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Pressure Transmitter

Discussion in 'Sensors and Actuators' started by Pushpraj7453, Jun 2, 2021.

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  1. Pushpraj7453


    Jun 2, 2021
    I want to connect resistor instead of pressure transducer.
    Pressure transducer output is 2 wired 4-20MA and it is connected to PLC.
    Its urgent help
  2. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Wow, @Pushpraj7453! You are just a fountain of useful information that some of us may have been able to use to solve your problem... if only you would tell us WTF you are trying to DO!

    Oh, really? Just unplug the pressure transducer and replace it with a resistor? What is that supposed to do?

    What value resistor will you substitute for the transducer? What will supply the 4 ma to 20 ma current that is necessary to create a voltage drop across the substituted resistor? Are you trying to calibrate the pressure transducer? Do you want to replace the transducer with a "dummy transducer" for test or prototyping purposes?

    If it is so urgent, why not provide us with ALL the details of what you are trying to DO?
    Harald Kapp likes this.
  3. Pushpraj7453


    Jun 2, 2021
    Yes i want to create dummy for 4 mA with resistor for calibration.
    What value i supposed to use ??
    Can i apply ohms law ??
    Pressure transmitter is 24 V loop powered
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You can try that as a starting point. However, the pressure transducer will generate a controlled current between 4 mA ... 20 mA depending on the pressure and depending not so much on the 24 V available from the PLC. When you replace the sensor by a resistor the current becomes dependent on the 24 V supply voltage. Any variation in this voltage will show as a variation in current.

    As @hevans1944 asked: what are you trying to do? You don't want to replace a faulty sensor by a resistor to trick the PLC into operating a machine, do you? The pressure sensor certainly was inserted to check the operating conditions of the machine. Bypassing it by a resistor is a bad idea. The PLC will no longer be abler to verify the correct operation of the machine. Damage to the machine may result, worst case people's lives are in danger.
    Have you heard the recent news from Italy? A cable car crashed, 14 died because a safety feature had been disabled...
    You don't want that to happen to you.
    hevans1944 likes this.
  5. Pushpraj7453


    Jun 2, 2021
    My PLC has programmed in such a way that without pressure transducer it wont run but in my case i want to use same program but i dont need pressure transducer and my boss is forcing me for cost cutting thats why i want another solution alao dont want to change the program
  6. Kiwi


    Jan 28, 2013
    The system consists of a closed loop from the output of the pressure transducer to the PLC input, through an internal resistor in the PLC to the input ground, and back to the ground on the transducer.
    The pressure transducer creates a variable current in this loop of between 4mA and 20mA depending on pressure. Usually more pressure equals more current.
    There is a converter in the PLC that measures the voltage drop across its internal resistor, and converts it to a pressure reading.

    What you have to do is trick the PLC by creating a current source to replace the transducer.

    This may work, but could have issues. Try at your own risk.
    The quickest "dirty" way to do this would be to connect a large value resistor between the PLC power supply and the PLC input.
    The input's ground terminal will need to be connected to the PLC ground to complete the loop.
    Connect an ammeter in series with the resistor to view the current.
    Lower the value of the resistor to increase the current to get the reading you require.

    As advised earlier, only do this if bypassing the pressure transducer will not create a possible dangerous situation for the machine, operators, or other people, etc.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    @Pushpraj7453: It would help us to help you if you would upload some images of the usual pressure transducer (like the one you don't want to use) along with a datasheet that describes the pressure transducer and the current-loop interface to the PLC.

    As @Harald Kapp and @Kiwi have warned you, defeating a PLC input that the PLC programmer thought was necessary should require a re-write of the PLC program to eliminate the pressure transducer as an input. If that isn't possible, a competent electronics technician or an electrical engineer can substitute a dummy resistor to replace the pressure transducer, after making certain electrical measurements to determine the appropriate resistance value.

    And, yes, Ohm's Law may be involved here, as well as some current and voltage measurements. But, why does your boss think you are qualified to make modifications to an existing PLC-based system? Who will be responsible if modifications result in undesirable or dangerous operation?
  8. shumifan50


    Jan 16, 2014
    This sounds like an extremely bad idea. You might end up being liable if anybody gets injured/killed because a safety feature had been removed and your boss might walk away without consequence.
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