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Any ideas for low-cost Thermocouple Interfacing methods?

shabaz

Note: See here for a project that resulted from this discussion: BLE EasyTempProbe 

Hi,

Since thermocouple measurement ICs are getting expensive/hard to find, I wished to use a single channel ADC, for thermocouple measurements (actually, I want to use a dual channel ADC for two thermocouples, but it's likely the same problem just doubled!).

The trouble is, the cold end of the thermocouple needs measuring too, and I was thinking of using a thermistor for that because that's easier to obtain (and cheaper) than an IC sensor. In summary, I wished to multiplex a thermistor and a thermocouple.

I've come up with the diagram below so far and wish to use it in an environment where the cold junction might be in the range of -40 to +50 deg C, and the thermocouple might be in the range of -40 to +400 deg C (maybe a Type J thermocouple). I think it will have an error of a few deg C. The ADC is 16-bit, and I likely won't be using the whole range of it. The ADC has a PGA of up to 8X, so it will be set to 8X when measuring the thermocouple.

The main benefit of the proposal below is that it is cheap since it just needs a couple of transistors and a few resistors. I could think of more complex circuits for a more accurate measurement, but it would be nice to see if this is good enough, or if it could be tweaked to be good enough unless anyone has other suggestions.

If it works, this would be a cheap way (under $5, ADC included) to have two thermocouples each with their own compensation, with the drawbacks that accuracy might be a few degrees at best, and no isolation either, unfortunately. 

Any ideas would be gratefully appreciated, since I'm sure I may be missing some great techniques, missing the wood for the trees, etc! Has anyone come across any low-cost methods to do such a thing? Any mistakes I'm making?

image

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  • Could you use a microcontroller with built-in temperature sensor to avoid the thermistor for the cold junction? TI might even have an MCU with 16 bit A/D and internal temp sensor.

  • in reply to dougw

    Hi Doug,

    As a slight variation, instead of an internal temp sensor, I was thinking of using (say) an internal 8-bit ADC in the microcontroller for measuring just the thermistor, which could be fine over a limited temperature range, but over -40 to +50 degC it's not going to be granular enough, no matter if I try slightly linearizing the thermistor with resistors. A higher-res ADC in the microcontroller could be the answer to that, but I'm hoping that using the external ADC could be as good or better, and eliminates needing particular microcontrollers.

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  • in reply to dougw

    Hi Doug,

    As a slight variation, instead of an internal temp sensor, I was thinking of using (say) an internal 8-bit ADC in the microcontroller for measuring just the thermistor, which could be fine over a limited temperature range, but over -40 to +50 degC it's not going to be granular enough, no matter if I try slightly linearizing the thermistor with resistors. A higher-res ADC in the microcontroller could be the answer to that, but I'm hoping that using the external ADC could be as good or better, and eliminates needing particular microcontrollers.

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  • in reply to shabaz

    Thermistor linearization is a bit tricky, but well documented and can get within a few degrees without too much problem, especially if you have a high resolution A/D. Your solution should be a good low-cost method.

    The TI MSP430F20x3 family (with 16 bit A/D and internal temp sensor) is listed by TI as under $1 in 1K qty. So it may be competitive to the cost of an A/D chip plus MCU plus thermistor. Although it does not have an amp.