as the title indicates, the max temperature I can read with a LM35 sensor attached to a heater is 75C, but a thermocouple reading is 120C, and my hand also thinks the heater temp is above 75C by far. The exact number chip is LM35DZ, and the datasheet says the maximum temperature is 150C, so 1200 mV are expected for the heater.
I've seen dozens of examples around this sensor, but almost all of them are room thermomethers (around 25C). I've tried reading directly to the chip, and through an opamp non-inverting amplifier, and the max temp for both are 75C. It seems the chips is stuck at such reading. By the way, the power supply is 5V. Maybe it's a faulty sensor, so I will need to get a newer one ASAP. It's a frustrating issue because it's such an easy chip to use.
What could be the cause of this difference in readings?
Best Answer
The LM35 is very simple to use, but there are still things you can do wrong. It is voltage-output so you are connecting to an amplifier (like an op-amp) output.
As such, putting excessive capacitance on the output can cause high-frequency oscillation, which you may read as a strange DC voltage.
See, for example, the datasheet figure :
The LM35 is not necessarily stable for >50pF loading, so if you have a long cable or a capacitor on the output, that is probably the cause of what you are seeing.