Temperature sensor LM35 and long leads

Noise spikes can latch an LM35, especially if the supply voltage is above +5. Current-limit the supply, bypass Vcc at the LM35, and use a series resistor between the LM35 output and the cable, as you suggested.

The LM35 will work below 0 C if you pull its output below ground, in which case the likelihood of latchup goes way up. You could use an LM34, which works to 0 F with no pulldown. Better yet, use an RTD.

The LM35 has lots of personality.

John

I used about a dozen LM35's in a hyronic heating project. The App Note had a current-loop design such that it ran on 2-wires, period.

The current loop did introduce a fixed error of 1C, as I recall. I was looking at the differential & didn't much care.

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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!

I am using a 8V battery (dying 9V) in a noisy environment, and have run the LM35 to at least 210C on a number of occasions and have seen zero problems.

...and, one can always subtract out that error.

Robert, why would I have to calibrate the LM35? Their worst case error is =B12=B0C

A single point calibration is all you'd need to get rid of the bulk of the error on an LM35 (or an LM34). An ice bath is the easiest fixed point to set up, but it isn't much use for the LM35 though it would be fine for the LM34 already recommended by John Larkin.

The NBS procedue for setting up an ice-bath can be found on the web. I've followed it - more or less - and got stability to something like

0,001C, We used de-ionised water to make the ice and for the waer in the bath, but I had to break up the ice-cubes with a hammer on the tiled bench top, so I can't be absolutely confident about the actual temperature

--=20 Bill Sloman, Nijmegen

If the LM35 and resistor are just dangling at the end of the cable you may find that self-heating eats up a fair bit of your error budget.

Steve

Or use an LM335 which reads out in Kelvin.

Steve

The LM35 is tolerably good as far as self-heating goes - around 300uW. A thermistor dissipates less heat (10uW is typical, and they tend to become unstable at higher dissipations) but offers rather higher thermal resistance to ambient. It is a lot easier to stick a decent heat-sink on a TO-220 packaged LM35 (Farnell order code 431-6721) than it is to improve the thermal resistance to ambient of a thermistor.

--
Bill Sloman, Nijmegen

The datasheet suggests otherwise.

John

LM35's came in a zoo's worth of species; some were much tighter specs than others, some had a wider temp range, etc etc.

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A host is a host from coast to coast.................wb8foz@nrk.com
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Unless the host (that isn\'t close).........................pob 1433
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There is percision, accuracy, and error; three different things.

Milliwatts???

Suggestions and experience are totally different animals.

Do you know more about the LM35 than National does?

John

Yes....the OP suggested a 12V supply and 2.2k resistor...thats around

50 mW for the sensor and resistor together - that will warm up a TO-92 and small resistor quite noticeably. Using the TO-220 package rather than the smaller options, a small heat sink and putting the resistor at the other end of the cable will all help greatly of course.

I've just been playing with an LM335Z circuit. I saw around 0.5 degree warmup drift while testing with the sensor soldered to a small PC board (partly from sensor self heating and partly from heating from the 100 mW or so dissipation in other the components on the board). The warm-up drift is insignificant now the sensor is mounted by itself on a heatsink.

Steve

If you can handle a bit wider tolerance, you might consider the LM50.

National must have figured out how much trouble capacitance loading is with the LM34/35, because they made the LM50 with an output that can drive much more.

The LM50 has an offset in the output so that you only need a positive supply for both positive and negative temperatures.

Only disadvantage for me was the tiny package.

Gary Peek Industrologic, Inc.

The resistor is in series with the output, to isolate the chip from cable capacitance, so dissipates no power. But yes, self-heating needs to be calculated, especially if the thing's not on a pc board.

The cable itself makes a decent thermal antenna!

John