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Ask Slashdot: Wooden Chasis and EMF

Posted by Cliff on from the now-thats-just-a-cool-idea dept.

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.

2 of 196 comments (clear)

  1. Re:EMF protection for a wooded pc chassis by Ares · · Score: 3

    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).

  2. Wire mesh is good start, ground it well by S_Walker · · Score: 3

    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.