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LM35: LM35

Part Number: LM35

Hi I am using LM35 precision centigrade temperature sensor in our 32A dc controlled three phase AC Solid state relay. Just while running the temperature rise test hooked up to picolog to find the temperature rise. one of the temperature sensor out of 2 fails after 30 minutes. followed by thyristor failing. Would this short happens because of thermal tapes used or some short circuit. 

  • Hi Shreelekha,

    Could you please provide a little more information on the failure you are seeing with LM35? When you say it fails after 30 minutes, what are you seeing on the VOUT pin of the LM35? Could you please provide details on these temperature tapes and the temperature rise test?


    -Alex Thompson

  • Hi Alex I am not sure how to measure the vout. I have just hooked up the data logger to SSR on top of thyristors and temperature sensor. So what I could see is first the temperature sensor fails and the next is thyristor. But I can see in graph after 15 minutes there was a drop in temperature from 20C to 5C like that and again back to 20C and the temperature never improved while the other temperature sensor shows 58C. After 30 minutes the thyristors are blown one after the other. 

  • Hi Shreelekha,

    You could measure the analog output of the LM35 with an oscilloscope or an analog input pin on an MCU, to see directly what the part is doing. But this is enough information for now.

    I noticed from your schematic that you are wiring the output of the LM35 directly to the positive input terminal of U4 to buffer the output signal. I recommend following the suggestions on page 14 of the LM35 datasheet, by adding a filter resistor and capacitor to the analog output before it enters the buffer amplifier. By having the filter resistor and capacitor on the analog output, that should help the output remain stable when connecting to high impedance input pin on the buffer.


    -Alex Thompson