Simple Thermocouple Amplifier circuit

Posted on May 7, 2012

Continuing with the thermocouple interface concept, the next step is to amplify the TC's millivolt signal into a more readable analog voltage, on the order of 0 to 5VDC. This simple circuit fits the bill. The LM358N is a dual op-amp IC. I'm quite sure any op-amp IC would do fine, just be sure it can accept a Vcc which is compatible with your battery pack. The IC draws ~ 1.0 mA. The resistors form a feedback loop into the op-amp, with a gain described in the schematic, and based upon the resistances R1 and R2. I used 100K and 1K respectively for the breadboarding, delivering a gain of roughly 100. This works well with the TC's range of up to 40 mV, with the output then being from 0 to 4VDC.

Simple Thermocouple Amplifier circuit
Click here to download the full size of the above Circuit.

I was surprised to find the circuit as stable as it was. Wired to a 6V battery, I attached a speed 300 motor simulating the starter motor to the same V+ and ground. Initial spooling of the starter motor caused a small hiccup in the signal, but it quickly restabilized, and it shouldn't cause any problems in operation. The C1 (0.1 uF ceramic) is essential to minimize this interference. The polarity of the thermocouple is important. If wired backwards, the circuit drops low and stays there, and could be confused with an operating TC at room temperature. A more elegant solution would be to offset the output so that if wired backwards, the output would deliver 1.0VDC, with the upper limit now 5VDC rather than 4.

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