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Ask Slashdot: Wooden Chasis and EMF

Cliff posted more than 15 years ago | from the now-thats-just-a-cool-idea dept.

Hardware 196

Red Leader asks: "Hi. I'm writing in the hope that some electrical people will be able to help me out with the nitty gritty aspects of shielding a computer case. You see, I'm building myself a computer case out of wood. I have already built one, but it was merely a wood case (replacing the plastic) which overlayed a standard metal chassis. Well, that was too heavy, and I also want to use my own layout for the 'guts' of the box. So aside from heat, noise and grounding (which i've pretty much figured out) - I'm worried about the electromagnetic interference aspect of this new machine. It's most likely going to be a dual Celeron based Ultra-Wide SCSI2 system - which I think (!?!) will generate quite a large electromagnetic field. I've been thinking of shielding the case by lining it with this with copper fabric or something similar - but don't know how it sizes up to a solid metal case. Any help would be greatly appreciated - and I'll be sure to post pics of the case (well it's actually more than that) in progress and when completed.

Sorry! There are no comments related to the filter you selected.

Conductive Paint (0)

Anonymous Coward | more than 15 years ago | (#1848742)

You can get conductive paint ( powdered Nickel ) at electronic supply stores. It's meant for just this kind of thing. You've probabaly seen it, it's the grey stuff inside high-quality plastic cases.
Cool thing about wood-- where you need a grounding strap, just nail it on :)

Cowardly as ever

We are Linux. Resistance is measured in Ohms.

Sounds good so far (0)

Anonymous Coward | more than 15 years ago | (#1848743)

Copper fabric would work fine, as would aluminum foil (assuming you could get a good electrical connection to it) or even conductive paint (this has been used on some plastic cased computers).

As stated elsewhere, the real trick is to make certain that all shields are properly grounded and that there are no large gaps (ventilation is of course okay).

The trick in EM shields is not the thickness of the shield, but rather its conductivity and the size of the openings (smaller openings cut off lower frequency radiation better).

Seems to me there's no reason that a wooden computer case couldn't be both well shielded and beautiful as well.

Good Luck!

ElectroDAG is what you want (0)

Anonymous Coward | more than 15 years ago | (#1848744)

do a search for "Electro Dag" this is a conductive
paint type substance used to do EMI shielding
on plastic enclosures.

Re:Faraday cage? (0)

Anonymous Coward | more than 15 years ago | (#1848745)

Faraday Cages work both ways. It is this that allows Foreign Embassies (ours and others) to keep people from reading the EM from the electronics without having physical access.

Re:Shielding? (0)

Anonymous Coward | more than 15 years ago | (#1848746)

Not quite the point. I'm not sure whether of not computer equipment falls under FCC part 15 (since it's not designed as a transmitter), but they state that along with accepting any interference from other devices, the device in question must not *cause* any interference to others. Hence the need for shielding.

it will work (0)

Anonymous Coward | more than 15 years ago | (#1848747)

just remember to make it with no gaps and grounded to your power supply. Also use copper if you can. If you want to make sure it is grounded to the metal, there is a rf gasket that radio's use that is like steel wool and rubber mixed together. It will make a good electic ground.

Measured in Oms? (0)

Anonymous Coward | more than 15 years ago | (#1848748)

Whoa, that's heavy. :-)

Re:Sounds good so far (0)

Anonymous Coward | more than 15 years ago | (#1848749)

>(smaller openings cut off lower frequency radiation better).

You misspelled "higher"

Re:That's along the lines of something I'd like to (0)

Anonymous Coward | more than 15 years ago | (#1848750)

I know one of the SoundBlaster AWE64 Gold [soundblaster.com] cards has a SPDIF connector. That would eliminate a lot of the noise! Of course your equipment has to accept it. It would be great for converting mp3's to DAT!

I did get this cheap all-in-one pentium motherboard (PcChips I think) which has an SPDIF input from the CD-rom but the sound was somewhat distorted.. maybe it was in the wrong format or something. Not like I could hear any difference between pure digital and an analog cable with those cheap computer speakers.

Re:You could always try lead. (0)

Anonymous Coward | more than 15 years ago | (#1848751)

But _do_ stay away from the arsenic, please.

Re:Topless Dancing (0)

Anonymous Coward | more than 15 years ago | (#1848752)

It might matter if you have other computers around it. I think they might get upset about that. I think most of the interference is going to be the power supply, which is always enclosed in it's own case. I seem to remember some guy who claimed he could crash computers from some distance by using a high-power radio transmitter. Probably not very safe for the user at the time.

Re:That's along the lines of something I'd like to (0)

Anonymous Coward | more than 15 years ago | (#1848753)

Use a sound card with S/PDIF digital output (coax or optical, optical removes electrical connection interference possibility) and purchase an outboard D/A converter. Run that to your Video 2 or whatever input, and you're there. No noise added by the PC, except in the digital processing, and fine sound due the the relatively higher quality of outboard D/A converters. Have fun, sounds like a cool project.

Chris

Re:iMac has bad shielding (0)

Anonymous Coward | more than 15 years ago | (#1848754)

I can just see Steve spending his many hours at Apple and Pixar playing with the Toy Story interactive CD-ROM on a lime iMac.

Re:Wire mesh is good start, ground it well (0)

Anonymous Coward | more than 15 years ago | (#1848755)

So, is my PowerBook G3 a big antenna?

Re:iMac has bad shielding (0)

Anonymous Coward | more than 15 years ago | (#1848756)

Do you somehow want some kind of ego validation from the rest of the slashdot readers by:
A)trashing the iMac
B)parading your linux.com status?

Back when I were a lad. (0)

Anonymous Coward | more than 15 years ago | (#1848757)

Painting the inside of the case with a metallic based aerosol sould work.
Back when I had an Acorn machine it used to interfer with the TV something horrendous (UK safety reqirements are somewhat more las than US).
Spray painting the inside of the plastic case with car paint and then earthing it worked a treat.

