How to Design OPAMP circuit for this requirement

Discussion in 'The Projects Forum' started by Santhakumar, Mar 7, 2016.

  1. Santhakumar

    Thread Starter Member

    Mar 17, 2015
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    Hi all,
    i've been looking for a circuit may be based on OP AMP solution.
    My requirement is that i am feeding varying input voltage (0 to 24V) to an circuit (could be op amp) it's output would be 0 to 3.3V.
    So thing is my output should be correlated with input that means
    1. Say input is 0% (0V) my output shout be 0% (0V).
    2. Say input is 50% (12V) output should be 50% (1.65V)
    3. Say input is 100% (24V) output should be 100% (3.3V)

    Do anyone have idea on how to have circuit for this.
    Thanks,
    -Santhakumar.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    All you need is a two resistor voltage divider if the 3.3V load is a high impedance.
    For such a load you could use a 1.62kΩ resistor to ground connected to a 10.2kΩ resistor going to the 0-24V source. The junction of the two resistors gives you 0-3.3V (nominally 0-3.29V).

    What is the load?
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Two resistors in a voltage divider across the 24v supply, 16K, 100K, will give you 1.65v @12v, 3.31v at 24v.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Did you read my post?
    My resistor ratios give an output slightly closer to the desired. :p
     
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  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    My resistor is better than your resistor-? Really?!?!? Are you running for president?

    ak
     
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  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Those old resistors are losers.
    We'll build a better resistor. You'll love it. It will be great!
     
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  7. #12

    Expert

    Nov 30, 2010
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    1% resistors almost always arrive at a more accurate solution, but I have a 100K and a 16K in my parts bin. It's a matter of convenience vs accuracy. Pick one.
     
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  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    images.jpeg
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    cmartinez and #12 like this.
  10. crutschow

    Expert

    Mar 14, 2008
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    Hell no. How did politics get into this? :eek:

    But for those interested nit pickers (and I assume you're not one), for an arbitrary resistor attenuation using 1% resistors there are generally many combinations of resistor values that give close to the desired value but some are closer than others. To determine the optimum combination I wrote a short visual basic program a number of years ago that looks at the combinations and lists them according to how close they are to the desired.
    Here is a screen shot of the values to get the desired attenuation of 3.3/24 = .1375
    The optimum is 1.62k and 10.2k.
    Of course that delta doesn't include the effect of the 1% resistor tolerance.

    upload_2016-3-7_18-17-47.png
     
  11. ifixit

    Distinguished Member

    Nov 20, 2008
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    How about: 24V to 18K+2.7K with 3.3K to gnd. The output (0 to 3.3000V) across the 3,3K can be buffered with a X1 op-amp if needed.
     
  12. Santhakumar

    Thread Starter Member

    Mar 17, 2015
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    Hi All,

    Thanks for the Answers and i can use it in my circuit.
    Is there any other ways we can make it without resistor divider. May be by using OPAMP comparator.
     
  13. #12

    Expert

    Nov 30, 2010
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    No. A comparator is not a voltage divider.
    Why do you want to avoid the cheapest, simplest, most accurate way to do this?
    I think I can do this with an op-amp, but why add the extra parts?
     
    Last edited: Mar 8, 2016
  14. Santhakumar

    Thread Starter Member

    Mar 17, 2015
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    Hi #12,

    Any way i'm gonna use opamp in my circuit so that i just want to hear more suggestion on this to have other options.

    -SK
     
  15. Dodgydave

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    Jun 22, 2012
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    Last edited: Mar 8, 2016
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  16. #12

    Expert

    Nov 30, 2010
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    My circuit has more useless parts than your circuit.:p
    and it requires a dual voltage power supply.:cool:
     
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  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    @#12,, me thinks he wants to make it more useless, may need a Bigger op amp like a TDA2040 :confused:
     
  18. #12

    Expert

    Nov 30, 2010
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    I think he's learning about op-amps and just has to see one in action.
     
  19. Santhakumar

    Thread Starter Member

    Mar 17, 2015
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    Hi #12,

    Thanks for your circuit, really appreciate your opamp circuit.
    I would use resistor divider rather using dual opamp and dual supply.

    And this output of divider would goes to ADC.. I would like read consistent values from the divider and so ADC.
    care can be taken at software level. but hardware level how effective i can accomplish it..

    thanks.
     
  20. #12

    Expert

    Nov 30, 2010
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    The consistent values start with your power supply. I don't know whether the ADC tracks the power supply voltage or not. I don't know whether to instruct you on generating a stable voltage reference or expect the ADC to adapt if it is connected to the same 5V line as the resistors. You just stepped outside my limitations.
     
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