Monday, February 28, 2011

Simple Thermocouple Amplifier

A couple of my friends needed a two channel thermocouple amplifier for their mechanical engineering senior project, but lacked the time or budget to try and get a commercial unit, so they asked me to build them one over the weekend.
My final result turned out pretty nice.  This uses a high precision Linear dual op-amp to amplify the signal from two thermocouples.  Using the LCT1051 low offset op-amp means the input offset of the amplifier is only 5μV, meaning we can skip all of the zero offset circuitry normally needed for these types of DC amplifiers.  The two potentiometers are used to calibrate the thermocouples to the target 10mV/degree Celsius output from the amplifier.

The one concern with using thermocouples is that they create a voltage signal (which is very small, on the order of 60uV/degree) based on a temperature difference.  This means that to find out the temperature at the end of the thermocouple, you need to also know the temperature at the base of the thermocouple, and then add these two temperature signals together to get the actual temperature at the bead of the thermocouple.  Luckily, Linear also makes a very nice thermocouple compensator, which does exactly this.  I did little more than build a pair of the reference applications from the LT1025 datasheet.

I am only using a positive power supply, so this amplifier cannot measure negative temperatures, and I wouldn't trust it much below room temperature.  This was designed to measure temperature elevation from machining, so this limitation is fine, but make sure this is alright for your application.
Parts list:
  • 2x 255k 1% resistor ($0.112 Digikey)
  • 2x 1k 1% resistor ($0.112 Digikey)
  • 2x 100Ω high turn pot ($0.92 Digikey)
  • 5x 0.1μF capacitor ($0.16 Digikey)
  • 1x 100 μF capacitor ($0.17 Digikey)
  • 2x 8DIP3 socket ($0.13 Digikey)
  • 1x LT1025 thermocouple compensator ($5.10 Digikey)
  • 1x LTC1051 dual low offset op-amp ($8.10 Digikey)
  • 9x screw terminals (Salvaged)
Building this on perf board was probably not ideal, but considering that this is a DC amplifier, as long as the connections to it are decently noise-free, this should be fine.
 Top (The wiring is all 32 gauge wire wrap wire)
Backside
Before I soldered everything together, I wanted to double-check that the resistors and pot were right (they should be; I pulled them from the datasheet), and that the circuit would work overall.  I did this by measuring the voltage out of a K thermocouple in boiling water, which should ideally be 1V (10mV/C * 100C).  Oddly, even with the amplifier gain turned ALL the way up, the voltage out was only reading 0.93V.  I spent almost an hour tinkering with the circuit on a breadboard before I finally gave up and turned off the hot plate.  Of course, as soon as I turned off the hot plate, the voltage swung up to 1.15V.  Turns out, since I had the hot plate and volt meter plugged into the same power strip, the hot plate was detuning the volt meter by more than 10%.  Once I realized this, it was trivial to turn the hot plate on and off to test the circuit, to make sure that my friends could calibrate it (their thermocouples were more of a pain to work with than mine, so I'll let them go through the difficulty of actually calibrating the amplifier for their project.

Nice little trick I picked up recently; if you use open frame sockets, you can actually fit a small ceramic cap inside the DIP socket, making it much easier to fit a filter cap near the power pins of the op-amp.

2 comments:

  1. I really like your projects. I follow your RSS and i feature the in my site. Although i was never a fan of perforated boards, you make them look nice also. As for the ideas, i think that you are one of the few people who make prototype circuits (for others to copy over and over again). congratulations.

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  2. They use to make IC sockets with bypass capacitors built-in for the 74 series ICs. Nice to see the trick isn't completely lost. :)

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