Should I choose MAX31855 for FPGA control?

• Should I choose MAX31855 for FPGA control?
  Posted by aifang on 18 Apr, 2017 09:46
• My uncle is making his own smoke house, and to make the smoke he decided on using a water heater element. Over the Easter Day we were discussing this, and it gave me the idea for my final project. So, my final project will be creating a controller/display for a water heating element. The goal is to use the DE0 to control the power source to the heating element, so that the wood chips used for smoking, will burn and smoke but not catch fire.
  
  Thermocouples will be connected between a DE0 (on the GPIO header) and the water heating element. The DE0 will read and display (the later on a 16x2 LCD add-on) the temperature of the heating element. The power source for the heating element will be connected to a relay and the control of the relay will be connected to the 5v pin on the DE0’s GPIO header. Upon flipping a switch on the DE0, a LED light will turn on, indicating that power is being sent to the relay and that the element has power. Once the heating element has reached the desired temperature a 15-minute timer will start. This timer will be displayed on the LCD. Once this 15 minutes has passed, the 5-volt signal going to the relay will be cut, the LED turned off, and a new timer will be set for 15 minutes. Once this 15 minutes has passed, the heating element will be given power and upon hitting the desired temperature a 15-minute ON timer will be set. This sequence will be repeated until the original switch is flipped off, or until 4 ON-cycles have passed.
  
  As for reading the temperature of the thermocouples I have found that I will require a digitize of sort. This is what I have picked out, MAX318551. My concern is the relay. I am not sure if I am understanding the terminology correct.
  
  What I need is the 5v GPIO of the DEO to send the 5 volts to the relay when called. I am correct in thinking that this particular relay requires 5 volts to turn on and can pass upto 220volts?
  
  
  If I am correct, is there anything special I should know about hooking up it at the FPGA?
  
  Lastly, does anyone have a recommendation of relay for my needs?
• #1 Reply
  Posted by ealex on 18 Apr, 2017 12:28
• hello
  
  i think a micro controller is better suited for your task.
  i imagine you will need:
   - SPI peripheral to communicate with MAX318551 - might need bidirectional data
   - calibration logic
   - lcd interface
   - user interface
   - some safety devices - if something locks up you don't want to keep the element on until it gets red-hot and starts a fire
      - watchdog in the code
      - hardware WDG as well - if the "heater on" stays on for more than 10min then cut power to the entire system.
      - maybe some sort of heater current monitoring - if the relay contacts fuse / the solid state relay fails short you sound an alarm / disconnect the main relay / etc
  
  you could use a solid-state relay to switch the load - look for a properly sized one, the heater's cold resistance might be quite low - you can get 10x initial current spike.
  
  another thing: i don't know if normal water heater elements will have a long life at over 100 deg(Celsius).
  you can find electric stove elements that can get hot enough.
  
  
  
• #2 Reply
  Posted by rstofer on 19 Apr, 2017 16:29
• In general, you don't want to use a mechanical relay because, among other things, the uC probably can't control it directly.  So, buy a decent solid state relay.  These are easy to control and can respond to cycling indefinitely (more or less).  It's pretty easy to find units rated 10 amps up to 70 amps:
  
  https://www.watlowdistributor.com/HTML/Watlow-SSR-Relay.php
  
  There are other sources...