Noise. This board is noisy noisy noisy. There are 3 main sources of noise to squash and 2 of them do not take that much effort to fix:. You need new caps, a couple of diodes.

So the noise on the thermistor lines was, attrocious, horrendous, wait-there-is-a-signal-in-there? Because the caps are cheap, I replaced them first. The replacements I picked up were 68 uF, 35 V vs the 22 uF, 16 V. Overkill, but I went for ripple current and lifetime ratings and these came out as some good ones. These caps regulate the power on the 3.3 V line for the PCB. The majority of the noise is now about 50 mV ptp, sans the dirtying of the ground from the 12 V supply.

Next up is suppression diodes. The MOSFETs overshoot as expected and if you want to read why, click here. I had a spare 1N4001 around, but given a 12 V PSU had spikes of 37-38 V with the MOSFETS, a 50 V diode wouldn't be sufficiency if a 24 V PSU is used. So I picked up a pack of 10, 1N4004s and gave it a shot. Unfortunately these only cut the 37-38V peak down to ~26 V, but the ringing was eliminated though. Sorry, but I didn't check the current characteristics. Putting 2 in parallel only lowered it another 2 V.

I wasn't happy with that and I happen to have some 1N4448TAP laying around from another project so I soldered one in. Right off the bat 17 V peak with a hint of ringing. They have a max surge rating of 2 A, lower than preferred, but the result is almost ideal given that the max RMS current draw is ~3.3 A. Now I do have a current droop occur right after the MOSFET switched on, but when it switches off, I get an equivalent peak so nothing is "lost".

I looked at the thermister signal once again. I still had about 350-400 mV of ptp noise that appeared to be common mode, I place my hand on the chassis and it attenuates, but the rest of the signal was more or less clean. Despite this, a choke on the thermistor lines didn't attenuate the noise signal specifically. I'm going to drop a ferrite bead or two on the power supply to see if I get an amicable result.