 # LM35DZ negative voltage

Hi there!

so I have a few of these: http://www.ti.com/lit/ds/symlink/lm35.pdf

when operating above freezing temperatures, the wiring couldn't be simpler, just measure the positive voltage, and the reading equates to the temp.

negative, however, is giving me a bit more trouble, as it outputs a negative voltage when the temperature of the sensor is below freezing. They give only a single example for this, which has me a bit lost, as I don't have any diodes or knowledge working with them. I was wondering, is there any way this voltage range could be rectified to 0v (being -55 degrees C) to 5v (being 155 degrees C)? is this something which could be done with a voltage divider, for instance?

Get some signal diodes, 1N4148 and 1N914 are cheap, ubiquitous signal diodes with
identical specs, here just used to level shift the supply to the LM35 - you then simply
measure both voltages as figure 18 indicates, and take the difference, no negative
supply or negative voltages needed.

well, I am hoping to find a way to do this using only one analog pin per sensor, as I have 12 of them I would like to use. I was hoping to find a way to merge thesignal into one standard 0-5v output :)

I don't have an easy solution for you...

An op-amp can be used to make a differential amplifier (taking the difference between the + & - outputs and giving you one positive output). Or it can be used to make a summing amplifier so you can add a fixed positive bias voltage.

In order for op-amps to work at around zero volts, they need dual (positive and negative) power supplies. And with 12 signals, you'd need 3 quad op-amps.

Note that single-stage summing amplifiers are inverting amplifiers, so you'd need a negative input-bias to create a positive output-bias, and you'd have to "invert" the readings in software.

P.S. There's probably a way to create a virtual ground for the temperature sensors. (Again, with an op-amp.*)

For example, if the temperature sensor's reference (virtual ground) is +1V, when the temperature sensor puts-out -1V (relative to it's own reference), it will be zero-volts relative to the Arduino's true-ground.

That should also eliminate the need for a negative power supply voltage, since true-ground is now negative relative to virtual ground.

As long as the op-amp can supply enough current, you'd only need one op-amp, since all of the sensors can share the same virtual ground.

• That would be a voltage divider (2 resistors) to set the reference voltage followed by a non-inverting op-amp buffer (an op-amp with 100% feedback and no gain-setting resistors).

rytcd: well, I am hoping to find a way to do this using only one analog pin per sensor, as I have 12 of them I would like to use. I was hoping to find a way to merge thesignal into one standard 0-5v output :)

You can share the biasing diodes with multiple sensors, they only shift the voltage. Then you only need one reference reading to subtract from the individual readings.

If you have a lot of temperatures to read the DS18B20 is an alternative to consider, one digital pin for many sensors.

A LM35*DZ are not for negative temperature. **D* letter means range [0°C + 70 °C] Under négative température it can works or....... not.

LM35, LM35C, LM35A, LM35CA are good for negative temperature. LM35D is the only one which is false.

Look at TMP01, gives you the absolut temperatur in degrees Kelvin. Always positive. 1,49 volts at 25 C and 5 mV/degree.

Pelle