Inexpensive Linear Thermometer

  
As a sensor in the design of this linear thermometer, the LM34 is used. The output is the difference between
Inexpensive Linear Thermometer - schematic

two base-emitter voltages AVeb °f two transistors operated at different collector-current densities. where the current densities are IC and let, k is Boltzmann"s constant, and q is the electron charge. Because all factors, including the ratio IciUci are constant, the output of the LM34 (National Semiconductor) is a linear function of in the range over - 50° to 300°F, which provides the output voltage of 10 mV/ °F with a max. nonlinearity of ±0.35°F. The output of the LM34 is amplified by a three-op-amp instrumentation amplifier. The fourth op amp gives the possibility to control the offset voltage of the amplifier. The gain A of the instrumentation amplifier can be set to any desired value by the choice of the resistance R2 only.




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