The LM35 is basically a thermometer on a chip, designed by National Semiconductor. Instead of using mercury or another fluid, it relies on a property of diodes that causes the breakdown voltage of the diode to increase with temperature. This relationship is linear and predictable, making the LM35 a very accurate temperature sensor, with a voltage output that is linearly proportional to temperature.
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Range and Accuracy
The LM35 measures a temperature range of -55 to 150 degrees Celsius. It produces an analogue voltage signal that has a linear relationship to temperature, with a scale of 10.0 mV per degree Celsius. At room temperature, the LM35 has a typical accuracy of plus or minus 0.25 degrees Celsius, and plus or minus 0.75 degrees Celsius over the full temperature range. In still air, it takes three minutes for the output to reach its final value; at one minute, the output is about 70 per cent of the final value.
Voltage and Current
The sensor normally takes a supply voltage of either 5 or 10 volts, but it can accept a supply voltage ranging from 4 to 30 volts. The LM35 draws only 60 uA of current from a single or bipolar power supply. If you use a single supply voltage, the chip will measure temperatures from 0 to 150 degrees Celsius, where 0 degrees outputs 0 V and 150 degrees outputs 1,500 mV.
Reading Negative Temperatures
There are two ways to read temperatures below freezing. The first way is to give the chip a negative supply voltage, in addition to the normal positive supply voltage. This is called a bipolar power supply. In this configuration, the chip will output below-freezing temperatures as a negative voltage. To do this, connect the output pin of the chip to the negative supply voltage through a resistor. Choose the value of the resistor by dividing the absolute value of the negative supply voltage by 0.0005 amps. The second method uses a single power supply. Connect the ground pin of the chip to ground through a diode, and connect the output to ground through a resistor. Again, choose the resistor value by dividing the supply voltage by 0.0005 amps. In this case, your actual reading is the difference between the voltage at the output pin and the voltage at the ground pin.
As of October 2010, all LM35-series integrated circuit temperature sensors are available in a TO-46 transistor package. The LM35C, LM35CA and LM35D variations are also available in plastic TO-92 transistor packages, and the LM35D is available in an 8-lead small outline package and a plastic TO-220 package. The device only uses three pins of the small outline package. Pin one is the output, pin four is the ground, and pin eight is the supply voltage. The transistor package pinout, from left to right, is supply, ground, output. The LM35C has a smaller effective range, from -40 to 110 degrees Celsius, but higher accuracy.
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