power supply for a 12-volt computer to use as a logging computer for amateur radio field operations at about 14 MHz. I do not hear any RFI from the computer or power supply, when running from 12 volts. Our systems are quite sensitive to this, with sensitivity running down to -130dBm.
When I use this computer in my motorhome, I do not hear it at all on an FM ham radio running at 147 MHz, whose antenna is only a couple of feet from the computer. I do get quite a lot of ignition noise in that radio, however, which I need to correct. The broadcast FM band is between those two frequencies where I have used this computer.
I have more problems with lines running across my cheapie touch screen, which gets its power from the 12-volt bus on this power supply. But I think that's the monitor. I only see it when it's being used in the motorhome, and I think the power line to the monitor is coupling with ignition wires (which are very close by)--that's probably the same thing causing the ignition noise in my 2-meter ham radio.
I have not put a scope on the output, but it seems reasonably clean.
Rick "who would eliminate other possibilities first" Denney
Yea, it's fairly conductive (< .5 Ohms According to the manufacturer, Similar to Germanium)
I'd put electrical tape on the inside of the cloth, then wrap the PS (maybe the whole computer box.
on a side note, People with tons of RFI, do you use a premade metal case? that might shield the RFI, being metal and all...
power supply should help a bit, but I would not use soft conductive materials. If the soft stuff sags into the circuit board, you'll get fireworks. You'd be better off making a box out of lightweight sheet metal. Even very heavy gauge aluminum foil (the "heavy duty" stuff) should be able to hold its shape well enough to prevent shorting out. I would not wrap the PS in an insulator--that will eliminate air circulation and there must be some heat loss from the voltage regulators. I would mount a cover such that it provided openings all around it.
As I said before, I do not get RFI from the computer. But I did use an all-metal enclosure that is grounded.
There is a significant possibility that the RFI is coming from the motherboard or hard disk and not from the power supply. To really know for sure, you would need to unplug everything from the PS and then power it up. Ideally, you'd put a purely resistive dummy load on the power supply outputs, but that will require some careful knowledge. At five volts, a 5-ohm resistor will pull one amp, and a 20-ohm resistor will pull 250 milliamps. At 12 volts, a 10-ohm resistor will pull 1.2 amps. These need to be 20-watt resistors--wire-wound resistors in ceramic will do fine. They are cheap at Radio Shack.
Given the data transients on the motherboard, I've never figured out how they can avoid RFI, and I suspect that a metal enclosure is absolutely required for them to get FCC approval.
For shielding the power lines to the monitor, which I suspect is the reason I'm seeing interference patterns there, I recommend a shielded power cable. I haven't done that but I will. Snap-together ferrite beads (also a Radio Shack item) on the power cable and on the VGA cable might work just as well. My cable snake to the monitor runs parallel to the main wire coming from the alternator on the engine of my motorhome, and the significant RFI I'm hearing from the engine in my 2-meter ham radio is suggesting that the engine compartment is putting out some serious RF.
The engine def. puts out tons of RFI, that's another reason why firewalls in cars are metal.
Just think of the amount of metal that spins at 1000's of RPMs and all the metal on metal rubbing... Add a turbo and it gets even worse.
another thought on shielding:
It could be as simple as covering the computer box with some sheet aluminum or even metallic paint?