Thermocouples are a common passive sensing element that respond to temperature in a measurable way. They are self-powered, requiring no excitation, and can operate over a wide temperature range (up to to 2000°C). They can respond quickly, with almost no significant delay in system operation.
The above thermocouple structure is simple and made by two wires of dissimilar metals. The resulting output voltage is small (~40μV per °C for a K type) and requires precise amplification. Otherwise external noise (especially when long wires are used between the thermocouple and the measuring circuit) may distort your signal. The below table shows common thermocouple types and characteristics.
Another issue is at the cold junction created when your thermocouple’s leads meet copper traces of the signal circuitry; this creates a second thermocouple in your circuit. To compensate for the effect of the cold junction, measure the cold junction temperature and add the thermocouple voltage (Vout) that would be produced by that temperature to the value indicated by Vout:
- Vtc = Vout + Vcj
where Vtc = voltage due to the thermocouple sensing
Vcj = voltage during sensing at the cold junction temperature
Below is the typical thermocouple compensation circuit. A temperature sensor is at the cold junction area to monitor and an ADC provides the output data at the required resolution.
For further detail about Thermocouple, please visit our TheCircuit Blog post about Types of Temperature Sensors.