2200 pF has a reactance of 500 ohms - a combination that
> will have
> little effect on the signal coming out of the box.
>
> Considering that a snap-on ferrite core will probably offer
> well under
> 10 uH of inductance to the line (that's being generous!)
> and that adding
> that ferried so doesn't add any additional shunt reactance
> to ground
> (like the cap would) you can see that at LF frequencies,
> adding that is
> ineffectual. Likewise, if you were to assume that the
> foil (or even
> coax-braid) shielding would add, maybe, 100pF/foot of
> capacitance (an
> overly generous amount!) you can also see that that's
> equally
> ineffective at LF.
>
> If you you need to modify a device that is radiating
> garbage from the
> power line connection to make it "LF" friendly, do what has
> already been
> recommended here:
>
> - Take an AC input filter from a dead switching
> supply. Often on old PC
> supplies, these are a toroid wound with twisted wire - one
> wire
> connected to each side of the line. They can also
> take the form of a
> toroid in which wire is wound separately on each half of
> the toroid.
> Finally, they can also look like a split-bobbin
> transformer, with a
> winding on each bobbin - but these are less-common.
>
> Having this "dual" choke is important as it forces
> common-mode RF
> currents to be canceled out. Since there are
> typically capacitors
> across the two sides of the power line on the "power
> supply" side of
> this choke, the two sides of the line are essentially
> shorted out at RF,
> allowing this choke to do its job.
>
> Unfortunately, these chokes often have only have about a
> hundred uH of
> inductance per leg, but some have far more: I've
> found that the "split
> bobbin" types (which are, unfortunately, a bit harder to
> find) usually
> have more inductance - sometimes in the mH range.
>
> - The bypass caps are usually a bit too small for effective
> work at LF.
> Often, these are 1000-2200pF and as the above example
> shows, these are
> too wimpy at LF.
>
> Good-quality high-voltage (400 WV or higher for a 120 volt
> AC mains)
> mylar capacitors in the 0.1-0.47 uF range are pretty easy
> to find and on
> the "line" side of the bifilar choke, one of these can be
> put between
> each side of the line (both the "hot" and "neutral") and
> the case
> ground. It is also a good idea to put one of these on
> the "power
> supply" side - between the hot and neutral.
>
> Now doing this poses somewhat of a hazard that is,
> unfortunately, a bit
> hard to avoid: Anytime you put a capacitor between
> line/neutral and
> ground, you are "lighting up" the ground with the reactive
> currents of
> those capacitors. If you were to measure between the
> "lifted" "ground"
> pin on a computer and the earth ground you'd see about 60
> volts (half
> the line voltage) of AC - but the current would be low,
> reactively
> limited by the 2200pF caps inside the computer.
>
> If you have, say, an entire workbench of gear, those types
> of capacitors
> in each piece can really add up, often causing a 60 volt
> "bite" if, for
> some reason, your ground wasn't really grounded. You
> can, therefore,
> imagine that replacing that 2200pF with a 0.22uF capacitor
> will make
> this even more "bitey"!
>
> Unfortunately, if you *really* need to effectively bypass
> the power
> supply at LF frequencies, this is hard to avoid - unless
> you can come up
> with some bifilar chokes that have several 10's of mH of
> inductance per
> leg which would still allow you to use the smaller (2200pF
> or so) bypass
> caps!
>
> 73,
>
> Clint
> KA7OEI
>
>
> Andy - KU4XR wrote:
> > What has been the experience from those who have tried
> this ??
> > The standard 3 prong black computer cord, wrap a small
> stranded
> > wire around the cord to use as a drain wire, then wrap
> the cord
> > with a layer of aluminum foil, place a clamp on
> ferrite core at
> > the outlet end, and at the computer end connect the
> drain wire
> > to the case, either directly, thru a capacitor, or
> inductor
> > which ever works, or leave it dis-connected if it
> makes things
> > worse. It would be a simple method, anyone have any
> sucess or
> > failure trying it ?? Of course at MW and LF
> frequencies.
> >
> >
> >
> > Andy - KU4XR - EM75xr - Friendsville, TN.
> > LOWfer Beacon " XR " @ 185.29866 KHz ( QRSS-60 )
> > Coordinates: N: 35* 43' 54" - W: 84*
> 3' 16"
> > http://www.myspace.com/beaconxr
> > http://webpages.charter.net/ku4xr/
> >
> >
> >
> >
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