Op-amp help

Discussion in 'The Projects Forum' started by geekoftheweek, May 9, 2014.

  1. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
    2
    Hello all!

    I am new to op-amps and have a few questions I either haven't looked in the right places or googled the right phrases...

    [​IMG]

    In this circuit R2 is actually a variable resistor with 68 ohms being the highest value I will be concerned with. R1 and R2 are a simple voltage divider to create a decent input. I plugged it into my breadboard to see how close things match up and was happy to find the values are close to what I had expected from my calculations. My meter shows 3.82 volts as Vout. I have not measured my resistors to see if they are at marked values which could be causing a bit of the difference along with whatever the op-amp is skewing things.

    Having said that... voltage for Vcc is 5V and 0V for gnd. I am using a lm324... mainly because I have 4 such inputs I want to monitor. My questions are... will Vout ever be more than Vcc? According to my calculations if I put 5 volts to Vin I should have a Vout of 65 volts correct? If Vout cannot be more than Vcc (which I assume is, but wanted a for sure answer) then the resistor/zenner pair on Vout won't be necessary as they were simply a way to limit what my PIC will see on the a/d pin. Would a different op-amp be more closer to ideal or a better choice? Like I said the only reason for this choice was because of the quad op-amp package.

    Thanks for looking this over and any suggestions / criticism you may have. I've been researching these for a while without trying anything (for why I have no idea!!) I just seemed to have a tough time getting a grasp on things... then I actually tried!! I am planning on trying different values for R3 and R4 over the weekend... or I may leave them be. I would like to see closer to say 4.75V for Vout with R2 at 68 ohms.
     
    Last edited: May 9, 2014
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi geek,
    The Vout of a LM324 will always be approx -1.5V less than its supply voltage.
    So with a 5V supply to the LM324 it will limit to approx +4V.

    Is this what you are asking.?
    E
     
    geekoftheweek likes this.
  3. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
    2
    Thanks Eric!
    That is probably the most important answer I was looking for. Is there a specification for that in the datasheet? I see a couple things in the datasheet that could possibly correspond to that but am not for sure. Is that a pretty common value to all op-amps? It looks like I'll be testing to see what the highest Vout I can achieve is and reworking my feedback resistors to bring things into perspective.

    One more step closer to my project becoming reality!!
     
  4. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Try using 8V as Vcc if Vout max does not currently suit your application, then re-calculate R1/R2 and R3/R4 accordingly.
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    Hi,
    Look at this image clip from the LM324 d/s.

    Also note that a 10K load on the output of the LM324 is required if you want the output voltage to go down close to zero out.

    If you have only 5V on your project, you could consider setting the +Vref for the ADC to +2.5V.
     
  6. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
    2
    I did begin to consider something along these lines as I do have higher voltages available, but in the interest of avoiding accidents I thought I would keep everything at 5V.

    Would the feedback resistors serve as this load or does it need to be a separate load itself? Thanks for the highlights of the datasheet.
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    If you used a higher voltage supply for the LM324 your 1K 5V zener would be ok, it would clamp the input to the PIC's ADC to ~5V, also the internal diode clamps in the PIC would clamp at approx 5V.. Its a common method.
    E
     
  9. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
    2
    Awesome link Bertus... I haven't stumbled across that yet, but it's bookmarked now. Unfortunately I really have to get this done when I get up this afternoon so I'm sticking with what I have on hand at the moment.

    I'm thinking the ~0 - ~4 Volt range I've observed through my testing so far will give me enough of a range to get a pretty good picture of what I'm trying to sense... If I go with Eric's suggestion and lower the Vref+ on the PIC to say around 4V then I'll still have almost all 1024 possible a/d converter values to work with and it will give me a really good idea where things stand.

    I'll rework the feedback resistors, put it all together, and get to programming... Hopefully by the end of the day tomorrow I'll have one more thing marked of the to do list!! If all is well I'll snap a few pictures of my progress for you all.

    Thanks again for everything!!
     
  10. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    Hola GOW.

    Next time try to post the image, like this. It is simple to get.
     
  11. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    And with the GIMP, you can easily enhance the contrast for better readability:
    http://www.gimp.org/

    [​IMG]

    Bertus
     
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