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ANALOG SYSTEM DESIGN

by George Martin

Start ý Considerations ý Transducer Interfaces ý Thermocouples ý ADCs ý Almost Thereý ý Sources and PDF

THERMOCOUPLES

Letýs turn to temperature and thermocouples. There are a lot of great references on the Internet for thermocouples, and you should do a search for a more involved discussion as to which thermocouple is best suited for each application. Basically a thermocouple is a junction of dissimilar metals (see Figure 3). The junction produces a voltage and that voltage is proportional to temperature, with 0 V produced at absolute zero.

Figure 3ýA thermocouple consists of a junction of two dissimilar metals. Well, at least it does until you connect it to your circuit!

Thermocouples have two basic characteristics. The first is that they produce about 40 ýV for 1ýC. If youýre measuring the temperature of a furnace, the thermocouple might make a lot of volts. But, I also suspect youýd be interested in the exact temperature of the furnace not just an estimate to the nearest 100ý. So, you need a high gain stage to get the microvolts into the A/D range.

The other characteristic is that there is a junction for the point of measure and also junctions when you change from the thermocouple wire to copper wire for the circuit board. These junctions also produce voltages that affect the measurement.

Going around the loop in a clockwise direction, you have Cu-Ch at Temp-El (a copper Chromel junction at the temperature of the electronics), Ch-Al at Temp-Eng (a Chromel Alumel junction at the temperature of the engine), and Al-Cu at Temp-El (an Alumel copper junction at the temperature of the electronics). This can be written as follows:

V = (Cu-Ch)_T-el + (Ch-Al)_T-en + (Al-Cu)_T-el

You can combine junctions at the same temperature, and note that reversing the sequence of the metals reverses the sign of the voltage:

V = (Cu-Ch)_T-el + (Al-Cu)_T-el + (Ch-Al)_T-en
V = (Al-Ch)_T-el + (Ch-Al)_T-en
V = (Ch-Al)_T-en ý (Ch-Al)_T-el

Because the voltage and temperature are linearly related, this reduces to:

V = (Ch-Al)_{T-en ý T-el}

In other words, the circuit directly measures the difference in temperature between the electronics and the engine.

In order to create a measurement relative to, say, 0ýC, you need to subtract the voltage produced at the electronics junction(s). This is called cold junction compensation. In the old days, we used to have cold junction compensation blocks. These magic devices produced a voltage opposite of the offending junction. Iýve paid as much as $700 for one that handled 16 thermocouples.

Another approach Iýd like to try is one in which you place a thermistor or IC temperature measuring device at the offending junction. If you know that temperature, you can calculate a cold junction compensation and have the CPU perform the correction. And, of course, there is an electronic solution. Maxim has a device to perform the correction.

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