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I am a mechanical engineering student and I'm quite bad at electronics so I decided to post here. For my dissertation, I require 2 circuits using the same 5.7 V DC power supply. One of the circuits is a temperature sensing circuit using an LM35. A previous dissertation I'm continuing on used this circuit:

LM35 filter circuit

The output is connected to a DAQ. The sensor correctly detects an increase or decrease in temperature but is not giving correct values. I've compared the temperature values to the temperature readings from a T-type thermocouple and they don't match. Help?

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closed as unclear what you're asking by Andy aka, PeterJ, laptop2d, RoyC, Finbarr Apr 8 at 0:08

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Of course they don't match. Why should they? Read the data sheets. \$\endgroup\$ – Andy aka Apr 4 at 10:23
  • \$\begingroup\$ @Matthew: Maybe add a note into your question (using the edit link) to explain why you expect the LM35 to give the same output as a thermocouple. \$\endgroup\$ – Transistor Apr 4 at 10:38
  • \$\begingroup\$ Readings never match, they have a difference. What is the size of the difference? What were you expecting the difference to be smaller than? What do the data sheets say about the expected size of the difference? \$\endgroup\$ – Neil_UK Apr 4 at 10:46
  • \$\begingroup\$ The data sheet specifies a 0.4 degree Celcius difference at most but I'm getting upwards of 3 degrees. \$\endgroup\$ – Matthew Darmanin Apr 4 at 10:48
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    \$\begingroup\$ Right, that gives you a different issue. What is your method for measuring? If you are just measuring ambient temperature, then this isn't a great method. Try an ice bath to get close to 0°C and take measurements from that. You need to come up with a repeatable test using known stable temperatures to take your measurements, then write your results down and compare. Is your Type T thermocouple thermometer definitely accurate? If both have small accuracy issues, then added together it can show large discrepancies \$\endgroup\$ – MCG Apr 4 at 11:42
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I've compared the values to the readings from a T-type thermocouple and they don't match.

An LM35 produces a linear 10 mV change in output for each centigrade degree change in temperature. A "T" type thermocouple produces a change of around 39 uV for a 1 degree centigrade change and is not linear. See the table below: -

enter image description here

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  • \$\begingroup\$ Just as a clarification, I compared the temperature values not the voltage output. \$\endgroup\$ – Matthew Darmanin Apr 4 at 10:44
  • \$\begingroup\$ You are not making sense. If you compared the temperatures (and not the voltage output) and both were at the same temperature, then they MUST match. If they are not at the same temperature then why bother comparing them? \$\endgroup\$ – Andy aka Apr 4 at 10:47
  • \$\begingroup\$ That is why I'm asking the question. They're supposed to match but the LM35 sensor is giving me a much lower reading. \$\endgroup\$ – Matthew Darmanin Apr 4 at 10:49
  • \$\begingroup\$ @Andyaka I am assuming Matthew is comparing the temperatures as measured with a thermocouple and suitable meter with the temperatures the measured voltage from the LM35 should represent. \$\endgroup\$ – Warren Hill Apr 4 at 11:12
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    \$\begingroup\$ I don't think this answer deserves a downvote. After reading the comments on this and the question, it seems OP wasn't quite clear about the issues. \$\endgroup\$ – MCG Apr 4 at 11:49
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Perhaps it's an issue caused by how your circuit is constructed. How are you measuring voltage? Have you tried looking at the LM35 output with an oscilloscope? Before and after the R-C filter.

I once build a thermostat using an LM35 and I did not add any filtering at the output. But I did make sure that my power supply was fairly constant, ripple on Vcc lines can cause bad readings. You could also try powering your circuit with batteries and see if that helps.

Also, page 14 of the LM35 datasheet says that it has limited capability driving high capacitive loads. Maybe those capacitors at the output are driving your signal down.

Try changing your circuit following the examples on page 14 of the datasheet.

Here's a link to a question I've found that is fairly similar to yours:

LM35 gives very high values in room tempratures, and decreases when we exposed it to a heat source!

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