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XTR115: Trouble making XTR115 current trasmitter provide proper output current

Scott Daniels
Prodigy 80 points
Part Number: XTR115
Other Parts Discussed in Thread: XTR300, XTR200, XTR111

Tool/software:

I have this circuit using an XTR115 and using R2 to test the current output. As you can see from the text in the schematic, the output current is only 58 microamps instead of 24mA. I am pretty much at my wits' end here. Do you see anything wrong with the circuit as shown? Or the testing of it? I figure I am missing something obvious but cannot see what.

Also, after watching the TI video Introduction to 4-20-mA current loop transmitters, I realize that I should possibly be using a 3-wire solution (I am providing output current from a PLC). However, neither the video nor the data sheet for the xtr300 make a lot of sense to me in how it is connected to an output device - e.g. the transmitter ground and device ground are shown as different (not connected?). Also, is there a reason the XTR115 chip cannot be used as I am attempting in this application (I am supplying VCC)?

The world of current transmitters is new to me, and I would like as simple of a solution as possible for my application.

Thanks for any help.

  • 0
    TI__Genius 15400 points

    Hi Scott, 

    Happy to help find a solution for you!

    First, I recommend taking a look at the E2E FAQ and article linked here to help understand the different transmitter options available. A main advantage of the 2-wire transmitter like the XTR115 is that the sensor is powered by the transmitter but you need to take caution in connecting IRET correctly as the sensor ground cannot be connected to the supply ground. 3-wire transmitters do not have this limitation but require a local supply.

    The basics of 4-20mA current loop transmitters - Planet Analog

    [FAQ] How do I choose a 4-20mA transmitter? - Amplifiers forum - Amplifiers - TI E2E support forums 

    Based on your requirements mentioned, it does sound like you are needing a 3-wire transmitter solution if you have access to a local supply. This is my assumption since I'm not sure what is connected to J1? 

    I recommend our newest 3-wire XTR, XTR200, as it is a simple current transmitter solution with an integrated output transistor.

    Please let me know which design path you would like to move forward with (either the 2-wire or 3-wire) and I can help with further questions for either solution.

    Thank you!
    Regards,
    Ashley 

  • 0 in reply to Ashley Kang
    Prodigy 80 points

    Thank you. The references were very helpful. That solved my testing problem.

    I really like the XTR200, but it is not currently in production. When do you expect it to be placed into production? What alternative do you recommend in the meantime?

  • 0 in reply to Scott Daniels
    TI__Genius 15400 points

    Hi Scott, 

    Great to hear! XTR200 is expected to release to production in October, but please note this timeline is subject to change. In the meantime, we are sampling the XTR200 right now online and have an EVM available to help with evaluation.

    Regards,
    Ashley 

  • 0 in reply to Ashley Kang
    Prodigy 80 points

    For now, I am planning to use the XTR111. I have a question regarding the MOSFET selection. All the recommended ones in the data sheet are rather large devices. Is there any reason I can't use a smaller one such as DMP3160L-7? What are some max ratings I should look for? This device is rated for Vds=30, Vgs=20, Pwr=1.0W, input capacitance=384pF.

  • 0 in reply to Scott Daniels
    TI__Genius 15400 points

    Hi Scott, 

    The MOSFET ratings are dependent on your application surge voltages. What are the maximum overvoltage ratings the device is expected to see? This will dictate the external component selection and if it's less than the breakdown voltage the example MOSFETs in the datasheet show, you can select one that is a lower rated device. 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 80 points

    Hi Ashley,

    Well I am not sure what the answer to that question is. This device is to supply a standard industrial 4-20mA current loop; basically what every other PLC would supply. I do know that industrial equipment operates on 24v and they use 10v for VFD control signalling. Also, I am supplying 24 volts to the VSP pin. I would prefer to use a common Mosfet in a SOT-23-3 package. Can you tell me or help me determine what my max Vds and power dissipation needs to be?  On power dissipation, I am guessing 24v * 20mA = 480mW would be my maximum power expected, so a 1.0Watt device would be adequate? The one I asked about has a Vds = 30V. Is that too low then, given the suggested ones have a Vds of 60?

  • 0 in reply to Scott Daniels
    TI__Genius 15400 points

    Hi Scott, 

    The power dissipations calculation of 24V*20mA is the worst case scenario where the voltage drop across the transistor would be 24V. This scenario would be for example a short, so your power dissipation would realistically be smaller for normal operation. 

    For your application, the MOSFET of Vds = 30V (DMP3160L-7) should be sufficient. 

    Regards,
    Ashley