Controller for a kiln/heat treating furnace

Here is a circuit for a temp. controller that I have developed for a kiln or heat treating furnace. It's a considerably modified version of a circuit that was published in the "Engineer's Notebook" section of an early 80's issue of "Electronics". The original used some components no longer made, hence the modifications. I also added some extra features of my own. I am using two, one to control an old pottery kiln I was given that I use to melt Al and bronze, the other a small heat treating furnace I built.

The output of the K type thermocouple (0-55 mV) is amplified 100X by U1B, 1/2 of a 358 op-amp chip, to give an output voltage of 0-5.5V. This is applied to pin 2, the inverting input, of the other 1/2 of the 358 which is configured as a comparator. The non-inverting input, pin 3, is supplied with a variable voltage (0-~6.5v) selected by the 10K temperature select pot. If the thermocouple voltage is below that set by the 10K pot the output of the comparator at pin 1 is high which turns on the triac controlling the heating element(s) via the opto-coupled triac driver. It also turns on the LED to indicate the status of the heater. Once the temp rises so that the thermocouple voltage exceeds the preset voltage on pin the 3 the the output of the comparator go low turning off the triac and LED. The cycle then continues. The switch gives a convenient, low voltage/current means of manually turning the heating element off. All resistors are 1/4W except as noted on the schematic. The triac is a 800V unit selected to have a current rating of twice the current draw of the heating element(s).

I hope someone might find this useful, either for a new project or to retro-fit an existing furnace.

For a large view of the schematic go to my album "Weyvern" and select the schematic picture "full size".

I used a PID furnace temperature controller and solid state relay, the PID learns the thermal inertia of the system over a few cycles and then coasts up to temperature without over-run.

I think it cost me around £18 including the thermocouple.

- Nick