BUS Temperature controller

Recently, we need to control the temperature of a dozen boxes, and the idea to make a controller for each box, and adjust/monitoring one by one is not ideal, so we resolve to make a temperature control on a bus based.

The module is to be the more universal and cheap possible, and the ideia to make it usefull and reuseable to use on another projects. Use a single power source (24VDC) to all modules.

Use a 4 wire cable to power and data to control all modules on network.

For the interface we choose RS485 and for the protocol the MODBUS RTU. It´s simple and practical.

This is what we made until now. 

We need a simple step-down circuit. Since we have a lot of LM2575-5 in stock, we use them.

The 5VDC, is not a problem, but we want also use a thermocouple type K. The simplicity to interface this is to use a dedicated IC, like the MAX6675 or MAX31855 but this ic´s are 3.3VDC, so, we add a small regulator to 3,3VDC.
If some day the stock of LM2575-5 is out, we can replace to a 3.3VDC power source, just using some shunts on some components.

For the interface to RS485 network, using SN75176.

To control the power resistor, the circuit can use two methods:
->An angle phase control. MOC3023 and a zero cross to control the angle phase.
->ON/OFF switching on zero cross. MOC3043.

For controlling all, a PIC18F26K22 is the brain with:

• • 4 jumpers to set the MODBUS address.
  • Control type :
    • Zero crossing comutation in case of ON/OFF control.
    • Phase angle control.
  • To control the temperature some type of PID or FUZZY firmware.
  • Thermocouple K probe.
  • Settings adjusted via MODBUS on all.
  • Monitoring all modules via MODBUS.

This is the schematic:

 

and the prototype:

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The Firmware is still under devolpment...

When is stable to realease a beta version, we put a link the the schematic, pcb and firmware.