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Thermocouples – Components Every Analog Designer Should Know

Maybe you’ve never used a thermocouple, assuming you don’t have to know how it works, but I disagree. I believe it is well worth spending ten minutes reading the relevant material. If you’re already very familiar with how it works, please let me know if I’m doing something wrong.

Thermocouples are temperature measurement sensors made of two different metals. They may be copper or iron, or they may be made of special metal mixtures.Two wires of different metal materials in oneNodeConnecting each other (this gives us the first important point) –no voltage at the nodewhich is like any two wires you are connecting, no voltage will be developed at the connection.

Now we know: When one end of a conductor is not the same temperature as the other end, a voltage develops across the wire. Yes it is true! Regardless of the resistance of the wire it appears as a voltage and no current flows – this is the Seebeck effect.If we use two different metals, there will be two different voltages, and theVoltage differenceIt can be measured at the open end, see Figure 1. Note that if you want to measure the absolute Seebeck voltage of a single wire of the same metal, the wires you are measuring will produce the same voltage in phase, and the measurement will be zero. You can only measure the difference between different wire pairs.

To complete this measurement, you will need to create one or more junctions on the measurement end (two junctions if neither wire is copper).we call itcold junctionbecause generally (not often) the temperature at the end where the measurement is made will be higher than the temperature at the end where the measurement is not made.

Thermocouple only measurestemperature difference.Note that in Figure 1 you measured the same voltage for both example cases.100°C between hot and cold junctiontemperature differenceA voltage of 4.1mV will be generated. The old-fashioned way to get an absolute temperature measurement on the hot side is to immerse the cold junction in a piece of icy water (another reason it is called a cold junction). The published thermocouple parameter sheet assumes a cold junction of 0°C.

If you want to know the absolute temperature of the hot junction but don’t want to soak it in cold water, you must know the temperature of the cold junction. This measurement can be done using a semiconductor sensor such as a TMP20 or ADS1118 (in conjunction with an A/D converter), thermistor, RTD or other semiconductor sensors that can measure absolute temperature rather than relative temperature. Based on the measured cold junction temperature (see Figure 2) a voltage equal to the thermocouple coefficient is applied.This work can be done in analog or digital form, calledCold Junction Compensation. If the cold junction is 0°C, the result of this summation is the voltage that would be developed.

If you need an absolute temperature sensor at the cold junction, why not just use that sensor to measure the hot junction? Thermocouples can measure a wide temperature range: low temperature measurements up to high temperature measurements above 1800°C, depending on the type. This may have other benefits, depending on the application.

If all wires produce the same voltage, why can’t we see this effect over and over again in our circuits? At normal temperatures for Electronic applications, the voltages are low and we usually use the same or similar conductors with relatively low Seebeck coefficients. When we use different metals, the temperature of the two junctions will usually be the same.

If you are measuring tiny DC voltages, you need to be especially careful. For example, when we measure the offset voltage of a high precision chopper amplifier, we must choose our components and materials such as connectors, and carefully route them. The mechanical layout must assume that the junction temperatures of the different conductors are similar to the other conductors and are thermally coupled. We use latching relays to minimize heat and keep other heat sources from becoming a low-level circuit. Measuring some micro-voltages correctly is a complex undertaking, and the accompanying thermocouple is often the ultimate limit of accuracy.