Some thoughts about leaving the cover off of a desktop computer.
What motivated me to post this topic in the first place was a recent bad experience I had.
True confession. I confess, I am one of those persons that sometimes leaves the lid off of my desktop computer because I'm still working on it is some way. And then I'm still working on it the next day. So I forget to put the lid on. An I forge why it is important.
Well, after getting my computer to work good again, it's time to make a new backup of the whole system. I have an external backup device that is well-made and has always been very reliable. But something has happened. It's not working right. Now Windows can't find the device.Try again, now the device is running very slowly. At that point I start recalling all the training and experience I've had with different kinds of electronic devices. So I suspect that RFI from my computer is interfering with my external hard disk drive enclosure. Sure enough, I close up the computer like it should be in the first place and my external hard disk drive behaves normally.
From previous research that I have done, I found that other sources say that most often the problem comes from the USB cable. Besides the cable itself, the connectors can play a part in RFI problems.
In one of the posts above I included a link to a PDF file I found from one of the electronic parts companies. They are selling a device to eliminate any kind of RFI or EMI problems with the USB connection. The device contains filters to eliminate both common mode and differential noise pickup and it also has a clamping device that will suppress static discharges.
Most of the time nobody would ever need such device. However, there is a need for such a device, and that's why they were manufactured and that's why they are being sold. In a few extreme cases there has to be a filter on the USB port to get rid of noise that is found in the environment.
Now back to the cover issue. When the cover is off of a personal computer the amount of electrical energy that starts flowing around the outside of the case increases by a very large factor. It will increase by almost 4 magnitudes. Which is another way of saying there will be about 10,000 times as much electrical energy on the outside of the case. With the cover in place most of the electrical energy is confined to the inside of the case.
Apparently some people don't believe this because you don't see it. Of course you don't see it, that's why it's called radio frequency. If it were visible light, the inside of your computer would glow like a an open barbecue pit fire.
Now a word about the FCC regulations. Yes, these regulations are very, very old. I studied these regulations when I was a youngster and got my first FCC license as a broadcast engineer. Since then, RFI has not somehow disappeared into the ether. It still exists. Any electronic device that uses any amount of energy will generate some type of radio frequency emissions that will radiate outward unless there is some kind of shield to prevent this from happening. However, there are no documented cases where the FCC barged into somebody's house and told them to put the cover on their desktop computer. Most likely it is because few people do such a thing anyway. If you want to prove this for yourself you can take a portable radio or TV set or whatever kind of device you like to use and bring it near your computer and see how much noise it will get off the computer. You will note that most of the radiation is coming off of cables that come out of the computer. The box itself is wary well shielded. Not snap off the cover off your desktop PC and noticed that the noise level on your portable FM radio will go up quite a bit. Yes, you'll still be able to hear local FM stations because that's the way FM works. FM has much better resistance to RFI. But do not assume this means that modern computers generate less noise than the older designs. That conclusion is wrong. Although newer computers are better made in many respects, they still radiate energy into space. The only way to stop this is to have everything shielded. Full shielding of the printed circuit board is not very practical. So the reasonable thing to do is to mount the entire thing inside of a metal box that has very tight fittings.
Of course, the box has to be ventilated. As has been mentioned by others, a desktop computer benefits from having forced air cooling that ducks the airflow over critical areas of the motherboard. Trying to keep the entire contents of the motherboard cool is not the objective. The parts the motherboard that need attention are the CPU and the memory modules. A good design will have forced air going over the CPU and at least one other fan pushing air over the motherboard. In most designs the power supply also has to have its own fan.
What about computer cases with glass panels? Many home brew computers have been built with either glass or plastic openings to allow the owner to enjoy looking at the inside of his computer. In order to comply with FCC rules, THE glass or plastic cover must be of a type that reduces radio frequency emissions from the inside of the computer. I provided a link to at least one company that offers this kind of material for anybody that needs it.
As of this date, I have never seen any articles about the FCC imposing fines on the makers of computer cases. I suppose this is because homemade computers are only a very small part of the personal computer demographics.
The FCC in recent years has come down hard on some institutions and companies that ignore the FCC regulations. Apparently some institutions think that the FCC rules do not apply to them. There have been two well-documented cases where the FCC went after institutions that use a large number of defective light fixtures. These defective fluorescent light fixtures were generating so much parasitic radiation that it interfered with commercial cell phone operators. These operators, AT&T in one case, and Verizon in the other case, had to nag the FCC until something was done. It took a while before the guilty parties responded and finally did something about the defective materials found in the light fixtures.
The point being made here is that the FCC is still finding instances where uncontrolled spurious radiation from some devices are causing serious problems in radio communications. So to not imagine that somehow our newer devices are so pure that they no longer generate any kind of spurious radiation. Not true.
I would rather not post a whole bunch of links to a bunch of different articles about RFI and related issues. Most of them are just too technical and very long. But here's something you can do at home. Take off the case cover from your desktop PC. Notice how it fits. Is this it sloppy or lose? No it's tight and fits very well. Why do they do that? Because a tight connection between metal and metal is essential to keeping radio frequency emissions inside the case.
So am I telling you that the world will end if you leave the cover off of your computer? No, I never said that. My point was that there are enough reasons to keep the cover in place that you just ought to keep in place. What got me started on this was that somebody somewhere was spouting about how he thought it was better to leave the cover off the case to make it cooler. That guy, whoever he was, did not understand what he was talking about. If the manufacturer really wanted more ventilation for the computer case, he would've put a bunch of holes in it. What is funny is that I found at least one poster who said he was going to put holes in his case so it would ventilate better. Apparently there is no limit to the amount of ignorance that some few computer owners have.
At this point I'm going to stop making anymore additions to this thread.
EDIT: One last thing. You may like some images. Try this on a search engine:
Field strength imagesAnd find both pages and images that relate to how RFI is measured.
