Most diodes exhibit temperature dependency. This characteristic can be exploited to enable temperature measurements through electronic means.
In the circuit featured here, an ordinary diode (labeled as D1 in the diagram) is used as the temperature sensor. When a constant forward current flows this diode,
the voltage in the diode is proportional to its temperature. This means that in order for the electronic thermometer to function correctly, a constant current must be available.
This constant current can be taken from a constant reference voltage source. A very stable reference voltage is provided in the circuit by wiring an IC (labeled as IC1 in the diagram)
with an ordinary zener diode(labeled as D2 in the diagram). This combination works as a super-zener diode. A 5.6V zener diode is selected because this type is the least temperature
The temperature change at the sensor diode results to an output at the IC2 of around -2mV/°C. This voltage output is then amplified by IC3 and delivered to the meter.
Calibration: Potentiometer P1 sets the lowest temperature to be measured.
Potentiometer P2 sets the full deflection at the highest temperature to be measured.
Electronic Circuits volume 1.0 - Circuit Nr. 28
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
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