Temperature Sensor - The LM35

        The LM35 is an integrated circuit sensor that can be used to measure temperature with an electrical output proportional to the temperature (in ^oC)

The LM35 - An Integrated Circuit Temperature Sensor

• Why Use LM35s To Measure Temperature?

• You can measure temperature more accurately than a using a thermistor.

• The sensor circuitry is sealed and not subject to oxidation, etc.

• The LM35 generates a higher output voltage than thermocouples and may not require that the output voltage be amplified.

• What Does An LM35 Look Like?

• Here it is.

• What Does an LM35 Do?  How does it work?

• It has an output voltage that is proportional to the Celsius temperature.

• The scale factor is .01V/^oC

• The LM35 does not require any external calibration or trimming and maintains an accuracy of  +/-0.4 ^oC at room temperature and +/- 0.8 ^oC over a range of 0 ^oC to +100 ^oC.

• Another important characteristic of the LM35DZ is that it draws only 60 micro amps from its supply and possesses a low self-heating capability. The sensor self-heating causes less than 0.1 ^oC temperature rise in still air.

        The LM35 comes in many different packages, including the following.

• TO-92 plastic transistor-like package,

• T0-46 metal can transistor-like package

• 8-lead surface mount SO-8 small outline package

• TO-202 package. (Shown in the picture above)

• How Do You Use An LM35?  (Electrical Connections)

• Here is a commonly used circuit.  For connections refer to the picture above.

• In this circuit, parameter values commonly used are:

• V_c = 4 to 30v

• 5v or 12 v are typical values used.

• R_a = V_c /10^-6

• Actually, it can range from 80 KWto 600 KW, but most just use 80 KW

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• Here is a photo of the LM 35 wired on a circuit board.

• The white wire in the photo goes to the power supply.

• Both the resistor and the black wire go to ground.

• The output voltage is measured from the middle pin to ground.l

• What Can You Expect When You Use An LM35?

• You will need to use a voltmeter to sense Vout.

• The output voltage is converted to temperature by  a simple conversion factor.

• The sensor has a sensitivity of 10mV / ^oC.

• Use a conversion factor that is the reciprocal, that is 100 ^oC/V.

• The general equation used to convert output voltage to temperature is:

• Temperature ( ^oC) = Vout * (100 ^oC/V)

• So if  Vout  is  1V , then, Temperature = 100 ^oC

• The output voltage varies linearly with temperature.