Shielding Your PC (0)

Anonymous Coward | more than 15 years ago | (#1848758)

You're Worrying about the wrong piece of equipment. You could have you machine running in a cardboard box and the emf thrown out of your PC would be nothing compared to the interference caused by your monitor....

Re:iMac has bad...not really (0)

Anonymous Coward | more than 15 years ago | (#1848759)


What you describe occurs even with "well" shielded monitors. As you know, CRTs use a magnetic field to control the scan pattern of the electron gun. Because of this, monitors are very sensitive to magnetic fields. For example, if you position your monitor close to where the AC mains come into your house and/or near the circuit breaker panel you will likely see some "jitter" in the display - this jitter will get much more visable when large appliances (like the range or washing machine) are running. This jitter goes away as you position the monitor further from the mains. Sometimes you might see this in an office setting if your work area is close to an electrical closet, etc.

Now, a monitor will also pick up magnetic fields from other near by equipment - other monitors in particular, especially if the display scan rates are different on each monitor! That is likely what occured - the iMac was probably at a different refresh rate than your display.

You are correct that the iMac is, perhaps, slightly less shielded than a typical PC but the iMac must meet the FCC regulations just like any other piece of electronic equipment. And, if you really take a look at the iMac, you will notice that what you see through the plastic is mostly a metal frame - you dont really see much of the bare electronics - especially not the CPU, etc. which operate in the higher RF range and must be fairly well shielded.

Re:You could always try lead. (0)

Anonymous Coward | more than 15 years ago | (#1848760)

> Heat and airflow from the fans would probably cause quite some lead in the air coming out of the thing, doesn't sound too healthy..

I bet there is no case-elves using grinders on the lead nor the lead is in liquid state. Lead isn't exactly asbestos.

Re:iMac has bad...not really (0)

Anonymous Coward | more than 15 years ago | (#1848761)

Another example of this kind of inteference with monitors is to put a mobile phone near to the screen and watch when it rings :)

No need for earthing (0)

Anonymous Coward | more than 15 years ago | (#1848762)

If an electricaly conductive
shield surrounds the object completely
then all electrical fields are shielded,
without a need for earthing.
It is also true for magnetic fields, if the shield
is magnetically "conductive".

Re:A mesh would only block lower frequencies (0)

Anonymous Coward | more than 15 years ago | (#1848763)

I believe you are correct. As re the freqs blocked, as best I can recall from P-Chem,
wavelength = frequency * c

c= the speed of light= 3e5 kps =3e10cm/second

In this case ;) if the mesh was 6 mm (approx 0.25"), then the frequency blocked would be
equal to 3e10cm/sec/6cm or.. 5e9/second or 5 GHz.

Please someone, check my math.

Jim Gwyn


Suckered (0)

Anonymous Coward | more than 15 years ago | (#1848764)

This was a Flec-tron add, through and through.

I thought I would comment about how U.S. Defense Mapping Agency in Glen Echo, Maryland (you know, the agency whose satelites last flew over Yugoslavia six years ago) has a mil spec for wrapping their *entire buildings* in copper mesh, to protect their computers from incoming EMF in a nuclear war. (I forget the required db drop in EMF density, but they were up there and plain old screen door copper mesh with lots of mechanically strong grounding apparently does the trick.) I was going to suggest that (plain old screen door copper mesh would probably do the trick) until I saw the ad.

Bang. Ya got me. That was a flec-tron setup all the way.

try a guitar manufacturer (0)

Anonymous Coward | more than 15 years ago | (#1848765)

I am about as far from being a physicist as it is possible to be, but even so it seems to me that perhaps a maker of electric guitars would be able to give you some handy pointers on the materials and methods employed in the process of managing electronic emissions within a wooden structure.

Faraday cage and shielding (0)

Anonymous Coward | more than 15 years ago | (#1848766)

A faraday cage need not be "earthed", and a metal screen material purchased at the hardware store should provide good shielding. The problem will be to keep signal paths/shields and power paths/shield separate to avoid ground loops. Thay can be no end of joy. Remember, the shielding must have no gaps. The biggest problem will be, cables passing through the shielding which will allow EMI generated in the box to pass outside, and EMI generated outside the box to pass inside. Recommend careful shielding and use of filter circuits to provide the isolation(at a particular frequency). Good luck.

Re:Shielding Your PC (0)

Anonymous Coward | more than 15 years ago | (#1848767)

More important is shielding your computer from your monitor

How about my TV! (0)

Anonymous Coward | more than 15 years ago | (#1848768)

When I have my TV and my box, which are about a meter apart, My cable reception is terrible. I tryed turning off the monitor and putting it in to suspend mode. (Two of the four fans go off) All this doesn't make a diffrence. I can't move it further away since I have a really small room. The Bios option about emf emission is truned off but this doesn't make a diffrence
Topless or not doesn't matter either. And no I don't have a cheap plastic case i have a heavy metal chassis. The main's aren't grounded so I thought that would be the problem. But again no it wasn't. I took a cable and connected it on a part on the topless chassis. I then hooked it up the the gas pipe used for the heater. So That is surley grounded.
The sparcs were pretty neat, when I connected the cable and forgot to turn it off.
I even hooked the coaxial cable shield from the TV to my "ground" No diffrence at all.

So if you got any help, suggestions, tips, hints or whatever...
And yes the Source is my box. And it doesn't matter what OS i am using.
oliver@mediaone.net [mailto]

degrees of mechanical freedom from ... humidity? (0)

Anonymous Coward | more than 15 years ago | (#1848769)

With metal cases, the usual problem is thermal expansion of the board relative to the case as it heats up. So, if you look at a good build, everything has freedom for relative expansion-contraction during warmup, without stressing the plastic layers and insert connections, memory cards included. On a metal case, relative expansions are probably on the order of 10e-3 inch, but even those small changes can produce tremoundous stress densities at critical points, such as at the processor socket, and cause a separation in the board.

