Reflow Toaster Oven - Part 6 - PWM Fun

MAJOR WARNING NO 1: THIS COULD KILL YOU!

No, seriously, mains voltages can kill you, keep that in mind whenever you work with it. Having the cover off doesn't make it much safer, there's plenty of exposed contacts with 230V on them. That's lethal.

So now that we've cleared that up, let's have a look at what's happening here. I've replaced the thermostat with a solid state relay. There's a thermocouple connected to the heating element and plugged in to the Arduino. The Arduino monitors the temperature and turns the relay on when it's too cold, and off when it's too warm. The target temperature can be set using the serial interface. There's a bunch of other thermocouples in and around the oven to monitor the rest.

MAJOR WARNING NO 2: SOLID STATE RELAYS REQUIRE HEATSINKS!

I know it says "25Amp" on the cover, but if you check out the datasheets for these kind of relays then you'll notice they're only rated about 4A without the heatsink. That's why I've connected a thermocouple to the bottom of the SSR, so I can monitor its temperature (heatsinks are on their way). So on the thermometer I'm monitoring the following from top to bottom:
-oven temperature (large thermal mass probe)
-oven air temperature (small probe)
-oven top temperature
-SSR temperature

So after increasing the temperature a couple of times this is the result:

The blue line is the heater temperature. The red line is the target temperature and whether the element is on (high) or off (0). As you can see, lower temperatures require less frequent heating (duh) and it overshoots the target temperature sooner than with higher target temperatures.

This was to be expected, but it's good to see it in hard numbers. Next step is to also track the air temperature and see what kind of influence that has on the required dutycycle.

And yes, the SSR runs warm, 40 deg C and rising, target temperature is 250. Air temperature was 176 and the top of the oven was nearly 70 deg C.