Re: Temperature sensor LM35 and long leads

Pandora wrote:

I know this topic has been discussed previously but I wondered if
anyone had personal experience of an application similar to mine.

I wish to monitor the air temperature (-5 to +30 deg C) at various
points in a void below the office floor. The void is normally
inaccessible, but the floor is being re-laid at the moment so I have a
"one-shot" opportunity to put the temperature sensors in place.

For each monitoring point, I plan to use National Semiconductor's LM35
three-lead temperature sensor with a 2K2 resistor in series with the
output lead. This sensor/resistor will be soldered to one end of a
two-pair screened twisted pair cable (Belden 8723) such that the output
and ground signals are carried on one pair, and the power supply will
be carried on the other pair (the two grounds will be commoned at the
sensor).

The max. length of the cable will be 5 metres (16 feet). I will power
the IC with either a 9V battery or floating 12V d.c. supply and use a
1M-impedance digital multimeter to measure the output voltage. When the
output is not being monitored, the LM35 supply will be switched off.

I require an accuracy of about ±2 deg C

Has anyone achieved good results in similar applications using the
above approach? My particular concerns are whether the capacitance of
the twisted pair cable causing an error in reading and whether the LM35
will latch up.

Do I require any bypass capacitors at the sensor end?

1) A shielded pair will do the job: common, power and signal.
2) AFAIK the LM35 is not sensitive to capacitance, but a series 1K or 10K from the LM35 output and the cable would give more than sufficent isolation.
3) You will have to calibrate each and every LM35 used to get +/- 2C accuracy; finding a standard and getting a constant temperature stirred bath that can be varied over that range will both be challenging, unless you are willing to spend a fair amount of $.
4) OTH, they are not bad to 210C!
.

Relevant Pages

  • Re: Temperature sensor LM35 and long leads
    ... I wish to monitor the air temperature at various ... I plan to use National Semiconductor's LM35 ... three-lead temperature sensor with a 2K2 resistor in series with the ...
    (sci.electronics.design)
  • Temperature sensor LM35 and long leads
    ... I wish to monitor the air temperature at various ... "one-shot" opportunity to put the temperature sensors in place. ... I plan to use National Semiconductor's LM35 ... three-lead temperature sensor with a 2K2 resistor in series with the ...
    (sci.electronics.design)
  • Re: Temperature sensor LM35 and long leads
    ... I wish to monitor the air temperature at various ... I plan to use National Semiconductor's LM35 ... three-lead temperature sensor with a 2K2 resistor in series with the ...
    (sci.electronics.design)
  • Re: Temperature sensor LM35 and long leads
    ... I wish to monitor the air temperature at various ... I plan to use National Semiconductor's LM35 ... three-lead temperature sensor with a 2K2 resistor in series with the ...
    (sci.electronics.design)
  • Re: Searching for Spice Model - National LM35
    ... This model will simulate the temperature dependent output of an LM35, ... Temperature Sensor ...
    (sci.electronics.cad)
Quantcast