Thermometer with a LM335

Posted on May 5, 2012

The circuit consists of two parts: The LM335 and its adjustment. The output of the LM335 is 10 millivolts per degree C, with 25 degrees C corresponding to 2.982 VDC. A reference circuit provides a zero reference voltage. It is adjusted to (2.982 volts - (25 degrees x 10 millivolts/degree) = 2.832 volts. To read the temperature of the LM335 directly in degrees C, connect the + lead from a high impedance DVM to the output pin and the - end of the DVM to the 2.732 volt pin. The factor of 10 millivolts per degree C is equivalent to 10 millivolts per degree K, since a change of one degree C is equal to a change of one degree K. The difference in the two scales is only their offsets. The melting point of water ice is 0 degrees C and 273.15 degrees K. The boiling point of water is 100 degress C and 373.15 degrees K.

Thermometer with a LM335
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Calibration: Set the voltage at the "Output" pin to 2.982 volts when the LM335 is at 25 degrees C. Adjust R2 to obtain 2.732 volts at the 2.732V output -this second adjustment is (ideally) independent of temperature. If you can get your hands on metal film resistors for the 2.732 volt divider, so much the better. Otherwise, adjust the 2.732 volt reference at the ambient temperature in which the thermometer is intended to be used. Use a good quality pot for R1 because the circuit's calibration depends on it. I used a Beckman Helitrim pot, dug out of my junk box, as R1, because of this excellent mechanical stability with temperature change and well as a low thermal temperature coeficient of resistance. The LM335 is made by National Semiconductor and STMicroelectronics. I found the ones I bought at Arrow ( for only 65 cents each (plus the shipping charge). They can also be found for sale on the National Semiconductor web site among others. I recommend the data sheet on the National Semiconductor site as the illustrations are clearer than those on the STMicroelectronics site.

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