Noise from PoE (power-over-ethernet) adapter

Just in case anyone else contemplates doing this, I swapped my RS&Boom to being powered by the PoE adapter shown below, since it was connected to a PoE-capable network switch, just to neaten the cabling. It worked nicely, except that the PoE device seems to emit interference that was being picked up by the helicorder, as shown below in the first screenshot. Unfortunately the PoE adapter came with only a very short 5V power cord, so it wasn’t possible to place it further way. I reverted to using the official RPi4 power supply, and the noise went away, as shown in the second screenshot.


That is unfortunate… I would be tempted to return it, and buy a different version. There is no good reason why it shouldn’t work as well as the ordinary PSU.

Well… except … if you pull apart a few old plug-in power supplies (wall-warts), a lot of them are filled with resin to prevent inspection, but other are not. Some you will see empty holes on the PCB marked with labels indicating that they might conain various capacitors and inductors.

As far as I can tell, these are the filters to suppress what you see. My guess is that they are occupied on the samples supplied for FCC testing. Once they have the certification, they save a few cents by leaving out the filtering on the production version.

Unfortunately, there is no simple rule, like pay more and get a better product. Sometimes the cheap ones are absolutely fine and the (very) expensive ones complete garbage. You may have to go through the buy-return loop a few times.


Hello timchurches,

Thank you for sharing your experience with that particular PoE unit. Definitely unfortunate.

If you want to try again (also minding Philip’s sound advice), you could check this post out: Power over Ethernet (PoE) - #10 by Hadders that describes a workingPoE unit that is not showing the noise you have highlighted.

Yes, the POE device itself works fine, and I am using an identical one to power a BirdNET-pi RPi4 bird call monitoring station (which works a treat), and there is no additional noise. But that uses a USB condenser microphone on a 2m long USB cable, no helicorder coil to pick up stray electrical fields. Never mind, mains power for the official RPi PSU is readily available inside the off-grid solar shed where the RS&Boom is located.

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Yes, I’ll take it apart when it inevitably fails… and now at least I have a spare for the BirdNET-pi station where its electrical noisiness doesn’t seem to matter.

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