help with lm35 circuit needed

Discussion in 'The Projects Forum' started by ddonn, Feb 22, 2010.

  1. ddonn

    Thread Starter New Member

    Feb 10, 2010
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    I have set up a simple temprature measurement circuit as shown in the link provided. I am using a 5V supply and a 500K resistor. At room temperature i am getting an output of .2mV instead of the approx 200mV i expected. Does the lm35 chip need to be calibrated and can anyone tell me if the circuit shown is correct?
    http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/TempLM35.html
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Have you used Google to get a data sheet?
     
  3. ddonn

    Thread Starter New Member

    Feb 10, 2010
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    Yes, I have a copy of datasheet. I checked the calculations for resistor values and it says to divide Vs by 50uA instead of 10E-6 stated on the circuit i constructed so i'll try that
     
    Last edited: Feb 22, 2010
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Ask your instructor/professor why the circuit on the website does not agree with the datasheet for the LM35.

    Ra is only used when a negative supply is also used. See page 1 of the datasheet.
     
  5. ddonn

    Thread Starter New Member

    Feb 10, 2010
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    Unfortunately it is a research only project so help from lecturer is not an option. I need to measure in the minus degree range also so i take it i need a plus and minus supply???
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    That is correct; you need a bipolar supply. (+V and -V)

    Whatever supply you use, you will need to calculate the resistor as the datasheet states:
    R1 = -V/50uA
    So, if you had a +/-5v supply, then:
    5v/50uA = 100k Ohms - or basically, 20k Ohms per volt of -V.

    [eta]
    Actually, there are more applications circuits in the datasheet starting page 7. Figure 7 might appeal to you for its' simplicity, but the diodes' Vf will change over temperature, and I frankly doubt the circuits' accuracy. If the temp range isn't extreme or not critical, you might just ignore the variations.

    Remember that the LM35 requires at least 4v across it in order to give a valid output. A 1N4148/1N914 diode will drop about 460mV with a 50uA current, and about 380mV with a 10uA current at room temperature - so that circuit would require 5V minimum.
     
    Last edited: Feb 22, 2010
  7. ddonn

    Thread Starter New Member

    Feb 10, 2010
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    0
    Thanks, i'll give that a try
     
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