Wooden case and EMI?

grindlestone · Post by **grindlestone** » Thu Dec 04, 2003 6:01 pm

Hello,

This is my first substantive posting at this site since I stumbled across it a few weeks ago. Since that time I've made a few mods to my machine that have made it a little quieter and a fair bit cooler.

But I can't go any further until I get rid of this cheap, cramped, tinny case and noisy PSU that I have.

I have an idea to build a wooden case for myself. I have reasonable woodworking skills and a good supply of tools. I like the idea of a wooden case and think I could make something that exactly suits my needs and will be more attractive than a mass produced one. It would be designed from the outset for noise reduction and airflow. I plan a standard shape mid sized case with space for 120mm fan at the back under the PSU and a large intake area at the front. It will contain at most 2 optical drives and two HDDs so it only will require a 5.5 inch drive cage at the top (HDDs to be suspended inside with elastic cord). I don't have floppy drives anymore.

My problem is that I need to find out about electro magnetic interference (sometimes also called radio frequency interference?). I have searched a few forums, including these here but have been unsuccessful in finding solid information.

I need to know whether an all wooden structure will allow my computer to interfere with radio and TV reception. This is important because the pc is in my living room and is becoming the cornerstone of all my home entertainment. Music, DVDs etc. Can anyone point me to a site/article that will fill me in on EMI and what I'd have to do to reduce it?

mrzed · Post by **mrzed** » Thu Dec 04, 2003 6:13 pm

I don't have any links for you, but I can say that the easy answer is line the insides with some light metal mesh or foil.

Many people also choose to ignore RFI. Those acrylic cases are as transparent to RF as they are to visible light.

Zyzzyx · Post by **Zyzzyx** » Thu Dec 04, 2003 8:56 pm

Hmm... can't say anything regarding radio/TV interference, but I've had a system running wide open basically sitting on the desk for months now, no obvious effects. But then, I've got crappy radio reception here even with all my systems off.

Elliot · Post by **Elliot** » Thu Dec 04, 2003 9:41 pm

Can EMI from a non-shielded case cause CRT monitor to buzz?

crisspy · Post by **crisspy** » Fri Dec 05, 2003 2:22 am

I run wide open, and have never encountered EMI, although if you do a search of these forums you will find a few references to this issue, with a few posters mentioning cell phones and other things as possible problem areas. My case is the Milk crate thingy here in the galleries.

I have seen conductive paint available for shielding plastic electronics enclosures, and it would work well on primed wood. Foil, mesh, etc. seem like an awefull lot of fuss, since you need a continuously conductive enclosure to get the EMI.

Good luck with the wooden case thingy, I think there are tons of cool possibilities.

grindlestone · Post by **grindlestone** » Fri Dec 05, 2003 4:11 am

Thanks for the responses guys. Seeing as crisspy isn't having any problems with his milk crate machine I may not worry about it so much. Alternatively, if I do go ahead and build the case I'll try glueing a few layers of tinfoil on the interior surfaces to see if that works. I'm quite keen on this project and hope to come up with something stylish and elegant - and quiet of course. Nice thick sheets of MDF and a big slab of teak or somthing to make the face plate out of will do nicely I think...

Phill77 · Post by **Phill77** » Fri Dec 05, 2003 4:19 am

I've been using a wooden trunk as a case for about the last six months, with the hard disk and PSU on one level and the motherboard in a seperate compartment above. The rom drive and floppy drive sit on top of the trunk at the moment, still to get round to making a seperate case for them. Been meaning to post some pics for a while, shall do it later.
I haven't any shielding in mine and have had no problems at all. Even had an amp I am building spread in bits near it and the PC didn't cause any hum, but a mobile from twice the distance did.

I put my PC in the trunk so I could use the top as a table to save a bit of space, the fact that it runs cool an quiet is a real bonus. You have loads of room for sound insulation and to duct the air flow in and out.
With a NF7-V2.0, XP2500+ and 7200.7 at full load, I am seeing Case temp of 30, CPU 47 and HD 40, so loads of scope for turning the fans down even more...

pangit · Post by **pangit** » Fri Dec 05, 2003 6:04 am

grindlestone wrote:I'll try glueing a few layers of tinfoil on the interior surfaces to see if that works.

One layer of foil should be enough. Two or more won't make any difference. Just make sure the foil is all internconnected, and then connected to earth and it will be an effective EMI barrier. Technically you will have built a Faraday cage.

Having said that, you will probably find it unneccessary unless it is right next to the TV/radio.

halcyon · Post by **halcyon** » Fri Dec 05, 2003 1:40 pm

Wood is not a good EMI shield for emissive or absorbative shielding.

Ideally you'd want to use mu-metal as sheets to dampen frequencies up to microwave region.

Tin foil will not achieve same amount of shielding and you'd have to use much more of it.

It's not that you'll have problems automatically, if you don't shield, but you may experience problems (with communications and audio gear).

How about building a wooden case on top of your metal case?

This way you get the damping of wood and the shielding of 1 mm thick metal.

regards,
Halcyon

Bean · Post by **Bean** » Fri Dec 05, 2003 2:42 pm

My cordless phone's base unit was right next to my computer case and resulted in poor reception. I bought the cheapest phone the store had so I'm not saying all phones would act this way.

Maybe you could test your setup with your cordless phone :lol:

grindlestone · Post by **grindlestone** » Fri Dec 05, 2003 5:03 pm

Once again thanks for all these great responses guys.

Crisspy: Btw, please tell me you were joking about getting cancer from your pc...

Phil77: I'm interested in your trunk pc. It'd bee cool if you could post your pics. I had been thinking about separating my new case into compartments too. A top one for the psu and optical drives and a lower one for the rest. Each compartment would cool itself independently of the other. I'll do a little sketch and post it later on.

Because of all your posts I've been more successful at finding information. I think I'll definately try to work some sort of shielding into the design - if only for a little piece of mind

joe_merchant · Post by **joe_merchant** » Fri Dec 05, 2003 9:09 pm

Keep us posted on your wood case adventure, I've considered producing them commercially, but I don't think anyone would want to pay what I would want to charge, and if they did - a factory outfit would come swoop my business away with lower cost alternatives. Also, for commercial production, some kind of metal innercase would be required to meet EMC standards - not a big deal for hobbyists as you see here, but if you sell them, there are some rules....

Some things to consider:

Metal acts as a heat transfer conduit - wood is an insulator - you will be 99% dependant on your airflow for cooling.

You might concentrate your cooling efforts in big passive heatsink devices internally, then get a significant (yet still quiet - you do own a router?) airflow over those passive radiators.

I really like the idea of the Zahlman heatpipe hard drive mounting bracket, there are also some good looking passives for the Northbridge and video cards. If you have a P4, it will likely still need a direct on CPU fan due to the massive amount of heat it needs to dissipate.

The guy suggesting mu metal is in extreme overkill mode. Light gauge sheet steel would be nearly as effective for EMI.

Post by **Ralf Hutter** » Sat Dec 06, 2003 6:10 am

I have 2 different cordless phones in my house, a 900MHz and a 2.4Ghz and they both suffer from a clicking noise if i get within a few feet of my system and the side cover is removed. If I replace the cover the noise goes away.

Thierry BERNARD-LOOF · Post by **Thierry BERNARD-LOOF** » Sun Dec 14, 2003 8:35 am

hello

My case is in MDF (wood with resine) it is covered inside netting in wire with iron, I do not have a problem for the TV and the radio, the cellular phone on the other hand functions well in reception but not so good in emission ("clicks") is not a realy problem but is not perfect.

The case that I built myself is visible with:

http://informatixpc.ifrance.com

bye.