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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?

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    \$\begingroup\$ What inverting op amp are you using, and what is it's power rails? And what is the +Vs voltage at the LM35? \$\endgroup\$
    – rdtsc
    Jul 15 '17 at 1:54
  • \$\begingroup\$ I'm using a LM324 opamp, and the power rail is 5V, for both, the opamp and the sensor. A direct reading from the sensor is 0.75V while the thermoucople shows 120C. Wires are 30 cm long (1 ft, aprox). \$\endgroup\$ Jul 16 '17 at 4:51
  • \$\begingroup\$ The LM324 is not a rail-to-rail op-amp, so it's output will not go all the way to 0v and 5v. Test one of these out of the circuit and measure it's output while giving it an input voltage. It might only go from 0.02v to 4v. This may or may not be part of the problem, but it's a good idea to test this. \$\endgroup\$
    – rdtsc
    Jul 16 '17 at 13:20
  • \$\begingroup\$ Thanks, I'll try it out ASAP, although the opamp is not the problem. LM35 Vout never goes far beyond 0.75V, when it's expected 1.20V. I wanted to use the opamp in order to isolate the issue. Later today I'll get a newer LM35 sensor, and may be other opamp. \$\endgroup\$ Jul 17 '17 at 16:41
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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 :

enter image description here

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.

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  • \$\begingroup\$ I'll try with shorter wires, and I'll look at the capacitances. Wires are 30 cm long right now. \$\endgroup\$ Jul 16 '17 at 4:52

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