At my university there’s a resale store that sells/auctions off old equipment. I decided to try to snag a pizza oven that was being auctioned off, $5 starting bid. The listing said it was previously used by the electrical engineering department for reflow soldering, perfect! The only issue was the knob for the timer was missing. The oven had two knobs: a bake/broil/off knob and a timer knob. Bid on it, set a $10 max or something and a week later got an email telling me to pay for the thing I had just won. I paid $7 or so for the thing, since someone else had bid on it and raised the price, but didn’t bid past $7.
After I got it, I took it home and dismantled it. Here’s my first video on checking it out and planning out my mod.
From here I discovered a few things: the bottom switch simply chooses which of the heating elements to turn on. One, both or none of them. The timer switch is just that, a switch. When the timer is “expired,” the switch is open but it closes as long as it is ticking down. Once it’s ticked down, the bell rings and the switch opens again to turn the oven off. At this point, I already started planning out that I’ll use a cheap PID temperature controller from ebay to control the oven, with some cheap thermocouples that were also bought from ebay.
Now, part two where I do a bit more decision making as to the design of the control system.
At this point, I’ve realized that the relay inside of the temperature controller is not going to cut it at all. The temperature controller’s relay is rated to 3A, while the oven can draw around 12A. So I’ll be using the 3A relay inside of the temperature controller to switch a larger, 13A-15A relay which will actually switch the oven’s heating elements. What I’ll end up doing is replacing the timer switch with the relay, since the timer switch is just that… a switch! Finally, on to part 3:
So now I have it all put back together and all functioning! I run it through a quick test and it does, in fact, work. It holds a temperature pretty well and its temperature readings are pretty comparable to those from my multimeter’s thermocouple.
I ended up using a pair of tin snips to cut a larger hole in the front where the timer switch used to be, and shoving the temperature controller into there. This causes one issue: the cavity where the temperature controller sits gets extremely hot when the oven is in operation, which caused some pretty weird behavior when the oven was cooling down. The thermocouple attached to the temperature controller would be fully cooled down, but the controller would still read ~50+C for a very long time, even though the thermocouple was cool to the touch and very clearly not that hot. However, the oven itself was still pretty hot, which is probably what caused the issue. At some point I might shove a PC fan into the casing of the oven to help remedy this, but for now it’s not the biggest issue.
Another design flaw with the project is the fact that it requires three separate connections to the mains! One for the oven, one for the temperature controller, and a third for a 12V plug pack that powers the big 13A relay. Ideally, I could tap off the oven’s mains connection internally and use that to power the temperature controller and the plug pack, but it’s extremely difficult to get that deep into the oven’s internals without destructively disassembling it.
I’ve only reflowed a little breakout board with it, and it went pretty terribly. I used way too much paste and had a lot of trouble securing the thermocouples to the board and the board to the oven. However, I’ll be getting some boards soon from a PCB fab and will hopefully be able to use those in the oven to give a better demonstration.
Overall, this project was a success and I’m really happy with how it turned out, with room for improvement in the future!