LM35 Problem

[Rod'sInfoQuest]

Hi,
As a complete electronics novice I am struggling to get a LM35 to work
properly. I know it should be simple but for some reason it is not
working and wondered if anyone has any suggestions. I have a string of
6 sensors with a live wire from a 9v battery connected to each via a
rotary switch. The ground goes back to the battery and I measure the
voltage between the middle pin and the ground.
The output of the 6 is different by up to 100 mV at the same
temperature. The output often seems to be around 300 mV when the
temperature is about 20 C, and only changes very slowly with increased
temperature, to the extent that I have no confidence in the output.
Could I have damaged the units, or connected them incorrectly -
currently they are not effective as temperature sensors!
Thanks for any help,
Rod

 

[Tim Auton]

Hi,
As a complete electronics novice I am struggling to get a LM35 to work
properly. I know it should be simple but for some reason it is not
working and wondered if anyone has any suggestions. I have a string of
6 sensors with a live wire from a 9v battery connected to each via a
rotary switch. The ground goes back to the battery and I measure the
voltage between the middle pin and the ground.
The output of the 6 is different by up to 100 mV at the same
temperature. The output often seems to be around 300 mV when the
temperature is about 20 C, and only changes very slowly with increased
temperature, to the extent that I have no confidence in the output.

Slow response is expected in air - check the graph in the top-right
corner of page 5 of the National Semiconductor LM35 datasheet "Thermal
Response In Still Air". It takes about 3 minutes to stabilise at its
final value.

Not waiting long enough will give you inaccurate readings, which will
also make comparisons between units inaccurate.

I'd suggest connecting all 6 in parallel to your 9V battery (i.e. remove
the rotary switch). Ensure they should all be at the same temperature -
fairly close together, with no heat sources (lights, power supplies,
central heating...) very close to them. Then wait a good 5-10 minutes
before measuring the output.


Tim

 

[Lionel]

Hi,
As a complete electronics novice I am struggling to get a LM35 to work
properly. I know it should be simple but for some reason it is not
working and wondered if anyone has any suggestions. I have a string of
6 sensors with a live wire from a 9v battery connected to each via a
rotary switch. The ground goes back to the battery and I measure the
voltage between the middle pin and the ground.

Sounds like you've wired them up incorrectly, or possibly they're
oscillating because you've wired them to long leads without any bypass
circuitry. Check the full data sheet below, paying paticular attention
to the "Typical Applications" section:
<http://www.national.com/pf/LM/LM35.html>