Tiny reflow oven controller

I looked at commercial reflow ovens and saw that they are expensive. That is a pity, because the hardware in them doesn't cost anything (Yea, I know, the development costs).

Prototype of reflow oven controller

Prototype of reflow oven controller

So I made a board controller that allows you to temperature control anything with minimum components. It has 4A TRIAC to control the heater, temperature sensor and attiny13 to control the heating. Also a red button, LED and on line regulator. The BOM cost was about 6$ in single quantities from Farnell.

Debugging the prototype was quite "fun". On line regulator meant that my ground was actually quite close to line voltages, so the probing was pretty dangerous. To use oscilloscope I used one channel as a virtual ground and divided two channels with math.

I got the regulator and zero volt crossover detection working but the TRIAC drive circuitry didn't work. The code was finally working, but I managed to blow the TRIAC somehow. I used small pulse after each zero volt crossover to turn on the TRIAC for the next half cycle. It brought the projects energy consumption quite low, only couple of mA. This allowed me to use quite small (100 nF) mains rated capacitor for the regulator.

Board layout with high voltage area clearly visible.

Board layout with high voltage area clearly visible.

At some point I decided to stop this project. Debugging it was too dangerous and I had made couple of bad design decisions. Like believing screw terminal over 300 V rating. Line voltages and 3.5 mm pitch screw terminals shouldn't go together.

Get the source files from GitHub

The schematic (PDF)

Since I still needed a reflow oven: I took a sandwich grill, added a thermistor, a small 5V relay and simple atmega32u4 devboard. Half an hour of coding later and now I have a reflow oven. PID controllers and thermal profiles are overrated, everything works perfectly with "go to 220 degrees then shut down" logic.

Final oven assembly. Thermal glue is holding the thermistor and relay is soldered on the wires.

Final oven assembly. Thermal glue is holding the thermistor and relay is soldered on the wires.

4 thoughts on “Tiny reflow oven controller

  1. Any chance you can show pictures of the finished unit and the surface or how you made the 'cooking' surface? I have a unit with a handle that looks exactly like this, but the part where the sandwich would go is all curved and shaped, it's not flat, so you could put a board on it. Is your unit flat?

  2. Yea, you have to get flat sandwich grill. They are quite rare, I couldn't find any. In the end one of my friend gave me his.

  3. I think it works quite well. I now have a professional reflow oven but the one made from grill and devboard is always in the closet should I need it. I have used it on and off and it's okay.

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