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4-20ma to 1-5v op amp circuit

  1. Jul 28, 2010 #1
    I have found this circuit in my travels on the internet and it should suit my needs. I need to know what this configuration is called so I can calculate the resistors for the gain needed.

    If anyone knows of a similar circuit that has trimpots to adjust the 4mA to 1v and the 20mA t0 5v that would be greatly appreciated.
     

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  2. jcsd
  3. Jul 28, 2010 #2

    dlgoff

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    Welcome to PF ionut

    If you are wanting to convert a 4-20ma source to 1-5volts, why not just use take the voltage across a 250ohm resistor?
     
  4. Jul 28, 2010 #3

    uart

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    R5 is a "burden" resistor and the rest of the circuit is a differential ampifier. You should make R5 a fairly low resistance (around one hundred ohms) and the other four resistors much larger. Make R1=R3 and R2=R4 and R5*R1/R2=250
     
  5. Jul 28, 2010 #4

    uart

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    Yeah I was thinking the same thing, but perhaps he wants to have the curent loop terminals "floating" (well not strictly floating, but non-grounded).
     
  6. Jul 28, 2010 #5
    Thanks for the replies.

    Yes I need the input to be non-grounded. That is why I want to measure the voltage across the resistor(R5). I need to tap into the loop and then feed the mA to the ADC of a microcontroller.

    Where can a put a trimpot to adjust the 4mA to be 1 v and the 20mA to be 5v. I was thinking that this will need some adjustment for zero(4mA) and span(20mA).
     
  7. Jul 30, 2010 #6
    Does anyone have a circuit where the 4mA(zero) and 20mA(span) can be adjusted. I would like a way to adjust the two.
    Or can anyone point me to where I can find such a circuit. I have search the internet for a circuit but can not find one that suits my needs.

    I am new to 4 to 20ma signals and need to convert one for input to an adc. I can not use the 250 ohm resistor method due to loop requirements.
     
  8. Jul 30, 2010 #7

    berkeman

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    Why not just do the conversion after the ADC? You can easily adjust for zero once the 0-20mA signal is digitized. Just set 20mA as full scale, and do the conversion math in the uC after the ADC?

    And what do you mean you can't use the 250 Ohm resistor due to loop requirements. Could you please explain that a bit more?
     
  9. Jul 30, 2010 #8
    I do not want any reference to ground.
    I need to put the least amount of load on the loop due to wire length and other devices on the loop. With the schematic above I can use a 100 ohm or lower resistor for this.

    I do not want to set the zero and span in software when I can have two trimpots for this purpose. I know that there is a way to accomplish this but I do not know how and where to find it. All current loop devices have an adjustment for zero and span depending on what the load is on the loop.

    I have found that I need the following adjustments in the op amp.
    'zero' is offset.
    'span' is gain.

    I found the circuit above doing searches for a few days but there are no means for adjustment.

    Maybe I am just searching the wrong terminology and that is why I can not find it !
     
    Last edited: Jul 30, 2010
  10. Jul 30, 2010 #9

    dlgoff

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    I'm curious ionut. Where are you getting the 4-20ma signal? What is it's source? If it from some sort of transducer; most have zero and full scale adjustment provided.
     
  11. Jul 30, 2010 #10
    Yes one is a pressure sensor and the other is temperature sensor. Two separate loops.
    I can not adjust the whole loop with the transducer/ transmitter since it will affect the whole loop. I need my unit to have span and zero adjustment. I am trying to build the circuit myself with an op amp. I do not want to buy an off the shelf product. Call it a learning experience on how stuff works.
     
  12. Jul 30, 2010 #11

    dlgoff

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    You have pressure and temperature in one loop? :surprised
    Oh well, I'll look around for some loop conditioner circuits.
     
  13. Jul 30, 2010 #12
    First line...

    Yes one is a pressure sensor and the other is temperature sensor. Two separate loops.

    I guess my eyes are not the only ones getting crossed
     
    Last edited: Jul 30, 2010
  14. Aug 6, 2010 #13
    Can anyone point me in the right direction ??
     
  15. Aug 6, 2010 #14

    dlgoff

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    Omega makes http://www.omega.com/vhpc/" [Broken] that claim to be up to 1800Vdc isolation. And this is what they say about their outputs:
    Note:
     
    Last edited by a moderator: May 4, 2017
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