Hello again, wanted to post some updates on the progress of this.
I ordered a few T400 boards in a pretty red color and soldered one of them, looks nice!
For the oven I use a cheap SilverCrest one I got from Lidl some time back.
I previously removed all the circuitry from it so there were only the heating elements left, the oven didn't have any protection anyway. I had two SSRs that I was planning to use to control the top and bottom heaters but while mounting them i overdid one of the screw terminals and the screw broke off leaving half of it in the nut, it was the mains terminal, so I didn't want to risk it, I decided to skip it and use one relay for both heaters, just for testing.
But since the SSR was unusable, instead of turning it on, I took it apart So if anyone is interested what is in a SSR, here are some pictures:
The TO-220 is a BTA16 triac and the other package is a MOC3063 zero-cross optoisolators triac driver, neat stuff.
So I connected one of the mains wire to one side of the heating elements, the other mains wire to the relay and the relay to the other side of the heating elements. This way I could switch on the heating elements with a 5V input. For now I connected the relay control terminals to my bench power supply:
I made two holes on each side of the oven and put the thermocouples through them, I tried to have the thermocouples meet in the center of the oven. Here's another picture, please excuse the state of the oven, I used it for bread before, need to clean it up
I plugged the thermocouples into the T400, plugged in the oven into the mains and switched on my power supply, which switched on the relay and started heating up the oven:
Here you can see the first few seconds, the oven heats up rather quick. Note how in the second photo the "9" from "199" is drawn on top of the Y axis, seems to be some kind of software bug, I build the firmware myself from latest git sources.
Here the oven reached the 240-ish which is the usual peak temperature in some reflow profiles I've seen, you can again note how quickly it got there from 80 degrees C, might be actually faster than the profile requires.
At this point I turned the heating off and opened the door, you can see it cools down also very quick, thermocouple 2 is measuring a higher temperature because it shifted when I opened the door and was touching one of the walls.
When I closed the door, the temperature started raising again for around 15 seconds before it started to fall again, I changed the time base to 5sec and you can see it cools a lot slower with the door closed. This got me thinking that to implement a proper reflow profile, it won't be enough to just toggle the heaters on and off, I will need either a fan or some motor mechanism to open the door.
That concluded my experiments for today, I will continue to work on the reflow oven, I need to order more relays, need to solder the ICSP1 header on the T400, which will give me access to DATA1 and DATA2 pins which I can use to control the relays. I will also need to modify the software to toggle the IOs based on a profile I select.
Overall I have to say I'm very impressed by the T400, it seems to be accurate (at least the values shown from 2 thermocouples where close enough), it's really fast to notice the temperature changes in the oven and I really like the professional feel of the case, buttons and the LCD I think it's well suited for a reflow oven controller!