LM35 and instability below 6 degree...

[synsyn]

Hello,
I have make a digital termostat with a LM35dz. So all work well untile the
temperature around the probe stay above 7 degree, but when the temperature falling down under the 7 degree i will get a very instable measurement. I have tried to change the pics(16f676) the lm35 the cable the tension(5v) but i get alway the same instability below 6/7 degree. :frown:
Have you any idea where is the problem ? Maybe i need to put some condenser o resistance on lm35 pins ?
Thanks for read.

[Acetronics2]

The problem is you are much too close to the minimum the sensor AND the adc can measure ...
moreover you are close to 10 ADC counts ( IF 10 bits used ... :D ) and the jitter is ... +/- 1 LSB !!!

no need to talk about 8 bits conversion ... ???

so, guaranteed 10% error ... @ minimum !

try a LM 335 instead ( voltage will be 2.79 v for 6.0°C ... ) and the Pic ADC will work perfectly ...

Alain

[synsyn]

Hi Acetronic2,
thanks for your reply and sorry for my late to reply to you.
There's no adc problem because if i use a trimmer i can read very low voltage and sure i use a 10bit conversion.
In anyway i have partially resolved the issue with more readings(64) for time and now i get more stable reading, but at low temperature the LM35 stay very unprecise.
I have see that if i unpower the circuit(and lm35 too) and after few second repower it i will get 2 degree different read when the enviroment temperature is below 9 degree.
So i think i need to change temperature probe because lm35 is very unsatisfactory for me for this aplication.

[mark_s]

I found it was very important to have a bypass capacitor (0.1uf) right at the power leads of the LM35. If you don't already have one it might help the stability.

[synsyn]

I have try to do some experiment and i have find than if i measure the output voltage from lm35 with a multimeter i get the correct one. So the problem is not in the probe(thank's Mark for the tips with capacitor) but in adc reading of my 16f676 :-(
The strange thing is if i try to take some measure with a trimmer tied from vcc and gnd i will get always the correct and stable measure but when i try do measure the output voltage from lm35 i read different voltage variations +/- 2v and the measure is very unstable. I don't know more what i can try for have a decent measure from lm35.

[AvionicsMaster1]

In these two measurements are you using the same power supply? You say you use a pot and it was good but using the LM35 you get erroneous voltages.

I wonder if when troubleshooting you used different power supplies and the DC voltage may be from an analog supply in the first case and a switcher the second. If so the switcher may need an electrolytic on the supply to the LM35.

In the last part of your post, how and where are you measuring the voltage? Could your meter/probe be loading the output of LM35? It would be interesting if you'd show some sketches of how you measured those voltages and what's connected to the LM35.

Just some thoughts.

[synsyn]

Hi AvionicsMaster1,
For my measurement i have used a professional laboratory PSU.
For my experiment i have used a pic16f676 on a breadboard and with a direct connection to LM35.
When i try to read the output voltage from LM35, i make a minimun of 32 ADC lecture and take out the middle, and it work very good if the temperature stay around 10 degree but when it falling down under this value i get more instability in reading the voltage output from the sensor. Instead if i try to read the output with a multimeter i get alway a stable meause.
In the weekend i will try with a different circuit and with a different PIC sample.
In anyway for the future i will use a DS1820 chip for avoid any problems and for simply use of it with any kind of PIC.

[AvionicsMaster1]

If I understand you correctly, if the voltage is steady using a voltmeter to read the LM35 output but your readings once massaged thru the PIC are unstable then I'd blame the code or the inability of the PIC to read the output.

In post #2 Acetronics says you are very close to minimum reading and that may be the case. He also suggests a different sensor which may be what you have to do to get an accurate reading.