An interference coincidence?

Monday, April 8th, 2019 at 7:30 pm

Many years ago I started monitoring the power consumption of my house via a ZigBee dongle that communicates with my smart meter. The quick and dirty solution I did at the time stayed in place for a long time, until it started to play up…

The first symptom I noticed was that my script would hang, in itself not that bad, except that I was calling it directly from MRTG… which would cause the entire MRTG polling loop to hang, so I wouldn’t get any monitoring. In response I split the script, one to read the usage to write to a log, and another (called by MRTG) to read the latest entry in the log.

The reading script would still get stuck, so I added a generous timeout and stopped thinking about it. Until I thought it would be a good idea to also drop the usage data onto MQTT for Home Assistant to grab. Nothing wrong with this, except that I upped the frequency from once every five minutes to every minute. Sometimes it would fail for hours, sending an email with the error every minute…

I initially noticed the increase in timeouts around the same time that I upgraded the hardware in my linux box and did a fresh install of Ubuntu. Due to default permission changes I found that I could no longer simply cat the serial port, I needed to connect using Device::SerialPort, so I was thinking that there was something wrong with the usb-serial chipset or the driver.

That was until a few weeks ago, when I upgraded the hardware in my Windows desktop.

For a long time I have been aware of a slight ticking sound coming from that computer, you know the sound of a fan slightly hitting something or being slightly off balance?

The new build went great and although I didn’t really need the upgrade it let me do an upgrade that was required, to my parent’s computer. As I was handing down parts I thought that I could replace the power supply fan instead of buying a whole new power supply, so I powered it up and heard the same noise…

… coming through my speakers.

Oh, this is not a bad fan, this is a failing power supply.

So what does serial port problems in one computer have to do with a bad power supply in another one? Probably nothing, but since replacing that power supply I haven’t had a single timeout error, it is as stable as when I first picked up the adapter. I guess it is possible that the power supply was generating interference at around 2.4GHz…

Another thing I didn’t think was related was that the UPS that these two computers run from had started to shut down. Not in any graceful manner, but with a sudden power off followed by a loud continuous beep. I suppose it is also possible that a bad power supply could also trip the UPS…

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