My Toaster Reflow Oven Project - Part 1

In a previous article just over a year ago, I wrote about a concept that I had read about for electronic hobbyists to turn a toaster oven into a reflow oven for soldering.  Well I couldn't resist giving this a try when my mom mentioned to me that her toaster oven had broken. I really don't know what was wrong with it exactly, but I took it apart and quickly verified that the heating elements still worked - that's all you need.  I decided not to use the dedicated reflow oven controllers from The Silicon Horizon mainly because they didn't sell a package with everything necessary included (controller, relay, thermocouple), and I didn't want to spend prescious time searching for these components and debugging getting them to work in their setup.  I carefully watched "temperature controllers" on Ebay for several months, before finally deciding to buy the VTC-620 controller with a thermocouple and solid state relay included.  The cost was approximately $65 including shipping charges.  The wiring of these components together is fairly straightforward, but I also found a datasheet for the VTC-620 online.  The datasheet explains the terminal connections as well as provides operating instructions on how to set it up and use it.

The Black and Decker toaster oven that I used has a top and bottom heating element like most toaster ovens.  I decided that it would be wise to be able to use either top or bottom, or both depending on application, so I bought some switches.  Basically I wired the input AC hot wire through a master power switch, and I also wired the timer switch built into the oven in parallel with this switch so that the master power could be controlled by either that or my master power switch.  I did this so that if in some applications there was the possibility of me accidentally forgetting to turn the oven off, I could use the timer switch instead of the toggle switch.  Then I took the SSR AC output and split it off to two switches, one in series with the top heating element and the other in series with the bottom element.  This would allow either heating element to be used alone or together.  The hardest part of the construction getting everything to fit together in the oven control area where the original control stuff was.  I definitely made it functional, and not pretty, but even so, it was a challenge to put the oven back together again.  Here are some pictures.

The first experiment I tried was to run the oven at a fairly low temperature.  I set the controller to 90 F and let it run.  It quickly ramped up to over 120 F using a very low frequency PWM (on the order of seconds) turning the heating elements on and off.  Then it was solidly off as the temperature fell to about 80 F.  The temperature appeared to oscillate in this fashion then between about 80 F and 100 F and it appeared the controller could not maintain very stable control.  There are some settings in the VTC-620 to adjust the PID control parameters, and I did not play with these yet.

The second experiment was to see how hot the oven could get.  So I set the controller to over 1100 F and let it run.  After about 10-20 minutes the oven was over 550 F.  At this point the display went to EEEE which according to the datasheet is a temperature out of range error.  The error condition stayed until the oven cooled back down to room temperature.  I actually thought something was broken with the controller but I tried it again after it cooled down and it worked again.  I repeated this experiment at the same thing happened, this time when the temperature was closer to 500 F.  I looked at the datasheet more closely and it said the ambient temperature range for the controller was 0 to 50 C.  I would expect that with the controller mounted right next to the oven, and with the oven at over 500 F, the controller's ambient temperature is probably getting above 50 C.  I'm not 100% sure if this is the problem yet, but I again ran the oven at a lower setting (124 F) for over 35 minutes and it did not error out which I think is consistent with this problem.

I have decided to pursue an external box to mount the controller in.  I still haven't decided exactly where to divide the components, whether the SSR should stay inside the oven compartment or it too should move out to an external control box.  I'm leaning toward the latter, but if anyone has any thoughts or advice, I would appreciate it.