With a wooden case, we add new orders of magnitude to this problem. Based on humidity changes from summer to winter in my basement, it would not surprise me that an oak case (coeff of expansion for moisture = ??) might expand a quarter of an inch or so. Not only will the case itself need the required degrees of freedom and degrees of strength, per laws of carpentry (ugh), but the entire mb-cards will need lots of float in the wood case, and all of the card inserts will need lots of differential float relative to the mb and the case, each of which are expanding in different directions. The back plane (holding pinouts) will tend to expand and contract in one direction, and the side plane (holding mb) in another.

The emf is no challenge. Copper mesh with soldered grounding. But the interesting challenge will be stress free mounting of the board while at the same time having a strong case. Would like to hear a follow up on how he sets the grains of the case, works out the aesthetics, and achieves the requried mechanical freedoms.

Re:Shielding? (0)

Anonymous Coward | more than 15 years ago | (#1848770)

Also, it's not like the FCC will knock on your door...

Not to up the flamage, but if you have a ham/CB/etc... radio operator, small-craft (or big) airport, or other place that uses radios (next door with their portable phone perhaps), then yes, you are inviting the FCC over for supper... (If they get a complaint, they'll investigate; Watch for the white van outside! ;-)

If you're using the computer in a basement, or are at least about 100 ft. away from annoying anybody, go ahead. Leave the case open... I don't beleive the crap about the health effects of EMI myself...

Re:You could always try lead. (0)

Anonymous Coward | more than 15 years ago | (#1848771)

Does lead chip easily? That might be a factor against it.

I wonder how much lead lead paint contained. It would be interesting to know if a room painted with lead paint reduced EMF...then again, I seem to be pretending to remember that the lead in the paint served a purpose that made painting or paint making easier.

Anyways, just thought it would be ironic that if the lead in lead paint was at high enough levels to reduce EMF from getting out of a room, that we phased it out in the late 70s, about the time electronics was booming and home computer use was increasing.

Shielding? (0)

Anonymous Coward | more than 15 years ago | (#1848772)

Who the hell worries about shielding???? I've mounted PC's on a piece of plywood.

Re:That's along the lines of something I'd like to (0)

Anonymous Coward | more than 15 years ago | (#1848773)

This sounds like something I'm thinking about building as well but I don't have the audio requirements you do, so I can't help you wrt the cards (plenty of others have).

But regarding the control of your box, look into the VNC program. It's free, works on a variety of platforms, and allows your to control your headless computer remotely by replicating what would be your displayed desktop (is your desktop, if you had a monitor).

Re:iMac has bad shielding (0)

Anonymous Coward | more than 15 years ago | (#1848774)

Oh, by the way, no real work gets done on the iMac--it's just there to verify Web sites and to, well, look cute and attract investors. Heh. No one deserves the torture of having to work on an iMac :)

So you bought a machine to attract investors and look cute for your company? A linux.com user, if true, selling out. Heh.

btw, not Apple's fault you couldn't figure out the thing. This is the "Gee, I needed a B&W but bought a 603. Why does it run slow?" syndrome. Other examples include, "I need to do high end graphics but stuck with the 64 megs. Why's my machine so slow?" Or the one we hear a lot, "I needed a high end server and bought NT".

Plenty of people do "real work" on iMacs. Maybe you didn't account for what you needed to work on when one of you folks ran out to buy it. That's your stupidity for wasting money. Shoulda just spent $60 on a 586 outfitted with 32 megs, 230 mb drive, slap a network card, and painted a mini tower case with some Rustoleum and Lexan model paint if you wanted a web terminal. Would cost you $100 max plus some break time and monitor.

Liquid metal (0)

Anonymous Coward | more than 15 years ago | (#1848775)

The supply company Grainger (did i spell that right?) offers a liquid alminum that you can apply with a brush. They also sell a aluminum putty that works equally well. If I'm not mistaken I think they offer copper versions as well.

emf screening and mechanical expansion/contraction (0)

Anonymous Coward | more than 15 years ago | (#1848776)

Expansion of wooden parts of the case may mean each part will need its own shielding and mechanical ground.

The wooded pieces will probably expand and contract differently than the emf shielding, of whatever form, and the emf screen and ground system may also need mechanical float. (If a single wrap of screeing is glued to two perpendicular pieces, it could become separated with repeated expansion and contraction from humitidy, and lose its ground.) That probably means an organized bundle of groundwires running around the case.

By adding some interior ducting for intake air on an atx positive pressure system, the sound deadening might be really cool. We're so used to the louder and louder whirr from all these new fans.

Re:I don't think you have to try (0)

Anonymous Coward | more than 15 years ago | (#1848777)

Is there a reliable, quantitative, and sensitive way to measure EMF for people looking to build their own cases? Cheap too. Or do I need an EMF proof room first?

Been thinking about building a plastic case lately since almost all non-custom PC cases (there are a couple of exceptions) absolutely suck in looks. On the other hand, maybe just buying a used case and painting it would be cheaper and just as neat looking.

Sorry if this was already covered; I'm sorta whizzing through the comments and haven't quite figure out this nesting stuff.

Re:Shielding? (0)

Anonymous Coward | more than 15 years ago | (#1848778)

Those of us with radio-based equipment in the house... like say a television (for Discovery and cartoons) and radio (duh).

I'd say to go with a grounded cage. the ground is just to help keep it from resonating.

- josh, another moron who lost his password
jbm@intertek.net

shielding... (1)

Anonymous Coward | more than 15 years ago | (#1848818)

The two things to remember are to make the shielding continuous (i.e. no gaps in the corners, etc.) and to ground the shielding inside the case.

If you fail to ground the shielding, you defeat the purpose of it, which is to keep the voltage potential constant.

Shielding RF (1)

Anonymous Coward | more than 15 years ago | (#1848819)

The ability to shield RF with a faraday cage is easily accomplished. The key element to keep in mind is what frequencies to I want to shield? In this case you are trying to keep frequencies of approximately 400mhz(or below from other chips) inside the box. Since the wavelength at this frequency is in the range of 70+ cm, you should
if a wire mesh is used), have no gaps or openings
bigger then about 10cm(probably overkill). I forget the exact numbers but generally as long as any gaps you have are less then 1/4 wavelenght at the frequency in question, you should do a fine job of shielding.

