Irradiated EMI from DC fan - Page 1
Forum section: electronics / projects

author: tony359
author_rank: Frequent Contributor
timestamp: July 07, 2025, 10:45:02 pm
content: Hi all,I have a box which is a sound player based on an old Linux PC. The designer cleverly installed the Output board just above the CPU cooler. I replaced the cooler fan with another one which happens to be closer to the output board and now I can see the noise on the outputs.Unfortunately the board design is poor: I can see the traces of the output on the bottom layer and the whole design is UNBALANCED. The board is circled white in the picture, the cooler is underneath.The noise is not on supply, if I move the board away, the noise stops. The noise is present with the original fan too, just "more acceptable".I tried with some alluminium foil stuck at the back of the PCB (with isolation!) but I don't see much difference. The foil was connected to the chassis.Besides getting a slimmer fan, a slower fan, a different fan, try the fan the other way round, is there anything I could do to shield the outputs? I was thinking of an alluminium sheet, like 1mm, attached to the bottom of the output board and connected to ground?Thanks!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 07, 2025, 11:31:38 pm
content: I am guessing that the noise is magnetic induction. Aluminum will do next to nothing. You might get some benefit from a soft iron plate. The optimum shape for magnetic shielding would be a cup around the fan, but that obviously is the worst possible for airflow.A different fan might help. It would depend on the magnetic design of the fan. I haven't really searched but there may be some fans which are designed to minimize external fields and advertise this property. Probably not in the value priced range.

author: moffy
author_rank: Super Contributor
timestamp: July 07, 2025, 11:54:58 pm
content: As CatalinaWOW said, a grounded layer of mild steel would be your best next try, some thickness required, 1-2mm.

author: coppercone2
author_rank: Super Contributor
timestamp: July 08, 2025, 01:14:47 am
content: THE SO CALLED pure iron sheet on ebay is magnetic and soft and in a close thickness to what you want.It also electroplates iron OK from a iron plating bath. It seems to correctly reduce into green ferrous chloride. So it might actually be iron.Hymu sheet works too, but its much stiffer and less maleable, but should work

author: tony359
author_rank: Frequent Contributor
timestamp: July 08, 2025, 08:29:50 am
content: Thanks for all the tips, appreciated.Yes, I thought about looking for a fan with low EM emission. I am using Delta fans, they're inexpensive but the one I have has a large rotor (it's 6300RPM but 70K MTBF) and I guess that a smaller one might work better. I'll have a look at fans advertising low EM emission.And thanks for the tip about mild steel. There is a metalsmith nearby, I'll see if I can get a 2mm plate for a test. I'll update with the outcome.While looking online, some fan manufacturers claim the classic "fan grille" have some EMI shielding properties, would that be something I should consider?Thanks for now!

author: Someone
author_rank: Super Contributor
timestamp: July 08, 2025, 09:21:03 am
content: Quote from: tony359 on July 08, 2025, 08:29:50 amI’ve only seen grilles advertising higher frequency attenuation. If you have some examples then do post them.

author: radiolistener
author_rank: Super Contributor
timestamp: July 08, 2025, 10:29:15 am
content: Unfortunately, near-field coupling interference is difficult to shield effectively. Especially shielding magnetic fields within near field region from source is particularly challenging. While various shielding techniques and tricks may slightly reduce the coupling, some level of interference will still remain. At best, you can only mitigate it to a limited extent. Therefore, it’s generally better to either move the sensitive circuitry away from the interference source or relocate the source itself. In the case of fan-induced interference, one possible solution is to use water cooling with an external pump, placing the fan outside the enclosure far away from sensitive board.

author: tony359
author_rank: Frequent Contributor
timestamp: July 08, 2025, 10:53:54 am
content: I got a 2mm sheet from a local shop (for free, a scrap) and I fitted it to the board. IT WORKS!There is a mild leftover which is negligible. The 2.6KHz is something else - it's there even with the fan disconnected. These boxes are notoriously noisy unfortunately. I will look into that.(RTA was set to a longer average on the final test so the jagged line is not the noise I am talking about, I mean the spike at around 200Hz and relative harmonics. The metal plate is temporarily attached with only 2 screws, there will be 4 eventually.)

author: moffy
author_rank: Super Contributor
timestamp: July 08, 2025, 09:32:59 pm
content: Congrats, especially on scoring a free piece of steel, nice guys at the work shop. That's quite an improvement in noise.

author: tony359
author_rank: Frequent Contributor
timestamp: July 08, 2025, 09:54:54 pm
content: Indeed, I wasn't expecting the free piece of steel!The lady said "they shouldn't leave me in charge" 😀But I will need more so I'll be happy to pay for them.Now I'll see if I can spot where that 2KHz is coming from but it might be some bad PCB design on the manufacturer's end.Thanks for all your help, I wasn't aware aluminium is not good at shielding.

author: tony359
author_rank: Frequent Contributor
timestamp: July 10, 2025, 12:06:51 pm
content: One last question: does the shield need to be connected to ground?

author: moffy
author_rank: Super Contributor
timestamp: July 10, 2025, 12:31:46 pm
content: Quote from: tony359 on July 10, 2025, 12:06:51 pmYes, that way it is both an electrostatic as well as magnetic shield. Think of it as a ground plane with magnetic field shunting capabilities.