Amplification of LM35 mV Signal using LM741CN opamp

Discussion in 'The Projects Forum' started by pochd, Sep 24, 2010.

  1. pochd

    Thread Starter New Member

    May 6, 2010
    13
    0
    I intend to read the output of several LM35 temp sensors connected to the surface beneath solar panels from a cable length of at least 65 meters using an Agilent 34970A DAQ interface. The LM35 puts out 10mVDC/deg. Would it be alright to use a non-inverting opamp circuit to boost the output?

    Below is a circuit designed to boost a 0.3 DC input by a factor of 6. It works fine in Multisim but when prototyped on a breadboard, 8.36VDC(pin6-1) is the resulting output instead of the expected 1.8VDC. Any idea why?

    [​IMG]


    Also, I intend to use shielded wire to carry the signals due to its bundled nature. Is this ok?

    I would really appreciate your help.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Your greatest problem is in the horrible old 741 op amp. It may be the first "real" op amp, but that was 40 years ago.

    Your op amp in the simulator has the negative voltage pin tied to ground. A 741 is only characterized for a dual supply, and that is +/- 15 volts. Try substituting an LT1006 for the 741. The LT1006 is a single supply op amp and should give the result you wish.

    In general, never use a 741. There are thousands of newer and much better op amps.
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You circuit will not work, not in real life anyway as you have found out.

    The simulator is not giving you the correct result. It is not the simulator's fault but mostly likely because of the Opamp model used.

    The voltage input range of the LM741 opamp is power rail minus 2V on each rail, effective limiting your input range to 2V~7V. Any input voltage outside this range will mean trouble. So your 0.3V input fails.

    In your case, the solution is simple. Use an opamp that has input voltage range that includes 0V. There are many to choose from. The LM324 is a common one, as with the MC3307x or the MC3407x series, where X=1,2,4 or A.
     
  4. pochd

    Thread Starter New Member

    May 6, 2010
    13
    0
    Would the use of the opamp help overcome the cable length problem? Is it ok to use a shielded wire outdoors if protected by a plastic conduit?
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    I've got a lot of LM335 sensors (same device) on our roof connected to common CAT5 cable. I send one Vcc & GND and have the current limiting resistor at each device. The output signals come straight back over the CAT5 cable without any buffering before they hit the op amps in the actual control circuitry.

    Longest run is several hundred feet and all I've got is 0.1 uF caps as a bypass at each end of the signal lines and at each power tapoff - no problems at all.

    Modern CAT5 cable is already PVC so I don't even bother running it through anything, then again I've got plenty of places to add tie wraps to keep the wind from blowing it all over the place.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    Could you sketch that out? I'm curious about the current limiter and the bypass caps. I've had bad luck putting a bypass cap on my board across LM35 output to ground - I hoped it might quiet things but it caused the voltage to rise and no longer give accurate temperature. As the datasheet says, the LM35 doesn't like to see a capacitive load. Resistive load is good for reducing the effect of cable capacitance, and I've connected output to ground through 220 ohms near the op-amp.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    One industrial solution to this problem is to convert your voltage to a 4-20mA current loop. That eliminates the effect of voltage loss across long cable runs. You'd just need to convert your 0-1V (assuming 0-100°C covers your range) to 4-20mA. There are current loop meters that could then read temperature directly off that current. I did something like this recently and can supply my circuit diagram if you're interested. BUT, I'd try other suggestions here first. They may be much simpler.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Apparently the LM35 doesn't like capacitive loads:
    http://www.national.com/ds/LM/LM35.pdf

    whereas the 8 year newer LM335 version doesn't seem to care, at least they don't mention it:
    http://www.st.com/stonline/books/pdf/docs/2158.pdf

    I'm using 4.7K resistors to limit the current from the +5V Vcc line. It's possible I'm only using 0.01 uF caps though, in what I consider to be non-critical bypass situations I sometimes just use what I've got the biggest bag of at the time.

    Once the signals get inside to the controller the first thing they hit are the non-inverting inputs of TL084 op amps with 100K resistors to ground and the outputs to the inverting inputs - in other words simple buffers. The major lengths of the CAT5 cable runs are all within a few inches of a metal roof

    [EDIT:] You might note that these are referenced to *K so they output a voltage under normal operating conditions.
     
    Last edited: Sep 24, 2010
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