I only ran into two issues with the board. 1st I forgot to measure the heater voltage to calculate power. That was an easy enough bodge with a voltage divider. 2nd I had issues with a negative thermocouple voltage spike (ground bounce?) after the heater turned off. In the attached image you can see how the thermocouple (blue) trace spiked down after the heater (magenta) turns off. It then rises as it returns to normal (see thermocouple trace on the left before heater turns on) as well as from heat transfer from the heater.
To combat this I first increased the capacitance. This resulted in the thermocouple voltage more slowly decreasing, but it would still go lower than it should. I kept increasing the capacitance until I no longer saw the negative blip on the scope, but the temperature readout would still show a 4-6C instantaneous drop, so the negative drop was still there. I went all the way up to 0.1uF and that’s when I saw the voltage started to oscillate. At this point I figured it needed ESR to prevent the oscillations, so I replaced the ceramic with a 0.1uF electrolytic, and poof, the oscillations were gone. Furthermore the 4-6C instantaneous drop was also gone.
Since I really didn’t like the electrolytic there, I put the ceramic back in and added up to 100R of series resistance, but I could never replicate the electrolytic behavior. So that is why the electrolytic is bodged in on top of the thermocouple amplifier section. 0.1uF is more capacitance than I want since it takes 2 cycles for the thermocouple voltage to stabilize, so I would like to find a solution with less capacitance. I don’t consider it to be a debilitating, though. I found 4 off cycles was sufficient for accurate, stable temperature readings, and that allows a control loop up to 20 Hz, which is plenty for a soldering iron.
If I had to do it over again, the changes would mainly be focused on aesthetics. I would make the display centered on the front panel and put the handpiece connector off to the side. I would also use a better display that was edge-to-edge or find a way to cover the edge of the current one. The original design was driven by the constraint of making it fit the OEM chassis and wiring as well as good PCB layout, so some compromises were made.
For mechanical changes I would look into doing the front panel differently. It took some effort and two panels to get the correct bend to match the chassis. Additionally, it was a total PITA to get all of the correct heights for components: display standoff height to be flush with the front panel, extremely tall tactile switches, stock hex standoffs. This was all driven by the handpiece connector height since I used the PCB solder pins. Future versions should just use wire soldered to the PCB to avoid this difficult constraint.
Overall it was a good project that I’m glad I did; I only managed to kill two tips in the process. Let me know if you would like further details on anything in specific.