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LM35: temperature sensor out of range in manuf. quality?

miki Dvir
Prodigy 40 points
Part Number: LM35

Hello

an existing old design (10 years in use) and over 3000 units produced.  about 5% fail of temp sensor out of range.  this old design , unipolar +5v supply has a resistor on the output, 100ohm. looks very small to my understanding.  according to datasheet, in a different config, the resistor is vs/50uamp = 100kohm...is it possible that there is too much current and the sensor heats up?  is this resistor needed at all?

  • 0
    TI__Mastermind 39845 points

    Hi miki Dvir,

    Are you saying your LM35 has a 100ohm resistor between its output pin and ground? This is a lot of load for LM35, but it does not exceed the Load Regulation spec of 1mA until the sensor is operated above 100C (100mV output.) As you noticed, the LM35 typically has loads in the kilo-ohm range. An output load is not required for LM35 operation, but may play an important role in the rest of your application circuit. For example, LM35 can't report temperatures below 0C unless it has a bias resistor below LM35's ground (See illustration on first page of datasheet.) I could review your schematic if you're able to post it.

    thanks,

    ren 

  • 0 in reply to Ren Schackmann
    Prodigy 40 points

    Hi Ren

    thanks for your reply. I thought 100ohm nay be unnecessary. I asked the manufacturing to remove the resistor and retest. let's see what happens...
    we are not measuring below 0deg therefore the unipolar supply. you pointed at the drawing in teh datasheet so maybe you can shed some light in the application showing the output measured between the out pin and two diodes on teh gnd pin. there is no explanation on this. any insight ?

    thanks

    miki

  • 0 in reply to miki Dvir
    TI__Mastermind 39845 points

    Hi miki,

    I assume you're asking about Figure 18. The diodes lift LM35's ground pin above 0V, creating a voltage offset. That way, the load resistor can be connected to system ground which is below the LM35's ground. It has the same effect as the illustration on the first page, but all output voltages from LM35 will be offset by 2*Vf of the diodes.

    thanks,

    ren