# Setting Reference Pin on LM35

Discussion in 'The Projects Forum' started by sureshparanjape, Nov 8, 2014.

1. ### sureshparanjape Thread Starter Active Member

Feb 10, 2012
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2
Sir,
I have taken up a small project of temperature control of a room, heated by electrical heater. I have a basic circuit with LM35, two IC 741 comparators- one for upper temperature control and another for lower temperature control. I don't like the idea of using POT and trial method to decide resistance values for connections from the DC source. Is there a mathematical way to determine what should be the resistance if one wants to control temperature between say 20C and 25 C? I wasn't successful in locating the formula.
I have a query- how does voltage at the reference pin of LM35 changes with resistance? Does "voltage divider created by the LM35 circuit resistance, resistance on connection from the source " make changes in the voltage at the reference pin?
I hope my question makes a sense!
My circuit, which is not very complicated, is working.
sureshparanjape

2. ### Lestraveled Well-Known Member

May 19, 2014
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sureshparanjape
The LM35 does not have a reference pin, so I am confused about your question about it.

The LM35 output is scaled at 10 milli-volts per degree C. So, 20 degrees C would equal 0.20 volts output.

3. ### DickCappels Moderator

Aug 21, 2008
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Lestraveled is correct except that when calibrated, the cathode of the LM35 is at a voltage that corresponds to absolute temperature (0° Kelivin = 273.16°C); still 10 mv/°C but 20° would read 10 mv x (273.15°C + 20°C) = 2.9315 volts.

Calibration: Set the voltage at the "Output" pin (cathode) to the absolute temperature that the LM35 is at when calibration is performed..

The other details:
http://cappels.org/dproj/lm335th/LM335thermometer.html

Last edited: Nov 8, 2014
4. ### Lestraveled Well-Known Member

May 19, 2014
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Dick
The LM35 is truly calibrated in centigrade. You do not have to subtract the Kelvin (273.16) offset to get Centigrade. The LM335 is calibrated in Kelvin.

DickCappels likes this.
5. ### wayneh Expert

Sep 9, 2010
13,408
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This is an accurate statement. If you need to go a bit lower, the LM34 goes to 0°F.

On the high end, to LM35 needs enough supply voltage to produce its output. In other words, you can't get past 50°C without more than 5V on the supply.

The LM35 cannot be calibrated internally. You take what you get.

6. ### DickCappels Moderator

Aug 21, 2008
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Thank you for catching my error Lestraveled.