Solder Reflow Oven Build

Here's a picture of what I have wired up so far.  The MSP430G2955 is a 38 pin TSSOP package device and is attached to an adapter board for easier prototyping.  I also used the Thermocouple Amplifier MAX31855 breakout board from adafruit, to let it do the math calculations for me.  When connected to a Type-K thermocouple, it provides the temperature result to an accuracy of 0.25 degrees Celsius and sends the data out by SPI so that it can be read in by the microcontroller.  I'll include more details in the future about the data format and the code I used to parse out the temperature result.

Type K Thermocouple: Thermocouple

adafruit Breakout Board:Thermocouple Amplifier MAX31855 breakout board (MAX6675 upgrade) [v2.0] ID: 269 - $14.95 : Adafruit Industries, Unique &…

MAX31855 Datasheet: http://datasheets.maximintegrated.com/en/ds/MAX31855.pdf

For the breakout board, the MAX31855 requires a Type K thermocouple. 

Here's a description from the manufacturer on the thermocouple that I am using:

CHROMEGA-ALOMEGA (ANSI Symbol K) The CHROMEGA-ALOMEGA “K” curve thermocouple with a positive CHROMEGA wire and a negative ALOMEGA wire is recommended for use in clean oxidizing atmospheres, The operating range for this alloy is 2300°F for the largest wire sizes. Smaller wire sizes should operate in correspondingly lower temperatures.

I chose a 20 gauge wire, so the max temperature will be considerably less than specified above.  The max range that I would expect to measure would be less than 350°C

MAX31855 data packet is 32 bits in which the thermocouple temperature is reported in bits 18:31.  Bits 0:17 contain some fault checking capabilities and the internal chip reference temperature.  I have parsed out the packet to only obtain the thermocouple temperature.  The 14 bit binary number is converted to a Celsius result by assuming each bit is equal to 0.25°C.  For example, a result of 0000 0000 0000 10 is equal to 0.50°C.  Since the last two bits contain the decimal portion of the temperature result, it was a simple process of just shifting the bits two places to the right, and then the whole number portion of the results can be outputted as an integer.  Then I could look at the last two bits separately to determine the fractional portion.

At first glance, it appeared that measuring a negative temperature would be a bit more difficult because of the way in which the MAX31855 handled these values.  Basically, the chip works backwards for negative, so a temperature of -0.25°C would be represented as 1111 1111 1111 11 and a temperature of -0.50°C would be represented as 1111 1111 1111 10 and so on.  Since this is basically the opposite of the positive temperatures, I found that if I just XOR'ed the result with 1111 1111 1111 11 (0x3FFF)and added 1, I could use the same method as I used for the positive results.  Here's a piece of my test code used to obtain positive or negative temperatures in Celsius:

   extern uint8_t volatile temp[4];  \\ temp[0] contains the top 8 bits and temp[1] contains the bottom 6 bits plus two unused bits which will be shifted out

   uint16_t volatile temp_result;

// Move the binary temperature result into temp_result using only the lower 14 bits

  temp_result = temp[0];

  temp_result = temp_result << 8;

  temp_result |= temp[1];

  temp_result = temp_result >> 2;

// The if statement checks the 14th bit.  Assuming there is no fault condition, this bit will be 1 if the result is negative.

   if ( (temp_result & 0x2000) == 0){

       sprintf(buffer, "Result is: \n%d.%d%cC",temp_result >> 2, (temp_result & 0x03) * 25, 0x7F);

       Dogs102x6_stringDraw(1, 0, buffer, DOGS102x6_DRAW_NORMAL);

}

   else{

  temp_result = (temp_result ^ 0x3FFF) + 1;

       sprintf(buffer, "Result is: \n-%d.%d%cC",temp_result >> 2, (temp_result & 0x03) * 25, 0x7F);

       Dogs102x6_stringDraw(1, 0, buffer, DOGS102x6_DRAW_NORMAL);    

}

Additional code would need to be added to check for faults prior to making the temperature measurement. 

I have added in some fault detection code.  This code will first check to see if any fault exits. If none exist, the program will proceed on to the previously discussed temperature measurement. If one does exist, the program checks which fault(s) are currently being reported by the MAX31855:

        /* Check Fault condition bit D16 (BIT0 of temp[1]). If 1, a fault exists
         and don't proceed with the temperature measurement. Instead determine which fault(s)
         exist and display the result to the screen.
         OC = Open Circuit
         SCG = Short Circuited to Ground
         SCV = Short Circuited to Vcc
         It is possible to have more than one fault at the same time.
         */
        if ( (temp[1] & 0x01) == 1){

         switch (temp[3] & 0x07) {
         case 0x01:
                sprintf(buffer, "OC Fault");
          break;
         case 0x02:
                sprintf(buffer, "SCG Fault");
          break;
         case 0x03:
                sprintf(buffer, "OC/SCG Fault");
          break;
         case 0x04:
                sprintf(buffer, "SCV Fault");
          break;
         case 0x05:
                sprintf(buffer, "OC/SCV Fault");
          break;
         default:
                sprintf(buffer, "Other Fault");
          break;

         }
            Dogs102x6_stringDraw(1, 0, buffer, DOGS102x6_DRAW_NORMAL);
        }
        else{

             //  Continue to temperature measurement

   }

Wow!  It's been 7 years since I posted here?  It might be time to finish this project!!!