This is the glue pot on the IMA edgebanding machine.

A customer of mine asked me to design a system that will control simultaneously three different heating resistors from 400 to 1000 Watts each, and will get temperature feedback from 3 different J-type thermocouples. The system will be used for a maintenance table for glue pots from IMA edge-banding machines. These machines have a unique feature that allows the operator to completely remove the glue pot from the machine for maintenance or servicing reasons.

The service table must have a temperature controller for the 3 (or 2) heating resistor sets of the glue pots. The glue must be kept in operating temperature (about 180^oC) or in stand-by temperature (about 140^oC), otherwise the glue in the pot will be solidified. Glue pots have 2 or 3 sections: The bottom section which has also the glue roller, the middle section which is called pre-melting, and an optional top section which is a pre-melting expansion. Each section has its own heating resistor set (800 to 2800 watts in total), as well as a J-Type thermocouple to read the temperature.

The controller that i design will have the following features:

Most important though, i plan to go into new technologies and techniques: This will be my first project that i will use an SMD PIC. Things are very nasty for me because i will use the 16LF1939 QFN/ML package, which actually has no pins on it's sides. Instead, all pins are underneath the chip. I watched several videos and read many articles explaining how to solder QFN packages, yet i still do not feel very comfortable. But i am absolutely thrilled to see the results of my first QFN/ML soldering job. I will use my brand new Aoyue 968 hot-air station for the job.

The QFN package is ridiculously small! Its tiny!	All 44 pins are underneath the chip, therefore a soldering iron is useless.

Additionally, the chip has maximum 3.3 volts supply, therefore i have to find ways to interface it with the LCD which uses 5V. Finally, this will be my first stackable board layout. What i mean is that the PCB that i will design will have a 16-pin female connector on top right and 4 holes on it's corners. This layout will be identical to the layout of a 20 by 4 LCD display PCB. I will use 10mm metallic spacers to fix and connect the LCD board right above my PCB - one board will be stacked above the other. Hopefully, my PCB will have the same dimensions like the LCD board.

K type thermocouples are very popular for one main reason: The temperature range. A K-type thermocouple can measure up to 1250 ^oC. But in lower temperatures, a K-type thermocouple is not very accurate. For this reason, the machine maker (IMA) decided to use J-type thermocouples. This type can measure temperatures up to 750^oC and its wires are made from Iron and Constantan. I will use the Maxim MAX31855J Cold-Junction Compensated Thermocouple-to-Digital Converter chip. This chip is the replacement of the MAX6675 that i used in my DIY Soldering Station project.They have the same pinout and they use the same protocol to interface with the microcontroller. Additionally, the MAX31855 series (suffix "J" stands for J type) provides more information to the microcontroller like the cold junction temperature, and it can also detect basic problems such as thermocouple connection break or short-circuit to earth/supply.

Each glue-pot has 3 sets of heater/thermocouple pairs, each pair with different setup:

12.8 amperes are too much to drive them through the PCB directly. There are of course methods to increase the maximum current through the PCB wires, but i prefer to use external load relays. I plan to use power bursts to control the output power, therefore the relays will be armed and disarmed many times. For this reason I chose to use Solid State Relays (SSR) to drive the loads effectively. I found a very interesting offer at ebay...

So, let's get to work...

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