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Questions about Using LM358

Discussion in 'General Electronics Discussion' started by letters109, Sep 18, 2017.

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  1. letters109


    Mar 10, 2017
    Well,I'm using an LM358AD op-amp.
    LM358AD Datasheet
    This is what I'm trying to design, if I change the current source from the 48uA to 8uA the output on the simulation changes but the output does not in physical testing which leads me to believe that something on the op-amp is incorrect in terms of hookups or what not.
    This is how I am currently hooking up the op-amp in terms of the pin-diagram of the LM358:


    What I did notice is that when I do not have the +V connection hooked up to 5V there is a small change whenever I vary the current but that seems to me that the current is entirely bypassing the op-amp entirely since when I short the inverting input and the output (Removing the 100K resistor) the difference is greater. When I do connect the +5V to +V then the volt meter jumps up to ~4V and just stays there, varying the current does not change anything so I'm completely stumped on why it seems to ignore the op amp without +V and only wants to take +V voltage and ignore everything else when it is connected.
    When either the 5V bias from the non-inverting lead or the +V pin is set to ground the op amp seems to not function and any current I deliver just goes straight through the resistor to the volt meter, changing the current does change the voltage but it's just a V = IR relation and nothing to do with the op amp.
    When the +V and the non-inverting lead have voltage bias then it decides to just hit some voltage value and stay there and no changing the current seems to do anything. So it seems like the 5V is just hitting the volt meter and drowning out anything else.

    Any ideas on what I may be missing?


    I've went ahead and provided 3.3V to the input and 5V to the rail so there is that 1.5V difference between them.

    So I've done the following to test it:
    1. Changed the 100k resistor that connects the inverting input and output to 10k because I thought that it was providing a voltage that was too close to the 5V supply and that might be causing problems and didn't change.

    2. Removed the resistor entirely so there is nothing connecting the inverting input to the output and the same thing, nothing changed.

    3. Looked at my circuit again but I don't see any shorts or anything that could be allowing a bypass so I'm stumped.

    Just so weird, if I don't have any voltage to the +V pin it bypasses the op amp entirely and I can measure the change in the current in the volt meter through resistances which makes sense because obviously the op amp isn't turned on. But when I do provide the +V pin power and try to keep the inputs 1.5V away from the rail it just seems to only output 1 single voltage value and ignores the current source from the inverting pin.

    Could you give me some suggestions? Thank you in advance!
    Last edited by a moderator: Sep 18, 2017
  2. Audioguru


    Sep 24, 2016
    1) The datasheet of the LM358 shows that the inputs do not work if they are within 1.5V from the positive supply.
    2) Your 100k pot does nothing and makes the non-inverting input always +5V because it is not a voltage divider that has your disconnected pin connected to ground.
    3) We do not know the polarity of your 48uA. If it is a positive current source then the inverting input is always almost +5V. It should be a current sink to ground. Since the feedback resistor is 100k then the current sink must be 5uA or less so that the inverting input is never higher than +3.5V.
    4) You said, "if you do not have any voltage on the +V pin ...." but which pin is that??
  3. dorke


    Jun 20, 2015
    First thing,
    it is better if you let us know what you need to accomplish in terms of input to output relations.
    What is the resistance of your current source?

    1. The output of your circuit is indeed a simple V = IR relation(inverting),
    why did you expect anything else?

    2. The LM358 is a dual amp. IC,
    Why not use the other amp. to create a voltage amp. of the output of the current to voltage "first stage converter" you have?
    There are many other ways possible as well.

    3. The bias of the non-inverting input should be set to 2.5V ,( 0.5*[+V supply] ).
    the way you biased it (set at 5V) causes your op. amp. to saturate.
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