I have a challenge that I would like to share with you. (at least a challenge for me)

The resistor R8 (10 turn potentiometer) can be adjusted from ~0 to 1K Ohm to output voltages from 3mv to 30mv. The divider labeled (400F) outputs constant 8.3mv that equals 400ºF for a K Type Thermocouple. All that worked great.

So later I decided to add a fancy LED (D2). The LED should turn ON when the temperature is above 400F and OFF when below. I suspect using a LM741 as a comparator is a poor design because of precision and low voltages. One option would be an AD790 comparator http://www.analog.com/static/imported-files/data_sheets/AD790.pdf A comparator in the .25mv offest range would be suficient for the precision I need.

Is there a simpler (cheaper) way to achieve this with precision and low voltage in mind?

Thermocouple tester

*UPDATE - HERE IS THE FINAL DESIGN http://iddevlabs.blogspot.com/2014/09/diy-thermocouple-testing-circuit.html

  • \$\begingroup\$ Huh? What's a 400 Farad divider, and how can 8.3 mv equal 400 Farads. \$\endgroup\$ – Olin Lathrop Sep 4 '14 at 18:10
  • \$\begingroup\$ The LM741 is not going to work there. It's not a very good single supply opamp. (Hopefully someone will suggest a better one.) The 741's input voltage range is 2-3 volts above/ below the supply rails. so for your 5 V of supply that means it will only work if the input voltage is between 2V and 3V. (You will also want an opmap with very low input offset voltage. typical for the 741 is 1mV.. that will cause an error in your circuit.) \$\endgroup\$ – George Herold Sep 4 '14 at 18:35
  • \$\begingroup\$ Should 400F read 40 degrees Fahrenheit? or possibly you mean 400 degrees F - which is about the melting point of an acetal co-polymer or does it mean 400Farad? Also, are you actually using a thermocouple that uses dissimilar wires and requires a reference junction or a thermistor in a voltage divider circuit? Its really not clear and makes your question impossible to answer. \$\endgroup\$ – JIm Dearden Sep 4 '14 at 19:13
  • \$\begingroup\$ @JImDearden, I am just making a circuit that outputs millivolts to test other circuits that read millivolts. \$\endgroup\$ – soldanr Sep 4 '14 at 19:51
  • \$\begingroup\$ @OlinLathrop I am sorry for the confusion 400ºF = ~8.3mv for a K Type Thermocouple \$\endgroup\$ – soldanr Sep 4 '14 at 19:51

The LM741 has mV of offset. If you want to get an accurate comparison you could use a precision op-amp as a comparator. For example, an ADA4051.

If you're going to build this, I would like to see the source impedance a bit lower, maybe use a 10 ohm 1% resistor rather than 165 ohms and make the resistor that is now 100K something odd in the tens of K-ohms. Generally thermocouple instruments assume a source impedance in the 100 ohm range, at most, and most have noticeable error at 100 ohms.

Note that you are not doing cold-junction compensation (or, more accurately, cold junction emulation), so you'll not be able to get a good simulation of a thermocouple-- you'll get one (approximately) degree of error for every degree your terminal strip temperature is different from reference temperature. The 8.3(16)mV voltage you're using assumes a 32°F temperature- I hope you're warmer than that. At a 70°F CJC temperature, 8.3mV would give you about 438°F equivalent.

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The approach you are proposing might look okay in a simulation. But it's very unlikely to work in real life (i.e. it will not work). 8.3mV is a weak signal. You can easily have more than 8.3mV of noise in your system. The noise will be tripping your comparator, and you will be having false detections all the time.

In real life practice, a thermocouple signal (tens of μV/°C) is amplified before something useful can be done with it. The amplification is usually done with an instrumentation amplifier. There are also specialized thermocouple front end ICs.

You should also look into Cold Junction Compensation of thermocouples. It's also sometimes called Ice Point Compensation.

There many application notes on thermocouple signal conditioning. Just search for these terms.

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  • \$\begingroup\$ Thank you! Yes, I am using an AD595AQ to do the amplification of the signal. Also it has a very nice cold junction compensation built in. However the challenge I have now is building a comparator that will light an LED when my mV simulation circuit reaches above 8.3mV I think I will have to amplify my signal and go from there. I should be able to come up with a simple circuit using transistors or voltage comparators to accomplish this. \$\endgroup\$ – soldanr Sep 4 '14 at 21:53
  • \$\begingroup\$ May be overkill for what you're trying to do, but if you want an accurate measurement, you should consider a specialized IC like the MAX31855. Hook that up to a tiny little microcontroller and you can control your LED based on any temperature you want. \$\endgroup\$ – Ryan Johnson Sep 5 '14 at 15:18
  • \$\begingroup\$ greelabs.blogspot.com/2014/09/… \$\endgroup\$ – soldanr Oct 17 '14 at 22:44

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