Toaster Oven Reflow Project - Part 3

This is a continuation of the article series on my toaster oven reflow project.  I had previously determined that the controller could not operate in the high temperature when mounted inside the oven compartment.  So I put the controller, the solid state relay, and the switches in an external control box.  The control box was an old cable set top box that I removed the insides of, and made some modifications to accommodate the new circuitry.  Here are some pictures of the new setup.

I decided to leave the indicator lamps in the oven.  As you can tell from the pictures, the plastic faceplate of the oven became broken during my disassembly and reassembly process to the point that glue would not even hold it anymore.  So I had to get creative with securing the top and side parts of the case.

Auto Tuning

As I mentioned in the past, the controller could only maintain the temperature within a +/- 10 to 20 F range with the default PID settings.  I ran the auto tuning process built into the controller for a temperature set point of 250 F.  Here is what the controller did in the process:

• Started at time 0 with temperature set point at 250 F and starting temperature at 100 F
• The output turned off at time = 3 min when the temperature reached 250 F
• The temperature overshot to 314 F at time = 5 min
• The temperature undershot to 239 F at time = 8 min
• Some time between 5 and 8 minutes the output turned back on
• The output turned off at time = 9 min
• The temperature overshot to 289 F at time = 10 min
• The temperature returned back to 250 F and the auto tuning process completed

After this auto tuning process the controller can now hold the temperature at exactly 250 F.  The heating elements are pulse width modulated with a frequency that is pretty low (period is on the order of seconds).

I performed the tuning with both top and bottom elements switched on, and I have not had a chance to test whether the tuned PID parameters are sufficient to control the temperature with just the top or just the bottom elements.

Heating Element Current Measurements

I took some measurements of the current in the top and bottom heating elements when they are turned on at 100% duty cycle.  

Bottom element = 5.46 A rms
Top element = 7.04 A rms

Both measurements were taken when the elements were hot.  The current actually seems to go up as the elements heat up, which I did not expect.  I was expecting that resistance would go up as the temperature increased, and thus current would go down.  I'm not sure what effect is causing this.

Crayon Melting Experiment

Now that the oven is finished I decided that the first project would be to melt some crayons and let the kids do an art project, in a scientific way of course.  So I had everyone guess the temperature of melting of a crayon.

• Kid A guess - 100 F
• Kid B guess - 90 F
• Mom guess - 300 F
• My guess - 200 F

We put a green crayon in the oven and set the controller to 90 F.  Nothing happened.  We ramped up the temperature to 100 F, and it overshot a little but the crayon still looked and felt very solid.  We ramped up the temperature toward 200 F. The crayon was visibly starting to melt at 165 F.  After the crayon was completely melted, we removed it from the oven to use in artistic creations.  Using an infrared thermometer I measured the temperature of the liquid crayon to be 150 F at some point while it was still completely liquidous but cooling off.  When the crayon was starting to solidify I measured the surface temperature at 147 F.  The conclusion is that crayons melting point is somewhere around 150 F.

Maximum Temperature

Recall from the previous article that the maximum temperature that I could seem to achieve with the plastic cover removed from the oven was around 500 F.  I decided to try to determine the max temperature again now that the oven is completely assembled.  When the oven reached 500 F it was still going up.  I did not have time to wait to see how hot it would get, so that will be left for the next experiment.  Also the next experiment will have to be to try to melt some actual solder.