scott216 / reflow_oven

Reflow Oven Controller

Reflow Oven Controller

I found a nice toaster oven when I when I was dropping some garbage off at our town's e-waste dumpster. It was in excellent condition. It has heating three heating elements on the top and three on the bottom . It had a microcontroller to control everything, so it was a bit more complex then a toaster with just a couple of knobs. My plan was to remove the micro controller PCB and replace it with my own. The heating elements are controlled by a couple of (triacs BTA26-600B). I hadn't used triacs before and had to do a little research on them. One unusual aspect of the toaster design is that the microcontroller board was floating on the 110 VAC line. This means that the 5 VDC supply voltage was over 100 volts if measure from mains ground. But the digital ground was only 5 volts less. So to the micro controller board this was fine, but the whole PCB was essentially about 110 VAC. I wasn't really comfortable with this, so I hacked the power supply board a bit so it output straight 5 volts. it also output -9 volts.

I wanted to continue to use the big start button from the front panel, so I took the original micro controller PCB and cut it in half to keep the start button and the LEDs that lit it up. Then I soldered some wires in a header that connected to the button and LEDs.

I designed a PCB with Cadsoft Eagle and milled it out on my ShapeOko CNC router. This was one of the first PCBs I did on the ShapeOko and it turned out pretty well (after a few revs). I had to figure out how to trigger the triacs properly. At first I didn't realize I had to trigger when the AC voltage crossed zero volts, this cause some magic smoke to be released from one of the resistors. This was easily fixed. Then I got a couple of Fairchild MOC3032 zero-crossover opto-couplers. I had a Nokia 5110 display from Adafruit which was a great fit for the existing display window. I used an Arduino Pro-mini (3.3 v) from Sparkfun to control everything. To measure the temperature I have a thermocouple from Adafruit and Adafruit's thermocouple amplifier.

For the Arduino code I started with code from here: http://github.com/CvW/Reflow-Oven-Controller/, which is based on code from Rocketscream. There was a bug in the debounce code I fixed and I replaced the MAX6675 with MAX31855 thermocouple amplifier. There a bunch of other little changes, nothing major. The code has two solder temperature profiles that you can choose between (by pressing the bake button) - one for leaded paste and one for a low-temp paste

This toaster is not a convection toaster where it circulates the air around, but I thought this would be a nice feature. So I used a little motor that I salvaged from an inkjet printer to power a 3.5 inch aluminum fan blade I ordered from McMaster (17545K63). I used a 3/16 bronze flange busing (McMaster 9440T63) to support the brass shaft through the sheet metal side. The fan is a bit noisy, but works pretty well. Since the reflow oven isn't an that long, not a lot of heat is conducted to the motor from the shaft.

I would like to have an interior light to illuminate the PCB as it's being reflowed, but I haven't figured out how to do that yet.

You can see some pictures on flickr or in the \photos directory of the repo.