The controller is intended for electrical kilns which use resistive heating. Power is controlled via a solid state relay (SSR) energizing the heating coil for a percentage of a heating-cycle. Through the use of a DPDT relay we have a redundant safety power disconnect when the kiln door is opened.
Using the latest PCB and a two phase 220VAC system, a hardware connection of the controller to the kiln looks like this:
The electrical schematic can be adapter to one's system easily. However for safety related reasons one should always include: overcurrent protection, relay + SSR, door switch.
WARNING: Wiring high voltage AC systems is a dangerous procedure if one is not familiar with mains power and safe practices. Always work with deenergized, disconnected systems. Use appropriate fuses/breakers and wires following local norms. The user is solely responsible for any damage or injury that may happen from inappropriate implementations.
The hardware design files in this directory are licensed under the CERN Open Hardware Licence Version 2 - Weakly Reciprocal. See ../LICENSE-HARDWARE for the full license text.
- Microcontroller: ESP32-DOIT-DEVKIT-V1 board
- Thermocouple Amplifier: MAX31856 thermocouple module board or an ADS1220 ADC converter.
- Thermocouple: Type-S Thermocouple (but configurable for B, K, R, N, E, J, and T types).
- Solid State Relay (SSR): To control the heating elements.
- Main DPDT Relay: Double pole double throw, disconnects main power.
- TFT Display: ILI9341 320x240 SPI display for the user interface (2.4 or 2.8 inch).
- Buttons: Three push buttons for on-device navigation (Up, Select, Down).
- Limit Switch: A safety switch for the kiln door.
- Reset Button: A reset button for the ESP32.
The PCB design files can be found in the /hardware/PCB directory.
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ADS1220 versionsis the current software supported version. The problem with this version is its missing some key details and I haven't done routing / manufacturing. The ESP32 pinout file is at PINOUT.xlsx. -
MAX31856 versionsare older PCBs I designed when the project utilized a MAX31856 and an SD card. The ESP32 pinout file is at PINOUT.xlsx.
Currently I am using the MAX31856 version PCB with modifications that make it reassemble the ADS1220 PCB:
- The MAX31856 SPI pins were rerouted to a small perfboard where I placed the ADS1220 in place.
- The SD card module is not needed and I modified it to use the kiln DPDT with a relay driver as follows:
Since these modifications are custom and would be a lot of work for anybody, there are currently only two approaches to using this project:
- Finish the ADS1220 PCB design and fabricate it, testing it works as expected.
- Use the MAX31856 PCB design. Software needs added compatibility of MAX31856 library (easy). Hardware will need a hack to use the safety relay (easy, use any pin to drive relay board directly).
The ADS1220 thermocouple software and electronics design are based on the following article: TI Precision Deisgns: Precision Thermocouple Measurement with the ADS1118
The article is very informative and definetely worth a read to better understand how the Electric Kiln Controller reads temperatures using the ADS1220 ADC.
For generating the necessary lookup tables I created a python notebook which uses the NIST ITS-90 Thermocouple Database. The generated lookup tables csv files are then uploaded to the data/ folder which is then uploaded to ESP32's flash. The notebook is quite self explanatory, includes a testing section for validation and can be extended to generate more than the available (K, R, S) thermocouple tables by simply loading the respective NIST table.
- Include 5V stepdown in board
- Do routing
- Create gerber files
- Potentially use 24V system with chineseium PCB powersupplies