[Lowfer] Sheilding power cords with aluminum foil ??

Mon Jan 25 17:54:52 EST 2010

http://www.apcmedia.com/salestools/FLUU-5T3TLT_R1_EN.pdf

http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=PMP1X%2DA 

Im investigating myself but above and I note the APC technical library has good info which can related to 

LF operation and the types of issues (some very confusing) -  Ive asked APC for a attenuation graph over frequency for the household filter.

Laurence KL 1 in Five

> Date: Mon, 25 Jan 2010 15:44:22 -0700
> From: turner at ussc.com
> To: lowfer at mailman.qth.net
> Subject: Re: [Lowfer] Sheilding power cords with aluminum foil ??
> 
> 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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