MAX31855 External Temperature drop down when I turn on the heater

Hello there, I hope someone could help me to solve it.

Briefly, I am working on a Arduino (Industruino D21G specifically) project where I need to heat a sample to specific temperature.

Industruino is powered with a power supply of 12V and 6A.
The temperature is measured using a thermocouple amplifier MAX31855 (thermocouple type K). It is powered to 3.3V using the μA78m33c voltage regulator, which is powered using the Vcc and Gnd ISO2 coming from 14 pin IDC expansion port.
The heater is driven by an CH1 analog pin out of Industruino of Indio board and its ISO1 ground, where the voltage change from 0 to 3.1V (Vout). This is then connected to an operational amplifier and then to a transistor BJT NPN that it uses the power of a second power supply of 5V and 60A to heat a resistor near to zero Ohm (heating element). Gnd ISO1 and ISO2 are not shared.

The problem occurs when I turn on the heating, setting the Vout to a voltage less than 1.5V the temperature recorded by MAX31855 drops down to circa -87°C. This problem occurs only when the heating element is connected. If I put a resistor of 3.3 Ohm in series to heating element the problem not occurs but obviusly not heating.

Here I attached an screenshot of that shows the behavior of the temperature when I change the Vout.

Vout = 0V the temperature is equal to room temperature
Vout = 0.5V the temperature drops down to -87°C
Vout = 1V the temperature increase, but it is shifted to negative temperature
Vout > 1.5V the temperature is fine.

Any idea? What could be the cause of this behaviour?

I really appreciate any help you can provide.

I found that the MAX 31855 is very sensitive to interference.

I seem to remember that -87 is special however. This is an error value that tells ypu something like the thermocouple is touching the heater.

If your process temperature is between -55 and +150 then it is infinitely easier to use a DS18B20 digital thermometer.

Does your K-type thermocouple have colour coded wires, that is red wire and yellow wire?

If so the red wire goes to the negative input terminal and yellow to the positive input terminal.

Also place a 0.1uF cap from the positive input terminal and the amplifier gnd.
Also place a 0.1uF cap from the negative input terminal and the amplifier gnd.

Tom.... :slight_smile: