Electronic – LM35 negative temperature sensing circuit

sensortemperature

I'm planning on using the LM35 temperature sensor to measure both positive and negative temperatures. The recommended schematic to measure negative temperatures without a negative voltage supply is this:

LM35 negative temperature

I understand that the role of the diodes is to lift/offset the negative pin from GND. What I don't understand is: What's the role of the 18 kOhm resistor?

Best Answer

A negative temperature is indicated by the LM35 with an output (marked in your circuit as +) level that is more negative than its ground pin (marked in your circuit as -). But the LM35 has no supply of a voltage that is more negative than its ground pin, hence it needs something to create that negative voltage from. That is the current supplied by the 18k resistor.

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