Magnetic Shielding 101

[ashbreth]

I am wondering how to effectively magnetic-shield an electrical motor, i.e. a fan motor.

Here's my dilemma:

I bought a small desktop highspeed fan to place next to my open case to cool the motherboard and improve circulation. However, as soon as I turned on the fan, my monitor image started shimmering and flickering (much like a monitor does when unshielded speakers are placed next to it). When I moved the fan closer to the monitor, the flickering got worse; as I moved it away, the flickering lessened and disappeared. I know I could move the fan to be away from the monitor, but with my desk setup, it is impossible, as the computer case sits on a shelf below the monitor, and can't go anywhere else.

So, is there an easy or effective way to shield either the fan's motor or to shield the monitor from the EM field generated by the fan?

[Richard_Cranium72]

http://www.exploratorium.edu/snacks/magshield/ http://wwitch.unl.edu/ammrl/archives/berkeley/ammrl/94/archive/0321.html "If the interference problem is minor, you may try setting your monitor to different refresh rates to find a level which is more comfortable. If you set it to 60 Hz, your jitter will disappear" http://www.emcs.org/ http://www.mushield.com/mnfall99.html "Tips on Using Magnetic Foil
If it is impractical to shield smaller components by fabricating rigid metal shields, foil makes an excellent alternative. When working with foil, the following information may be helpful.
To minimize fringing of fields, don't create sharp corners. If holes are required, use round holes or slots with generous radii on either end.
When covering a cylindrical object, overlap the foil by at least 3/4" in each layer. Make the first two seams 180° apart. Make the next layer at right angles, the next at 180°, and so on.
To improve shielding, space the foil layers by three or four thicknesses of masking tape.
Because foil has a high permeability, never wind it continuously in a spiral. If the material is spiraled, there is danger of creating a pole piece in the center of the shield.
When drilling foil, be sure that the drill is ground for cutting sheet metal and not for normal steel cutting. A normal drill will pick up the foil with a corkscrew effect. This bending will reduce the foil's permeability" <<= http://www.mushield.com/designgd.html

[ashbreth]

I tried shielding the motor with foil, but it didn't work (course, maybe i was doin it wrong :-) )
My other concern is even if i can fix the problem simply by changing the refresh rate of my monitor, the EM field will still present a malignant effect on any electronic and magnetic equipment within its proximity.

any other ideas? (i am still reading thru those links and trying out different refresh rates)

thanks richard, i appreciate it.

[Kruppt]

I don't remember quite how to wire it in correctly it's been to long ago since I did it for radio. But one thing you could do is solder in a large capacitor that can take the voltage&amps(110V@20amps or more). It is either across both leads or in line, can't remember, but the effect is that it cancels out the sine of the AC wave and doesn't have the oscillating effect. Also you can wrap the power cord around a ferrite torid and that also will cancel out the AC oscillation. A TV repair shop could probably give you the proper scoop on how to solder in the capacitor correctly. You can get the torids I'm speaking of from Radio Shack. You want to wrap it(power cord to fan) as many times as will fit around torid. You can get a ferrite rod from a old radio also if you have one.(the black rod inside the radio that is used as a AM ant.)

Another thought if your up to the hassle.


If you want the optimal cooling of case (mobo, cards, HDD). You want to
turn your case into a "wind tunnel", another words air is sucked in from the
front of the case and leaves the back of the case. The fans at back of case
should be sucking out(blowing out) and the fan upfront of the case should be
blowing into case(sucking air into). You should have at least two fans in
the back of the case, one with the power supply and another case fan. Make
sure they are both blowing out from case and located as close to the top as
possible. Then you have to do a little mod on the case its self, one all
holes and vents on both sides of case and bottom (and back if need be), have
to be taped up with cloth tape or masking tape, two, you have to drill holes
in the front panel of case as most stock ones don't have large enough acess
for incoming air from the front of the case. Drill neatly spaced 5/16 holes
( at least 64 holes) in the lower part of the front panel, for a larger
volume of air to enter the case at BOTTOM front. What this all does is the
air moves in and out of the case at a faster velocity and in one direction,
being drawn across all components inside the case and out. The case temps
will drop 10 to 15 degrees, and also cools cards, drives, ect. more in the
process. Also you can cut the stock case grills off that are in front of the fans with
a dremel (with cut off wheel ). This will allow for freer flow of air (more
volume) less resistance and quiet down the fan noise a bit. As far as
cooling the cards down, this works great if they are horizontaly positioned
in the case, but if you have a board that has them in a vertical position it
will not have the full cooling effect on cards that one would have if the
cards were in a horizontal position. Like I said this works excellent, I
have a 15 degree drop from my stock case temps to the modified version of
case. you can also direct the incoming air by taping across front of open, unused,
drive bays, and force the air to draw where needed such as across ATA66
HDD(s) ect.


[This message has been edited by Kruppt (edited 04-04-2000).]

[This message has been edited by Kruppt (edited 04-04-2000).]