A Thermocouple is a terrific way to measure temperature. The effects of temperature change on dissimilar metals produces a measurable voltage. But to make that measurement you need an amplifier circuit designed for the thermocouple being used.
While researching “Zero Drift Amplifiers” as a follow-up to my video on Instrumentation Amplifiers I noticed the little schematic the front page of the LTC1049 datasheet which is shown here. I thought it was an ideal example of an analog application where some gain and some “gain helper” were needed to accomplish our useful little application of amplifying a thermocouple probe.
In the video I don’t really talk much about the thermocouples themselves other than the type I see most of the time which is type K. If you’re not already familiar with the construction of these probes you can find an informative write-up on thermocouples and the different types on the Wikipedia page and you might also want to check out the Analog Devices app note if you would like to know more. What I will cover is a reliable and precise way to read from these probes, seen in the video below and the remainder of the post after the break.
Different thermocouples sensors have a different temperature coefficients meaning that they will generate different amounts of voltage for the same change in temperature, usually specified in volts per degree of Celsius (v/◦C). Knowing the temperature coefficient of a sensor is only half the equation, we also need to nail down the zero point, meaning that we establish a calibrated reference point. Applying a known temperature such as immersing the sensor in ice water would be a simple if inconvenient way to establish a known reference temperature. Basically we could zero out and measure the change in volts per degree C from there. Below is a graph showing
Alternatively we could use a Cold Junction Compensator (CJC) such as the LT1025, a chip made to not only replicate the different temperature coefficients of the various thermocouples, but also give us a pretty reasonable calibration.
Behind the scenes the CJC acts as another thermocouple or thermometer and changes the voltage as seen by the thermocouple, which is at room temperature in our case, and also corrects for some other non-linearities. With the thermocouple being driven by the CJC the output of the thermocouple is then fairly linear and fairly calibrated.
For this quick demo I went one step further and used a chipset from Linear Technology called the LTK001 (PDF) which is comprised of an LT1025 CTC and a matched amplifier known separately as the LTKA0x. A quick glance at the specifications for the LTKA0x shows a lot about why it works in this application: it has a high open loop gain and very low input currents and input current errors. Analog Devices has a good write-up on open loop gain (PDF), we can talk more about bias current and errors in the future.
The schematic I used is here and was a composite of a couple of different sample circuits, if I were to do this circuit as a production run I think I would include a trim pot as well (and a calibration procedure.).
The PCB layers are shown below and I can make the Gerbers available if anyone is interested.The bottom is a ground plane fill which is why ground traces aren’t easy to spot.
Using this circuit you can realize a simple thermocouple amplifier which should give you enough gain to interface to your favorite controller providing it has an Analog to Digital Converter (ADC). As this reads out in millivolts per degree Celsius you will have to do the conversion to Fahrenheit in software which should be straightforward.