PSA - GND your 0V Line from PSU with Thermocouples
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So after battling an issue for almost a year, I finally resolved it, - I really wish I’d know this years ago, so posting it here.
Ground your 24V / 48V power supply “0V” line to your ground. I had not done this until tonight, always thinking that I was avoiding a possible ground loop. Turns out, some sensor readings are impacted by this, particularly if you shield cables. I’ve never had the best of luck with Thermocouples, they’ve always been on the fritz. Turn out, this was the likely cause.
I recently observed some very odd behavior on my Thermocouple lines as documented here: https://forum.duet3d.com/topic/31601/mystery-help-4-tc-lines-all-error-at-the-same-time/9?_=1705372886695
At the time, I had a custom PCB with 240V passing through with SSRs - I attributed the noise to that, and ultimately redesigned my electronics. I have no regrets spending about a year doing that (well, mostly), but I was shocked to encounter the same issue with a new board, wiring, and most components.
After adding filters and ferrite clamps, the noise got cleaner but was still quite visible. I saw a good 0.8V peak to peak at 60 hz, not only on my thermocouple lines, but also the SPI lines and 24V input. Somewhere either the noise or the ground shield over the lines was interacting with the signal and making everything go haywire.
It’s worth to note that almost all other aspects of the Duet 3 boards worked as intended with no visible problems.
So…. In the event you have sensor issues, try grounding your 0V line on your power supply. There’s a good number of Reddit posts you can find that explain the merits and downsides of doing so. Personally with the Duet 3, I wish I’d done this ages ago.