In addition to wire mesh, any ferrous metal should do just fine, regardless of thinkess, since
the idea here is bouncing light off a mirror, rather then stopping a bullet with a wall, to use a metaphor.

One final note..if you're like most people, you are much more likely to encounter problems from RF getting into your computer then RF escaping (radios with a few watts of output power near unshielded hardrives can sometimes produce
unpleasant effects)

Jean B

General Radiotelephone Lisc., Advanced Amateur.

A mesh would only block lower frequencies (1)

Anonymous Coward | more than 15 years ago | (#1848820)

My days as a radar tech. tell me that all the other posters are correct.
A faraday shield is an EARTHED conductive material that COMPLETLY surrounds something, and electromagnetis radiation from either side will strike the shield and be drawn to earth.
If your equipments isnt earthed properly the shield does nothing, the electromagnetism will be re-radiated by the case.
As far as a mesh goes i beleive that a mech will block lower frequencies but not higher frequencies. If the wavelength of the elecrtomagnetic radiation is less than the diamater of each hole of the mesh then the shield will do nothing at that frequency and higher.
The above is how waveguides work (waveguides dimensions have to be a multiple of the wavelength)

This is probably way too much information, but i couldnt resits dredging up some old memmories

copper fabric done properly will do it (1)

Anonymous Coward | more than 15 years ago | (#1848821)

To do it right, you will need to cover every area possible (except for door openings) and you will need to make sure that _ALL_ of the fabric is _PROPERLY_ grounded to an _EARTH_ ground. This is not as easy as it seems.

I have some experience in this area, some years ago I repaired monitors that had very noisy switching power supplies and the FCC "busted" me because we blocked out the cop station next door with our interference. The solution was to encase our monitor racks in screen that we grounded. I put a little radio next to the racks and kept it on, if I got interference on that then I knew something was wrong and had to shut down. It worked like a champ!

Re:EMF protection for a wooded pc chassis (1)

Anonymous Coward | more than 15 years ago | (#1848822)

One other thing to remember about figuring out what size mesh to use when shielding PC's...harmonics. The PC bus may be using a 100 Mhz clock or the CPU running off a higher clock but that is not a sine wave, it's a square wave & square waves have harmonics, the degree of which depends on the rise & fall times. But since even Ku band satellite dishes (12-14 Ghz) have a mesh around the size of a window screen, those should be fine (window screen, not satellite dish ;-) ).

GOESMan (awaiting password)

I don't think you have to try (1)

ESD (62) | more than 15 years ago | (#1848823)

If I remember my EMC class correctly (which is about 2 yrs ago) even metal computer cases hardly provide any EMC shielding.

The problem lies in the fact that there are gaps in any computer case (disk drive, CD-ROM, anything that can be seen on the outside of the case). These gaps leak almost as much of your EM fields as having no case at all (at least at the clock frequencies used by your computer).

The only normal case I have seen that shields _something_ is my Alpha XL case, which is lined with EMC springs around every gap. This prevents the computer from disturbing my screen too much (and yes, it even makes the screen wiggle when the case is closed).

If you want EMC protection, you need a special EMC cabinet and knowledge about connecting cables to such a cabinet (you can't plug in cables just like into an ordinary computer).

Gtnx
Marcel de Boer, 3rd year student EE/IT

iMac has bad shielding (1)

palpatine (94) | more than 15 years ago | (#1848824)

Ugh. iMac's monitor isn't shielded very well. At the place I used to work, someone turned on the monitor for the iMac next to where I was working, and my monitor got funky for a little while. It's like I degaussed it for the first time in years or something.

Oh, by the way, no real work gets done on the iMac--it's just there to verify Web sites and to, well, look cute and attract investors. Heh. No one deserves the torture of having to work on an iMac :)

Re:Faraday cage? (1)

Bill Currie (487) | more than 15 years ago | (#1848827)

I'm not certain myself (been a while since I did anything electrical, and I never fully groked FCs anyway), but wouldn't this sort of thing work in both directions? ie fields outside don't affect the inside and fields inside doeen't affect the outside. I suspect it depends very much on your grounding.

Re:Faraday cage? (1)

Bill Currie (487) | more than 15 years ago | (#1848828)

Thought so. Thanks for the confirmation.

Re:That's along the lines of something I'd like to (1)

gavinhall (33) | more than 15 years ago | (#1848829)

Posted by viperx2:

The Diamond MX300 has a daughterboard, the MX25 I think, that is fully digital and I think I read that it is compatable with minidisk format, so I assume it is also DAT ready. They use the same cables and stuff. The MX300 was 50$ on pricewatch, and the MX25 is available from diamond for only 25$. I looked at this a while back, so my numbers are off. The MX300 works in linux, while the SBlive does not. The MX300 is truly a fraction of the price, and, from what I understand, a much better card.

ViperX2
-- The more you tighen your grip.... --

Re:Use thinner sheet metal. (1)

gavinhall (33) | more than 15 years ago | (#1848830)

Posted by iam1:

I would agree with the person above who said to check out the electronics setup in guitars. I am not sure about the copper "fabric", but a thin sheet of copper shielding will do everything fine. As far as grounding or "earthing" it, your power supply would do that for you.

Re:A mesh would only block lower frequencies (1)

Andy Dodd (701) | more than 15 years ago | (#1848833)

That sounds right... Even 10 GHz has a wavelength of a few mm, from what I can remember... Unless you're using that .25-inch or so mesh, you should be safe.

Beads (1)

Andy Dodd (701) | more than 15 years ago | (#1848834)

You can't overestimate the importance of beads. All the machines in my house have full metal cases, and kill TV reception, unless you put inductors on every single lead. (Especially the power cable, as I think RF goes down that and up into our TV amp.)

Depends on the application and the ground type. (1)

Andy Dodd (701) | more than 15 years ago | (#1848835)

You should see the ground systems some hams have. But this is a lot more than what someone needs to install just for an "adequate" ground.

It also depends on the conductivity of your dirt. A swamp is a lot better than a desert. :) The ARRL Handbook (Basically one of the bibles of amateur radio) has a LOT of information on this.

