Time for a status report!
Now things starting to come together. I have remounted the oven to the PD2005 and started testing. The PID loop regulates very well, much cleaner than the old thermostat. By experiments I have established that the old thermostat were shutting the heater power when the temperature is 70 degrees C and turning it on again at 65 degrees. So there is a hysteresis of 5 degrees. My PID loop keeps the temperature within about 0.3 degrees. The only problem is that the electronics in the oven creates rather much heat in itself, forcing different PID settings depending on if the PD2005 is on or off. Still, there is a tendency to put the temperature up or down by turning the PD2005 on or off. So the "always on" heater is a bit of a dud. But the same goes for the original design. The way to see it, is perhaps to regard the heater (when PD2005 is off) as a "pre-heater", shortening the time for the unit to reach stable temperatures.
My setpoint is 60 degrees, not 65-70. I can adjust the setpoint up to 85 degrees but I think 60 degrees is good enough as its a compromise between power requirements, "effort" for the PID, stable temperature and longevity.
I'm using two NTCs. One of them is used by the MCU controlled PID, the other one is a interlock using only analog circuits to shut the power when the temperature is off (interestingly, the hysteresis here seems to be about 5 degrees C), adjusted by a pot to about 70 degrees.
I had a hard time to fit it all in the unit. My first design where a small board intended to sit in the oven housing, but in my second design I've scrapped that idea and made a board about 75*90 mm. Then, the new transformer needed to be fitted, as well as the power board (rectifiers and caps).
I ended up using felt as insulation. The felt is inside the oven, and another layer on a paper roll (toilet paper!) that fitted perfectly on the oven.
The stability seems to be improved. Stupid enough, I didn't record the stability enough, so I will ponder this thread to get clues about how bad or good it was before. But during several hours, the max and min recordings seems to be 200 µV apart in CV mode (no load). I will do additional tests under load and CC mode.
Below is some pictures. Note the fitting of the transformer (is a 230->24 VAC @ 1500 mA used as a 115 -> 12 V. The max current used is just under 500 mA). The front panel neon light for the oven is replaced with a RGB diode. Blue is "too cold", red is "too hot", green is "hunky-dory", combinations of the colors indicates various error conditions, pre-heating and such). Also note the dial cover that was originally white but very worn. I'm used my girlfriends nail polish with a "Power Designish" orange color. I think its rather neat!
From the top of the oven (big red cylinder) comes all the signals from the heater. The tick green wires are power, red and green enamel are the two NTCs and the thicker white is an ordinary K-sensor to a external thermometer. Also note the outer felt isolation. Upper in the picture you can see the control board. Between the transformer and the front is the little power board. Its not secured yet and only sits there. I'm using a network cable to connect the oven to the control board.
On the control board (main pcb) you can see my little patch board I mentioned in a earlier post, with the TL431 reference.
Everything is rather ugly, a rats nest, but it works!