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LM318 Op-Amp and TMP36 Temp sensor question

Discussion in 'General Electronics Chat' started by Andy1845c, Nov 19, 2017.

  1. Andy1845c

    Andy1845c Active Member

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    Hello,

    I want to use an analog temp sensor like the TMP36 and interface it with a 0-10V card on a PLC. I just happened to have a TMP36 laying around so I mocked it up and am getting ~.7 volts from it at the target temp (basically room temp).
    I'd like to raise the voltage in a linear fashion to something more workable. I tried to use a LM318 (again, had it laying around) and hooked it up as shown in my attached schematic.
    I cannot get an output of any kind from the op-amp. I have my .7 going in and nothing coming out. I have tried several ICs trying to rule out having a bad one. I've never used an op-amp before so I may very well be missing something basic.

    I've studied AudioGuru's circuit (http://www.electro-tech-online.com/attachments/opamps-gif.16569/) and mine matches his non-inverting single supply circuit minus the voltage divider.

    Anyone have any suggestions?
     

    Attached Files:

  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    For a single-polarity supply, as you have, the LM318 cannot operate properly with an input voltage less than about 3.5V, according to its datasheet.
     
  3. Andy1845c

    Andy1845c Active Member

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    Thanks for the reply Alec. Where on the data sheet is the minimum input voltage specified? I see an "Input Voltage Range - Min 11.5v". I don't see the 3.5V figure anyplace. Is there a better op-amp you would recommend for this?

    Also, if anyone is so inclined - if I wanted to translate the .7 volts into a 4-20 mA signal - would that be exceedingly complicated? I don't know that I want to, but I have been pondering it in my head.
     
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    The LM318 is a very old design, and is not rail-to-rail in or out. Even the crummy old LM324 or LM358 would work in your application because it's common-mode input voltage range goes down to -0.5V with respect to the V- pin.
    There are now hundreds of opamps that are rail-to-rail in and out

    63.png
     
    Last edited: Nov 19, 2017
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    It's deduced from the -15V supply and the -11.5V figures.
    Does 0.7V correspond to 4mA or 20mA or ....?
     
  7. Andy1845c

    Andy1845c Active Member

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    What would happen if I change the supply voltage? I did try it at a variety of voltages up to 18v and got no output.

    0.7v would ideally be about 12mA or something. Mostly I am just wondering where I would start if I wanted to play with that route.
     
  8. Andy1845c

    Andy1845c Active Member

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    Ill see if I by chance have either of those around. I've never used op-amps before so I haven't really stocked up on them
     
  9. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    To make that circuit work with the '318, you would have to add a negative supply of at least 4V. This makes for split supplies, +15 and -4V.
     
  10. Andy1845c

    Andy1845c Active Member

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    Thank you Mike. I clearly need to study how op-amps work :)
     
  11. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    This is what it would take to make the LM318 work in your circuit:

    63a.png
     
  12. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Here's how you could do that :
    CurrentLoop.PNG
     

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