Re:good shielding is not that hard to achieve (1)

Andy Dodd (701) | more than 15 years ago | (#1848836)

As far as if you can't see the problem, no one else will - I have to agree with you on that.

But if there is a problem, it can possibly occur house-wide. The only thing I can think of is that RF is leaking through my AC mains and into our TV preamp. Whenenver any machine in my house is running, Channel 11 gets funky. Putting ferrit on the PS cable and a few others helped a lot, but it's still pretty bad.

Re:Faraday cage? (1)

j.e.hahn (1014) | more than 15 years ago | (#1848837)

If I remember my Electricity and Magnetism correctly, a farday cage is an equipotential. The idea is that since field lines (in either electric or magnetic fields) can't cross, then the fields inside the cage are contained inside, and those outside are contained outside.

So if you design something which is almost an equipotential you get an OK shield. Now, I'd think a fine enough mesh (something magnetic is probably best) would be suitable. But IANAEE and IANAP (an Electrical engineer or physicist).

Re:That's along the lines of something I'd like to (1)

Eric Sharkey (1717) | more than 15 years ago | (#1848839)

The Trident 4DWave NX based cards support this. They're still a little hard to find, but you can order them from Hoontech at http://www.hoontech.com [hoontech.com] . They offer quite a bang for the buck.

Trident actually wrote a GPL'd Linux driver for this thing, so it's a good card to buy.

Re:A mesh would only block lower frequencies (1)

Liquid E. (1864) | more than 15 years ago | (#1848840)

Ah yes, first year physics...

v = f * lambda, where v = c (speed o' light), f = frequency and lamba = wavelength.

c is more accurately represented as 299 792.5 km/s, or 299 792 500 m/s.

At a wavelength of 6mm (0.006 m), we have:

f = v / lambda = 299 792 500 m/s / 0.006 m = 4.99654 x 10^10 Hz

So, it's actually closer to 50 GHz (I think you forgot to convert 6 mm to 0.6 cm in your equation).

Then again, I'm in math - what do I know? :)

Wow, good old days... (1)

tjansen (2845) | more than 15 years ago | (#1848842)

Reminds me of my father almost 20 years ago, building a keyboard with a wodden case for the TI99/4A. It looked like the Flintstone's computer and was a long - running gag in my family...

SPDIF! (1)

Zippy the Pinhead (3531) | more than 15 years ago | (#1848843)

Dolby Digital, DiscMan and DAT all use SPDIF for digital I/O. More and more soundcards support SPDIF, but beware: some only sample and output at one speed (SBLive=48K). If you're going for analog I/O, spend money; some cards cost more for a reason.

SPDIF runs over fiber or copper, converters are available. The SBLive! forums seem to have good discussion of SPDIF issues. There are also cheap SPDIF-only cards available. Some are supported under Linux, I believe.

'twould be nifty to have a thinclient with speakers in each room, feeding off a MP3/netradio server. Of course, decompression lag would lead to echoey audio between rooms. Better to just use USB speakers.

Copperweld (1)

Stormbringer (3643) | more than 15 years ago | (#1848844)

Use copper-clad steel sheet if you can get it. That way you get the benefits of magnetic as well as electric shielding (an RF emission has both aspects).
If you really want to cut emissions, get some tin-snips so you can cut sheet stock to shape. Look at current cases to see how they even have petaled shielding cupping the keyboard connector to prevent leaks there.
You'll also want to get some clamp-on ferrite beads (All Electronics in LA is my source) for the cables. The beads are lossy inductors at the intended frequencies, so they eat signals that would otherwise be emitted (or injected: you're protecting the system from arriving interference as well as preventing it from emitting interference, after all).

Hmm, another issue will be thermal... (1)

Stormbringer (3643) | more than 15 years ago | (#1848845)

You'll have to be more careful to have good airflow throughout that box, compared to an equivalent metal box, because the wood is a better thermal insulator than metal, so the case itself won't throw off as much heat.

Re:EMF protection for a wooded pc chassis (3)

Ares (5306) | more than 15 years ago | (#1848855)

I seem to remember that a 100 MHz signal has a wavelength of 3m (c/100Mhz). Likewise, a 500 MHz signal has a wavelength of 3/5m = 60cm. Even at 1GHz, the wavelength is 10cm. I'd say you're fairly safe using a mesh (at least for now).

Just use aluminum foil (1)

aXi (6533) | more than 15 years ago | (#1848856)

Just stick aliminum foil to the inside of the
case, just do not forget to connect it to ground,
also connect the motherboard to ground. All the
individual cards need to be grounded too.

Just use aluminum foil (1)

aXi (6533) | more than 15 years ago | (#1848857)

Just stick aliminum foil to the inside of the

case, just do not forget to connect it to ground,

also connect the motherboard to ground. All the

individual cards need to be grounded too.

EMF protection for a wooded pc chassis (1)

MartinD (7344) | more than 15 years ago | (#1848858)

First of all , there are three "S"s in CHASSIS.

And secondly, you are all thinking too
complicatedly (sic), just take some metal mesh
(screen door?) and staple it to the insides of
the wooden computer case. And yer it is a
faraday cage, and yes it keeps emf out and yes
it keeps emf in. I should know this crap, I have
a degree in physics.

shielding (1)

Dale (10863) | more than 15 years ago | (#1848864)

I've used a a couple coats of carbon paint to shield against RFI in electric guitars and even good old heavy duty tin foil (what a bitch to tape down though). This might do the trick for you, you just ground it with a screw. Thin sheet metal would be best but if your case is solid hardwood the weight savings of paint would help.

Re:Topless Dancing (1)

1101z (11793) | more than 15 years ago | (#1848865)

I have used 4 to 5 five computers in the same room with the covers off with no problem. Sound was fine, moniters were fine, TV was fine, and radio tuner card was fine. Why would computers be bothered by the EMP created by computers, They are always inside the Faraday cage of the case.

The amount of interference from AC wall power is worse in my experiance then the interference then a computers with there covers off.

