Fixing a kiln

A while back I had put a new thermocouple into our kiln since the last one crapped out. This is not unusual. What was unusual is when the thermocouple braid shorted the controller against the 200v rail since all of the connectors inside the kiln are uninsulated. The controller was toast so I decided to build my own, not very difficult.

First I built a controller from a cheap Inkbird thermal controller with a solid state relay. It seemed to work well until it got to about 600C and then it would level off and just get hotter and hotter without giving a reading that it was above ~800C. So I tried to confirm it with a pyrometer to double check and yes, it wasn’t reading correctly. So I thought it was the thermocouple and I replaced it, no dice. I then made my own controller with a MAX6675 module. Still the same problem. This stumped me for a bit until I did some reading.

What I found was that polarity of the wires is very important. Each wire from a thermocouple has its own composition and this is important in proper application of the Seebeck effect. On a K-Type thermocouple, the wire that goes to the negative terminal is magnetic whereas the positive wire is not. The wires I had were made to the Japanese standard where the red was positive and white was negative whereas US standard has the red wire as negative. The kiln now works correctly.

Some things to note as well is that running a kiln off a cheap Chinese SSR can be a bad idea. The cheap SSR’s seem to limit the overall current, probably at the zero-crossing point where it cuts out. I used the bigger chonkier mechanical relay for this application and there are no problems. Also, turning on a heating element has a fairly large inrush current, it’s good to put some larger decoupling caps on any power supplies that might be feeding from the main AC source.

Once I’m done programming the controller to do ramps and timed cycles I may publish the code and circuit diagram so that others can make their own controller if they wish. That said, here’s a pair of electronics cats who assist by staring at me while I work.

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