Naked Science Forum
Non Life Sciences => Geek Speak => Topic started by: peppercorn on 20/06/2008 01:12:13
-
Q1. traditional PCs are in metal cases, which act as a Faraday cage, but so-called modded PCs use a whole host of materials; do these ruin the screening properties of the case?
Q2: In theory, would it be possible to make a case out of mesh (or a material with a mesh embedded in it - like the window on a microwave) with zero EM emissions?
Q3: Are PC's unstable if separate parts are not all earthed to a chassis? (ie mainboard, PSU, HDD were just bolted to wood, say). [case designers must have to meet specifications I'm sure, but how problematic would such Heath Robinson systems be, huh?]
Thanks!
-
An earthed metal case would screen EM radiation.
It is possible to screen a plastic case by painting the inner surface with electrically conductive paint,
or securely sticking sheets of foil to the inside: (if the foil becomes detached is may cause a short-circuit).
The purpose of this screening is to block electromagnetic interference (EMI/RFI) affecting other electronic devices,
(or preventing electronic eavesdropping).
The EM fields created by domestic appliances are not harmful to human health: the EM radiation is not ionising radiation.
http://en.wikipedia.org/wiki/Electromagnetic_interference
-
Though some would have you think its frying your brain.
Buy Q-Wave now! From all "good" homeopathic health stores! [O8)]
-
Thanks guys.
I have no concern about health risks, but...
The reason I ask is because for my own peculiar reasons my PC is currently operating with the key parts (the mainboard, PSU & hard drive) having no shared earth connection.
The mainboard is screwed to a metal backplate, but PSU & hard drive are just sitting on the desk.
In explanation: The reason it is all chopped up like this is I plan to repackage it in a wooden box !!
Maybe unsurprisingly in this configuration it is somewhat unstable sometimes reports no system found at POST & one time the OS actually corrupted & had to be reinstalled.
A further question arising as I think about it is: Are PC parts effected by not being earthed through a common chassis? Do you think a simple earth strap connecting the parts would be enough?
-
Wooden PCs are so passe [:)] ...
The wood doesn't offer much protection against RFI (radio frequency inteference) or ESD (electro-static discharge).
Foil glued to all the inside faces of the wood might help, but at present it only has earth wires joining everything up.
http://skyscraper.fortunecity.com/nova/229/woodenpc/
A possible alternative would be wood veneer or wood-effect Fablon stuck to the outside of a metal case.
http://www.maplin.co.uk/Module.aspx?ModuleNo=1728&doy=21m6&C=SO&U=strat15
http://www.valeveneers.co.uk/
Check out this microwave oven case, (it's well screened too, until you open the door)...
http://en.wikipedia.org/wiki/Image:Casemodding_microwave.JPG
There's an idea!: aluminium flight cases ...
http://www.maplin.co.uk/Module.aspx?TabID=1&DOY=21m6&ModuleNo=36796&criteria=
Remember your modified case must permit ventilation to prevent overheating.
-
>>Wooden PCs are so passée<<
Ho Ho!! Yeah, true!
Although I was thinking of something more like this...
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.propmeupprops.com%2Fphotos%2Fothercoolstuff%2Fzenith.jpg&hash=3c297efe7c8438ebdd67a4cc4fa70481)
....Love the microwave PC!!
-
If you used an old 1950s-1960s TV case you could replace the TV screen with a CRT monitor...
[ Invalid Attachment ]
http://www.sciencemuseum.org.uk/objects/radio_communication/1971-76.aspx
Here is a source of vintage TV cases (empty prop)...
http://www.harryposter.com/vintagetelevision.htm
-
1930s toaster PC...
http://slipperyskip.com/page17.html
-
Actually I've also got a 1950's bakerlite TV case, but the hole for the screen is too small...
-
A metal case is no good unless all the removable panels are electrically connected together at many points which will not be the case with painted panels. Also you would have to de-couple all the external connections with at least with capacitors to the metal case.
PCs and all sorts of electronic items can emit a lot of radio frequency mush at all sorts of frequencies which can interfere with sensitive radios. The overall energy involved is small though.
-
A metal case is no good unless all the removable panels are electrically connected together at many points which will not be the case with painted panels. Also you would have to de-couple all the external connections with at least with capacitors to the metal case.
PCs and all sorts of electronic items can emit a lot of radio frequency mush at all sorts of frequencies which can interfere with sensitive radios. The overall energy involved is small though.
I think capacitive de-coupling with external cables is part of the specification for the designers of the PSU & motherboards to take care of.
Also one has to wonder how perfect the average PC case is a completely suppressing the RF inside...
-
"One has to wonder how perfect the average PC case is a completely suppressing the RF inside..."
Hardly at all I would have thought as it has to connect with the outside world and capacitive decoupling is poor at high frequencies (if it is fitted) and it cant be done on such as USB ports. Might as well make the case of plastic. As about as good as a bucket with many holes in it.
-
But the overall level of RF radiation will be pretty small in terms of any possible risk. I have to site any antenna for my sensitive narrow band VHF radio system about 20 metres away from PCs etc and even then there are annoying weak carriers every few tens of kHz.
-
I think in the my case (no pun intended!!) the instability of my PC project may well just be down to the fact that the HDD is just getting on a bit...
I'm not sure what year it was made but as it's only 15GB it's not going to be a fresh of the shelf.
I did wonder whether if IDE ribbon cables suffer any amount of crosstalk if they are run too near the surface of the motherboard?
And I suppose the PSU might contain HF pulses on the DC lines (5V, 12V, etc) if they generated with switch-mode stepping...
-
Having designed quite a few circuit boards in my time, I would doubt that a lack of (chassis) earth connection would have much effect on system reliability under normal conditions. That said, operating the parts in a non-standard case and/or with the bits laid out on a desk may result in insufficient airflow to keep hard disks and processors etc cool, which could foreseeably lead to intermittent failures and reliability issues.
-
I run my PC quite close to my weather satellite receiver antenna and have no problems although my home built display unit (1984 vintage) requires careful adjustment to avoid causing interference
-
Q1. traditional PCs are in metal cases, which act as a Faraday cage, but so-called modded PCs use a whole host of materials; do these ruin the screening properties of the case?
Q2: In theory, would it be possible to make a case out of mesh (or a material with a mesh embedded in it - like the window on a microwave) with zero EM emissions?
Q3: Are PC's unstable if separate parts are not all earthed to a chassis? (ie mainboard, PSU, HDD were just bolted to wood, say). [case designers must have to meet specifications I'm sure, but how problematic would such Heath Robinson systems be, huh?]
Thanks!
Answer 1 to a certain extent YES.
Answer 2 NO not ever
Answer 3 It will work but might just fail occasiaonally due to induced spikes, possible ground loops and as above potential coolong issues.
EM emissions covers from DC to way past xrays, since all energy eventually ends up as heat (a form of EM energy) the amout of EM radiation will be the same as the input power.