Re:I don't think you have to try (1)

1101z (11793) | more than 15 years ago | (#1848866)

I don't understand why a dual cpu system would matter. Aren't you going to put heat sinks with fans on the cpus which are metal and the fans power is a source of ground. As for the SCSI card you are worried about interference there well if scsi had a problem with interference then it would not run inside the case where there is lots of interference, or outside the case in places like computer rooms.

I also remember reading in a computer manual some where that if you get interference to try moving stuff from behind the computer, they admit that the fan of the power supply is a great big EMP hole.

Re:You could always try lead. (1)

earthman (12244) | more than 15 years ago | (#1848868)

Heat and airflow from the fans would probably cause quite some lead in the air coming out of the thing, doesn't sound too healthy..

Re:Shielding? (2)

jetson123 (13128) | more than 15 years ago | (#1848870)

Running your computer without shielding is like turning your stereo all the way up. You are likely to disturb someone else's radio reception or amateur radio station. You might also be disrupting digital transmission (wireless printers, etc.).

Unlike acoustic noise, it's a lot of work for people affected by radio interference to figure out first of all that it is happening at all (rather than that their radio or digital wireless system is broken) and where it it is coming from.

Causing interference like that is also against FCC regulations, and people can track you down and have you fined.

Unless you live on a desert island, if you run your computer open, sooner or later you are going to cause other people a lot of unnecessary headaches. So, please shield your computer.

Topless Dancing (1)

detailer (13448) | more than 15 years ago | (#1848872)

Many of us run caseless w/o any problems. It really depends on what else you have around it. Neither wood nor plastic shield EMF. Sometimes running without a case is nice if you're lazy like me or just like blowing the dust around.

Re:Shielding? (1)

Tyrell Hawthorne (13562) | more than 15 years ago | (#1848873)

So, if I don't shield it and cause interference with my mobile phone the world will end? As long as you haven't got anything very sensitive I'd say it's it's OK to "cause interference", but of course, I'm just in tenth grade and know nothing about physics...

Also, it's not like the FCC will knock on your door...

Re:iMac has bad shielding (1)

bjwest (14070) | more than 15 years ago | (#1848876)

> Ugh. iMac's monitor isn't shielded very well.

Not many monitors are shielded very well. All but the expensive industry-quality monitors are encased in nothing but plastic.

Faraday cage? (1)

st. augustine (14437) | more than 15 years ago | (#1848877)

Never tried it myself, but I'd think a metal mesh of some kind would work about as well as solid metal -- after all, a Faraday cage doesn't have to be solid.

But wait, all a Faraday cage does is ensure there's no EMF inside the cage, which is sort of the opposite of what you want... someone with more electrical engineering knowledge bail me out here?

Re:iMac has bad shielding (1)

Compuser (14899) | more than 15 years ago | (#1848878)

>>No one deserves the torture of having to
>>work on an iMac :)

Well, there is one Steve who sure deserves it.

That's along the lines of something I'd like to do (2)

grappler (14976) | more than 15 years ago | (#1848879)

I would like a "music server" in my house. That is, connecting an old computer with no peripherals to a music system. I would control it by telnet or a web interface. Obviously, good sound quality is a high priority, and that might very well mean shielding.

Actually, though, what I would really like is an all-digital sound card, with any analog processing completely cut out of the loop. Ideally, it would output a digital signal that a good home audio system will decode, such as Dolby Digital, if that is possible. Or maybe the DAT format.

Anyone know how, with digital signals, shielding, or some combination of the two, you could make the best, cleanest audio signal possible? This wouldn't just be for MP3s, but also higher quality stuff. Any advice on this greatly appreciated.

Foil of sorts... (1)

Etriaph (16235) | more than 15 years ago | (#1848880)

I work at a printing company, and some of the packaging we have to do is done with wooden crates lined with a kind of aluminum foil where one side is, likely enough, aluminum foil, and the other side looks more like utility paper, or packing paper. The machinists in the building actually use the same foil to prevent an EM field, which causes nausea when working with the two pieces of equipment we do have that are highly electric. They were given this hint by some other printing company in Britain who uses the same devices.

I'm not sure where you can purchase it exactly but with a little poking around I'm sure you could find it. One side looks like normal aluminum foil (a little higher gauge however) and the other like a brown paper bag. If you try it, lemme know if it works, I might just comission a computer that doubles as an endtable. :)

All-digital sound card? (2)

Anonymous Freak (16973) | more than 15 years ago | (#1848882)

Well, the only one I know of (that's not a professional card) is the SB Live, full edition. With it's digital I/O card and the extra optical digital card, you can input digital, process the signal digitally, and output digital. It can even mix digital audio, internal digital CD, digital MIDI, one analog source, digital SP/DIF, and optical digital all at the same time. I use my computer as a piece of my stereo system, with the internal digital CD, two digital in's (MiniDisc and an external digital out CD player,) and MP3; all being processed for better sound (if you're running [god forbid] Windows, try using the "Concert Hall" environment for classical music.)

And, of course, it's 48Khz digital SP/DIF out is great for recording to MD or DAT. (Or going to a high-end reciever)

Use thinner sheet metal. (1)

Gumber (17306) | more than 15 years ago | (#1848883)

I don't know if copper cloth with cut it, but reatively thin sheet metal should do it. This is what they use for a lot of plastic cased PCs Make sure that the sheets are well sealed and are electically contiguous.

good shielding is not that hard to achieve (1)

wmeyer (17620) | more than 15 years ago | (#1848885)

Your computer will generate RFI, and very little EMI. Copper mesh will work well, and is far easier to connect to than aluminum foil. If you emit significant RF energy, you will run the risk of upsetting neighbors who are ham radio operators, or perhaps will interfere with TV reception.

If you know of any nearby hams, they can help you with practical tips on how best to shield your case. A ham can also scan some of the radio bands looking for your noise, if willing.

To check for TV interference, use a TV with rabbit ears (not cable) and check the low VHF channels (2-6). Those are the most likely to be affected. If interference is present, shut down your PC to be sure whether it is the source.

Also be aware that the noise signature of a machine will vary as a function of what it is doing. Disk access operations will change the RF compared to purely CPU based operation, so do a variety of things with the machine while checking.

