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  Simple LM335 Thermometer
An adaptation of an application note circuit for a 1 hour project.




Not too many parts. When a voltmeter is connected across the outside terminals of the output connector, its display reads out in degrees C.
I've been fascinated by the LM335 for some time -maybe my obsession with stability finally gave way to my fascination with a circuit with intentional thermal drift. I finally bought some the other day, and to check one out, I hooked it up as a thermometer, based the National Semiconductor application note.

The +5V on the "input" connector is as a convenience in case I want to power the circuit from +5V someday, or use the +5V as an output to drive a meter or microcontroller.

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) = 2.732 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.

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 (http://www.arrow.com/) 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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Dick Cappels' web version first posted in December, 2003. Updated February 12, 2006.

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