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magnetic shielding for monitors?

L

Loco3KGT

Senior member
I re-arranged my apartment.. and not thinking ahead of time discovered my monitor is within range of the electromagnetic field being thrown off by my circuit box (and the power going through/around it). Can anyone recommend a way I could shield the monitor (or the box?).
 
1. re-arrange your apartment again.
2. turn off your circuit box
3. put a field generator in between which will exactly cancel out the effects of the circuit box.
4. stick your monitor in a farraday cage (chicken wire will do)
5. scream bloody murder and give up
 
and I'm being serious here.. how do you build a faraday cage? ( i can't find any simple sources on the web )
 


<< and I'm being serious here.. how do you build a faraday cage? ( i can't find any simple sources on the web ) >>


Use a conducting material (duh), and create a cage where the holes (or space between individual wires) are half the size of that of the EM pulse you want to block.

Hint: a microwave uses the same principle to keep you from roasting your eyes when staring inside while it's on.
 


<< and I'm being serious here.. how do you build a faraday cage? ( i can't find any simple sources on the web ) >>



Basically, a Faraday cage is just a grounded conductive enclosure. I won't bore you with the electrodynamic theory of why it works, but they will filter out a good deal of EMI. They don't even have to be solid. As someone above mentioned, a cage of chicken wire would work. I'm not sure how practical it would be to put one around a monitor though. You may want to put it around the source (the box if you're sure that's the cause) of the EMI instead.
 


<< As someone above mentioned, a cage of chicken wire would work. I'm not sure how practical it would be to put one around a monitor though. You may want to put it around the source (the box if you're sure that's the cause) of the EMI instead. >>


Use whatever you find more aesthetically pleasing 😉
 


<<

<< As someone above mentioned, a cage of chicken wire would work. I'm not sure how practical it would be to put one around a monitor though. You may want to put it around the source (the box if you're sure that's the cause) of the EMI instead. >>


Use whatever you find more aesthetically pleasing 😉 >>




A properly built faraday cage(bloody hard to make. Stick a radio in there. If you can hear radio it's not good enough) will filter out EMI, but won't do anything about magnetic field.
 
A Faraday cage (screen room) shields from dynamic EMI, not static magnetic fields that cause color distortion and the like in a monitor. The most effective shielding you could use would be made from mu metal which is a special grade of sheet iron that is specially formulated to shunt away magnetic fields from an unwanted area but is very expensive and very hard to fabricate. In addition it would have to be large enough for you to get inside with the monitor and you would have to have some sort of ventilation system to allow breathing for you and cooling for the monitor.
 
I assumed he was talking about RFI-type interference. I doubt the magnetic field from the breaker box is the cause. Viewing the magnetic field source in the breaker box as a single long, straight wire, and a current of 100A (the max on standard residential service) at a distance of 30cm (about 1 foot) the field strength would be 6.67*10^-5 T. That's an order of magnitude smaller than the field of the earth (10^-4 T). The magnetic field produced by everyday currents in a wire aren't that much compared with other sources like electro-magnets and permanent magnets. Unless he has the power main for the whole building running through his apartment, RFI or some other field source is more likely.
 
Options:
Get an LCD
Get a medical grade monitor
Build a faraday cage, make sure it's well grounded otherwise it may re-transmit the EMI, a magnetic material may be required. (iron, cobalt and nickel. Steel contains iron)

Another option would be to put a shield on the wall by the panel, it wouldn't be as effictive but it might work good enough and be a bit more pleasing to the eye and easier to pull off.
 
Thank you, Jerboy. The observation that a faraday cage may not do much for magnetic field interference was good. Only 17? Precocious, at least.

A mag field drops off much faster with distance than an EM field, so just a little extra distance may help a lot.

I keep thinking that a detectable powerline induction from more than inches away may indicate some heavy current in the power ground circuits. The normal circuit current is completely balanced in each cable, or should be. If there is current in the ground legs, it will make a big current loop. This would cause just the effect you are experiencing. If the owner has a competent electrician buddy with a clap-on amp meter, it might be worth checking out. It might show up some other problems,too.

As far as shielding, mu metal is quite expensive and saturates very easily. ( Mu ~ 100k) Transformers are commonly shielded by using a shorted copper band around them. Something practical to try if all else fails: Get a small box of romex building wire. Make a big loop of several turns with it and connect it end to end - all the wires - to make a large shorted loop. Then try various positions with it near the monitor/box. If it doesn't work or isn't practical, you've got the wire to install a ceiling fan some day. 🙂
 
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