Also, RF, as with any field energy, decreases as the square of the distance, so move the TV within 6 feet or so for initial checking. If you can't see a problem there, chances are no one else will, either.

Re:No need for earthing (1)

Fatllama (17980) | more than 15 years ago | (#1848886)

While magnetic flux is shieldable, magnetic fields are not. Also, one must ground the shield to hold it at constant potential (Earth serves as the best source of such a voltage).

- Jeff Sherman
Physics undergrad, UT Austin

Shielding? (1)

Brad Andrews (18226) | more than 15 years ago | (#1848887)

What's the big deal with EM worries? I run an O/C Celeron 400 with all kinds of sensitive parts (SBLive, DVD Decoder, etc.) just thrown caseless into a tiny little wood cabinet and I don't get the slightest bit of noise or interference in anything. I always figured the shielding bit was one of those stupid FCC regs that could be safely ignored.

Re:EMF protection for a wooded pc chassis (2)

Chipaca (18396) | more than 15 years ago | (#1848889)

You might just be forgetting Maxwell's time-dependant equations, then (specifically $\rot{\vec B}-\frac1c\pder{\vec E}t=\frac{4\pi}c\vec J$). As someone said further up, the problem with meshes is that they effectivly act as high-pass filters, quietly letting through any radiation whose wavelength is less than $\approx1/\sqrt{5}$ of the mesh length. You can check this easily: look through it :) .
I find that most of the vox-populi notions about Faraday cages come from radio people, where you only have to worry about very low frequency (long wavelength) radiation.
Hey, where is the `submit as TeX' button?

Re:You could always try lead. (1)

BobBoring (18422) | more than 15 years ago | (#1848890)

Nice try but lead is not a ferro magnetic media. Steel wire cloth (window screen) would work better, be lighter, less toxic. You can cut window screen with kitchen shears and staple it to the inside of the case.

(Yes lead is a conductor, a poor one. Faraday cages are great for weak electric fields but leak magnetic fields unless a magnetic conductor is used to trap that nasty B field)

While your children are not licking the inside of your case the heat from the components in the case will cause vaporous lead and the trace mercury found in all lead to out gas. In and enclosed area like the typical bedroom you could get heavy metal poisoning in a few months of exposure.

Yup, some people use lead as a shield against radioactive particles. So it blocks Superman's x-ray vision. Funny thing is, things like cement, paraffin and water are better shielding per unit weight. It must be some 'other' property besides particle capture cross section that blocks Superman's sight. Otherwise he should not be able to see through a concrete wall.


Re:Wire mesh is good start, ground it well (1)

Ted Nitz (18875) | more than 15 years ago | (#1848891)

3-4 feet isn't enough in a lot of areas, I have a friend who just put a new ground rod in, and code required something like an 18' rod in his area, he had one heck of a time getting that all the way down.
-Ted

Any URL's to other wooden case projects? (1)

mostejo (20983) | more than 15 years ago | (#1848893)

I'd like to see how a finished one looks and how hard it is to make one. I'm getting ideas of a tower case that doubles as a couch endtable....

emf-l mailing list. (1)

aatu (23608) | more than 15 years ago | (#1848894)

The emf-l mailing list comprises laymen, experts, and other concerned citizens. The topics are just about everything you could think of in the field (pun) of the ever-increasingly "blue" world that we're all living in. Unfortunately the list moderator and maintainer is (as of today) on a two week vacation which puts the list on hold as well. But, you may still join now (OH, the work load of the /. effect. Poor Roy...). Just send an email with your concerns to rbeavers@llion.org and surely there will be someone who is able to help you.

/Adam

PS. BTW, don't mail me, as my mail server is currently broken. DS

A Whole Lotta Tin Foil... (1)

Uart (29577) | more than 15 years ago | (#1848896)

And don't forget your tin foil hat.... I never take mine off. Those aliens... err electomagnetic fields, aren't gonna get me!!!!

You could always try lead. (1)

Mikesch (31341) | more than 15 years ago | (#1848898)

Several places sell sheet lead about a 16th or 32nd of an inch thick for use in soundproofing. I've worked with it, and it is really easy to work with, incredibly flexible and easy to cut. The only problem with this is that it is *really* heavy. But for shielding, there is probably nothing better. Hell, superman can't even see through it.

Before anybody says it is dangerous for children/animals, do your children lick the inside of your computer case? I thought not.

Re:Shielding? (1)

Paul Johnson (33553) | more than 15 years ago | (#1848900)

As long as you haven't got anything very sensitive I'd say it's it's OK to "cause interference"


Well, as long as I don't throw anything poisenous I might as well chuck crisp packets and other litter around. But of course, I know nothing about the environment.


Paul.

Tempest Protection (1)

Silex (34738) | more than 15 years ago | (#1848901)

What you want is Tempest-grade sheilding. Do some research on tempest.

Re:Sounds good so far (1)

iwoj (35166) | more than 15 years ago | (#1848903)

>You misspelled "higher"

You misspelled "lower"

Re:Topless Dancing (1)

Iron Mike (35916) | more than 15 years ago | (#1848904)

It is my understanding that your chassis ground (case) is an electrical reference point. nothing more. If you decide that your chassis is going to be a metal grate nailed to your wooden case, that is fine (or just use a nail). Connect the ground wire from your power supply to your chassis metal grate (or nail) and anything else you want grounded. This grate is then your ground (or chassis or electrical reference point). The chassis ground for those who don't have a case is the metal ring on the end of the ground wire connected to the power supply.
The bottom line is that it is not manditory to have something to connect it to (though it can be handy). You can leave the wire loose if you want (but anything touching to it is then grounded).
Make sure you are consistant with your ground (do not have multiple chassis points. you can only have one reference point) or you may run into problems

As far as EMF goes, I don't believe it is that huge of an issue unless your circumstances are unusual. The EMF generated by your computer is so small that it does not interfere with itself or other equipment around it. AFAK.

