Building a small Faraday cage, need advice

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[nCn]Preacher

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Anyone ever build a Faraday cage?

I want to shield a transformer that is 4.5 x 4.5 x 1.5 ( with PCB & associate heatsinks etc) The transformer is actually mounted on the PCB and I don't feel comfortable enough with my meager soldering skills to separate the two and shield the transformer apart from the PCB.

It gets pretty warm to the touch so heat will be an issue.

I am picturing a copper screen cage inside a hobby box. The box will be vented into a duct that will use a fan to direct heated air out of the area. Sound do-able so far?

The ground for the cage is the biggest question. Can I ground it through the power source ground or will that defeat the shielding? If not, can I ground it through my computer PSU? The reason I ask is that both the transformer and the computer PSU will be attached to the same surge-protector and I don't know enough about electronics and faraday cages to say if this is good or bad.
 
hey man... why is it that you are so worried about shielding this device? Is it for being around other computer components?

You realize most transformers are set to be pretty efficient and in doing so they try to contain all of their magnetic fields to within their iron core. 99% of the other fields are cancelled out. If you are worried about being near your hard disks and such I wouldn't worry about it. Magnetic fields don't travel through metal (I'm pretty sure) really so your HD should shield itself I believe.

If you are set on shielding it you realize you could just enclose the entire thing in a metal box and it would be shielded.. if you needed a fan you could have a mesh screen with a fan.. that'd be easier than making an entire screen cage.
 
as was said up there.. you shouldnt have to build a cage for it.. shouldnt be an issue.. if its makeing your moniter bounce put a ferrite ring on the moniter cabe both at the back of the pc and at the back of the moniter

also... there is a copper spray paint out there that is conductive... you can spray the inside of the project box with itand it would be set... as for grounding it.. im a littel rusty but i beleave you only have to ground it if your protecting it frm interference, but if you trying to protect your pc/parts, you dont have to (again.. im not 100% shure) oh and im not shure on the brand of the copper paint eather.. i just know its out there some where.. (you could buy that window defogger fixer stuff and paint your own lattice)

thore
 
Thanks for the posts, guys, but let me go a little deeper into this to explain.

I am working on a custom case setup and am putting a device inside the case. The device is clearly marked "Keep at least 3 feet from computers and other sensitive electronics" After disassembling the device and checking it out at the equipment shop at work, it turns out that the transformer, on startup, sends out an RF spike, so I want to shield the computer from that spike. (Better safe than sorry!)

A Faraday cage will contain that spike and allow me to use this.
 
Shouldn't any conductor work? The reason that a mesh works is because the holes are less wide then a wavelength which effectively reflects them (EM waves that is). As long as currents can be induced. The idea is changing magnetic field hits the conductor inducing currents in it, rather than what's in side.

I'd get a metal project box and enclose it inside, any fans or vent holes I'd cover with a copper fine mesh which I'm sure wouldn't be too hard to find. Then take it to your shop at work and check the RF spike that you were talking about. I don't think that it will be able to penetrate. ( as far as B-field I don't think it is really possible to do better than a few milliTeslas 3-5 )

As for grounding it I'm not real sure on how to do this.. but I'm not positive that it will matter too much as long as there isn't a gap inbetween where the wire leaves the enclosure.
 
From what I remember of physics, the cage needs to be a particular material. I think's it's Iron.
 
joecool234 said:
From what I remember of physics, the cage needs to be a particular material. I think's it's Iron.
Click to expand...

You are thinking about magnetic shielding. A soft iron shield around a delicate peice of machinery will help shield it against magnetism because it offers an easier path to flow through.

A faraday cage can be any conductive metal, though copper seems to work the best. Solid copper sheeting is the best but Copper screen gives the advantage of acting almost like a solid (As far as waveforms are concerned) but also allowing heat to escape.

I have never played with one and don't know how well or how easily that translates into real world success, which is the reson I posted here hoping some smart kid would set this old man straight. :D
 
aL Mac said:
hey man... why is it that you are so worried about shielding this device? Is it for being around other computer components?

You realize most transformers are set to be pretty efficient and in doing so they try to contain all of their magnetic fields to within their iron core. 99% of the other fields are cancelled out. If you are worried about being near your hard disks and such I wouldn't worry about it. Magnetic fields don't travel through metal (I'm pretty sure) really so your HD should shield itself I believe.

If you are set on shielding it you realize you could just enclose the entire thing in a metal box and it would be shielded.. if you needed a fan you could have a mesh screen with a fan.. that'd be easier than making an entire screen cage.
Click to expand...

OH MY GOD. Magnetic fields don't travel through metal? Please tell me you are joking. How do you think a transformer works anyway?
 
joecool234 said:
From what I remember of physics, the cage needs to be a particular material. I think's it's Iron.
Click to expand...

Iron is part of it.

Its usally an alloy of somewhere around 80% nickel, 4% molybdenum, 16% iron alloy

Its usually refered to as Mu-metal (Mu being the measure of magnetic permeability).

But that is for magnetic shielding.

If he's sheilding a transformer, he probably wants both magnetic and RF shielding.

For RF shielding, you need the cage to completely encompas the object you are shielding. For magnetic shielding, you don't need as complete of a shield provided you are carefull with where the gaps in the shielding are in relation to the sensitive parts.
 
Magic H8 Ball said:
OH MY GOD. Magnetic fields don't travel through metal? Please tell me you are joking. How do you think a transformer works anyway?
Click to expand...


You don't have to be an asshole. The metal just has to provide an easier path for current to flow, therefore current is induced in the shielding rather than the electronic components.

How about you explain why the material has to be so much % iron etc. rather than be a total ass.

And since you asked how a transformer worked I will provide a simple short explanation. A trasnformer consists of 2 coils wrapped around an iron core. In a basic transformer circuit an AC generator produces current in the coil at the primary side. The coil at the secondary side has a different number of wraps. Therefore if the secondary side has more wraps it is a step-up transformer and if the primary side has more wraps then it is a step-down transformer. V_s_ = V_p_ * N_s_ / N_p_
 
i checked with my dad whos an EE... said the wire mesh in a project box should be good.. beshure you surround all sides and make shure the equipment isnt touching the cage or it will short (duh) and that the cage MUST run to earth ground (this was the part i wasnt shure about.. ) (if the case is grounded (by touching the psu), and the psu is grounded, and your running a three pronged surge protecter off modern house wireing, then all you have to do is run a lead from the cage to your case and you should be good to go)

thore
 
You would be better off modifing the circuit so it doesn't produce the spike. If it is a normal wall transformer it should be as simple as putting a mains filter on the input to the transformer, and a few ferrite rings on it's leads.
 
thore said:
i checked with my dad whos an EE... said the wire mesh in a project box should be good.. beshure you surround all sides and make shure the equipment isnt touching the cage or it will short (duh) and that the cage MUST run to earth ground (this was the part i wasnt shure about.. ) (if the case is grounded (by touching the psu), and the psu is grounded, and your running a three pronged surge protecter off modern house wireing, then all you have to do is run a lead from the cage to your case and you should be good to go)

thore
Click to expand...

Outstanding, thanks for checking!
 
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