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Current Limiting in 4-20mA loop

Muhammad Rehan
Intellectual 345 points
Other Parts Discussed in Thread: XTR108, XTR117, XTR115, LM324, XTR116

Greetings,

I am using a two wire 4-20mA current loop. Now I want to limit the input current to 2mA forcefully if any fault occurs in the system so that I could check if my supervisory circuitry is working fine or not.

Please suggest me any circuitry or IC.

Regards,

Muhammad Rehan.

  • 0
    Guru 22270 points

    Hello Muhammad,

    I would suggest the XTR108.  It has a fault detect and output that can be used to signal your system. It is programmable through SPI.http://www.ti.com/lit/ds/symlink/xtr108.pdf

    There are also the XTR115 to XTR117, but they do not have built in fault detection.  Take a look at the thread below for more information that may aid you in your selection.

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/188741.aspx

    For other 4-20mA devices, you can visit the Product page and do a parametric search for any oher features you may need.

    http://www.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=671&uiTemplateId=NODE_STRY_PGE_T

    Thanks,

  • 0 in reply to Greenja
    Intellectual 345 points

    Hello Greenja,

    Well I need an active current limiter circuit that uses op-amp to limit the current to 2mA. Its not like that the fault occurs in the loop system. I want to limit the current just to check the DAC or ADC at the receiving side. The main idea is to create a fault by limiting loop current to 2mA so that the supervisory IC detects it as a fault and hence resets the MCU and the system. Could you help me in finding that particular cct?

    Regards,

    Muhammad Rehan

  • 0 in reply to Muhammad Rehan
    Guru 22270 points

    Hello Muhammad,

    I believe I know which circuit you are referring to.  I have used it in a discrete 0-36mA circuit that used an LM324 Op Amp and transistor with a PWM input and it provided voltage regulation.  All using one Op Amp!.  It was probably one of the best circuits I have seen.  It also provided the option of 2 or 3 wire 0 to 20mA, 4-20mA and 0-5, 0-10V.

    Unfortunately I don't have that at my disposal right now.  I do believe there is a XTR or similar chip that does this.  You will have to wait on a response from the TI experts on that.

    Thanks,

  • 0 in reply to Greenja
    Intellectual 345 points

    Hello Greenja,

    Thank you for the support. Sure I can wait.

    Regards,

    Muhammad Rehan.

  • 0 in reply to Muhammad Rehan
    TI__Guru 63205 points

    Hello Greenja,

    Thanks for your help.  I would be interested to see the circuit you reference if you're able to find it.

    Muhammad,

    Since your request is rather unique we do not have an integrated circuit with a built-in feature such as the one you're asking for.  I do believe we can implement it pretty easily with an XTR117, an additional resistor and low-impedance switch.

    The circuit below uses two 250kOhm resistors in parallel with a switch in series with one to change from a 4mA zero-scale to a 2mA zero-scale.  See the images below.  With the switch closed, the zero-scale output is 4mA, with the switch open it is 2mA.  

    Hopefully this helps! 

  • 0 in reply to Collin Wells
    Intellectual 345 points

    Hi Collin,

    Thanks for the help. Is the above circuitry applicable with XTR115?

    Regards.

  • 0 in reply to Muhammad Rehan
    TI__Guru 63205 points

    Hello Muhammad,

    Yes, this circuit will work with the XTR115 and/or XTR116 as well.  The XTR115/6 are simply the XTR117 with a built-in precision reference (VREF) in addition to the coarse voltage regulator (VREG).

  • 0 in reply to Collin Wells
    Guru 22270 points

    Collin,

    If I showed you the circuit, it would greatly impact the sales of the entire XTR11x product line.  The last thing we developers need is for someone with your skills, knowledge and expertise entering into the freelance market!  I'll keep it under my hat for now :)

    Thanks,