-Mike
mailto:mmangan@intergate.bc.ca

Re:Shielding? (1)

Unclaimed Mysteries (38972) | more than 15 years ago | (#1848908)

FCC Section 15.102 directly addresses this issue.

The big deal is poorly shielded computers are the biggest generators of radio frequency trash that other services have to deal with. Not ionizing levels, that's another story, altogether.*

RF interference drives hams crazy (ier). They constantly have to track down leakage from bad casing, bad connectors, cables, etc. Don't care about them? How about hospital patients monitored by sensitive medical equipment? A clean RF environment is important for these devices.

The noise generated by computers already extends well into the VHF range. With the number of computers rising and CPU speeds going up and up, the noise is going to collide with more services such as cell phones.

C. L. Smith's Unclaimed Mysteries
http://www.unclaimedmysteries.net/
*All: "That's another story."

Should work fine (1)

BarneyGuarder (44042) | more than 15 years ago | (#1848915)

The key in shielding is to surround whatever you wand shielded with a conductor. EM waves don't penatrate conductors very well. I'm not even sure you need to ground it, but it is a good idea for other reasons if not this one (unless you like getting electicuted). As a previous post mentioned, the smallest opening should be smaller than the wavelength of the EM radiation, in this case a few cm should do it.

Take a look at your mocrowave door sometime. It uses the same idea. THere is nothing special about the glass, it just has a piece of metal with small holes in it. The EM (microwave) frequency is about 2.5 GHz IIRC. Those little holes are smaller than that wavelength, so nothing gets through.

radiation? (1)

JaySWF (59721) | more than 15 years ago | (#1848929)

The way I understand it, each device in a personal computer (i.e. CPU, motherboard, RAM, HDD, graphics card...) has to comply with FCC-regulations concerning electromagnetic radiation.

So, there is IMHO no need for any additional shielding, since even a traditional (metal) case won't shield any radiation, because it has slots for CD-ROMs, Floppies and on the back side for cards etc.

Thought on shielding (1)

elint (60317) | more than 15 years ago | (#1848930)

I'm not sure there's all that much EMF created by the PC...But don't quote me on that. First, is there any EMF shielding on the *shudder* iMac? And second, I've seen a clear plastic case on a celeron game machine...Although this might not apply since both of my data points are extremely weak systems, as opposed to your dual-celeron with u2 scsi...

Wire mesh is good start, ground it well (3)

S_Walker (60381) | more than 15 years ago | (#1848931)

Wire mesh is just fine for basic emf and rf shielding...
It should be roughly contiguous, although, that is hardly worth pulling out a micrometer of magnifying glass.

Just make sure each seperate piece of mesh when assembled makes good electrical contact with the frame? (if you have some sort of framework on this) or other conductive bus. To test if this is roughly adequate,
1. get a multimeter (measures voltage/resistance and usually current, sometimes mislabeled a voltmeter)
2. set it to measure resistance or continuity.
3. touch 1 probe to one portion of mesh, the second to a point opposite...as in as far as you can get inside the case from point 1
4. look at the resistance readout.
5. It should read 0 ohms (if yours has a floating needle, it may be a bit off, not to worry) If it reads anything > 1 ohm, you don't have a significantly conductive material to be useful as a shield/ground...
(For those who will argue here...I just finished measuring the foil from a pack of marlboro light 100's that came out to 200 mOhms , or 0.2 Ohms, in case you cared.)

Now for the final key point. Ensure that the power supply is properly grounded, your shield is properly grounded, and that your new case is plugged into an adequately grounded 3 prong receptacle.

For clarification...if you don't plug it into a grounded receptacle, what you've just configured is essentially a floating common attached nicely to a power cord which sould make a passable antenna for rebroadcasting all the stuff you just collected.

If you need to add a better ground to your house...i.e. you get lines in the tv when you run the vacuum...talk to an electrician unless you are comfortable in a circuit breaker box.
You can get a 3-4 foot copper clad steel rod, with copper wire, to act as a grounding rod. have the electrician hook up the copper wire to the ground bar in your circuit breaker box, then you attach it with a clamp to the grounding rod.

Note..you're going to need a sledge hammer...preferably a big one...
Apply the sledge hammer to thetop of the grounding rod until only about 3-4 inches sticks out of the ground....
now attach that wire, and your house ground should be much better.

Whew...wrote a lot more than I planned...but you get the idea.

Re:Shielding RF (1)

Ping1400 (60391) | more than 15 years ago | (#1848932)

Computer clock-frequency is around 400 MHz. The resulting RF-frequencies depent on the steepness of the ramp of the signals. At a processor-speed of 400 MHz the RF distortion will reach at least upto 3 GHz. Gaps should be smaller than few cm.

The plates that construct the cage must have good inter-connection on the egdes, closing the box. (otherwise the plates could become antennas!)

For professional EMC protection (not protecting the environment for computer RF, but protecting the computer against high-voltage discharges or lightning) the complete machine should be encapsulated inside a metal cage (preferably copper) with only one hole on one side. All cables should enter through that hole and must be filtered at the entrance. All common-mode wires (ground/shield) should be perfectly connected to the cage at that entrance-point.

Wooden case (1)

azi (60438) | more than 15 years ago | (#1848933)

That copper fiber does just fine if you just remember to connect it to ground potential. Power case will do just fine and some times there is screw for that.

Another thing is that copper layer must be as continuous as possible. Electro Magnetic Interferences goes from any hole big enough for the wave lenght.

Re:Faraday cage? (1)

Cougar_ (92354) | more than 15 years ago | (#1848936)

The problem you would face with a faraday cage however is that the power cables etc. would need to exit it, and this reduces the effectiveness quite dramatically.
If you put an electromagnetic field inside a faraday cage, it will insulate it perfectly, but stick another wire through the side of the cage, and you've effectively turned off the cage.

Re:Wire mesh is good start, ground it well (1)

AtomSuite (121772) | more than 15 years ago | (#1848937)

Are you sure the 0.2 Ohms you read weren't from the resistance of your test leads?

Make sure to check the resistance through your test leads (since we are talking such a low value) and apply that amount against your readings.

I like the grounding rod thing...sounds like fun!
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