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Thread: 4-20 MA transmitter signal

  1. #1
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    4-20 MA transmitter signal

    Field tranmitter has 4-20 ma output which is connected to plc. plc output supply to transmitter is 24Vdc. but in field transmitter side receives only 16.5 Vdc. What will be the causes for voltage drop.

  2. #2
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    130525-1508 EDT

    Loop resistance of the wire.

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  3. #3
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    Could also be 250 ohm series resistor in the loop. An older method, but sometimes local indicators with 1-5Vdc input are used in conjunction with the transmitter. The resistor creates a 1-5Vdc drop across it for input to the indicator.
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  4. #4
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    Quote Originally Posted by Smart $ View Post
    Could also be 250 ohm series resistor in the loop. An older method, but sometimes local indicators with 1-5Vdc input are used in conjunction with the transmitter. The resistor creates a 1-5Vdc drop across it for input to the indicator.
    yeah, used model is 3051 cg rosemount pressure transmitter with local indicator. my question is, if 16.5 voltage only occurs across the transmitters. will transmitter work properly. what are the test we have to do for transmitter working condition

  5. #5
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    put ammeter in series with 4-20ma line and measure it over full range?

    just scale the plc input to match?

    4-20ma circuits often have scaling adjustments called span & elevation or similar names.

  6. #6
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    139526-0739 EDT

    srinivasan:

    Look on the transmitter data sheet for the minimum voltage across the transmitter. The purpose of the current loop is to allow accurate data measurement over wire loops with varying resistance. The transmitter is a constant current source down to some minimum voltage across the transmitter.

    Experimentally adjust the transmitter to produce a 20 mA load. Increase the series resistance until the transmitter can not maintain a 20 mA load. Measure the voltage across the transmitter. That is your minimum transmitter voltage. Add some voltage to this as a safety factor.

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  7. #7
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    Quote Originally Posted by srinivasan View Post
    yeah, used model is 3051 cg rosemount pressure transmitter with local indicator. my question is, if 16.5 voltage only occurs across the transmitters. will transmitter work properly. what are the test we have to do for transmitter working condition
    Here's what the manual states...

    Power Supply for 4-20 mA HART
    Transmitter operates on 10.5 - 42.4 Vdc. The dc power supply should provide
    power with less than two percent ripple.

    NOTE

    A minimum loop resistance of 250 ohms is required to communicate with a
    Field Communicator. If a single power supply is used to power more than one
    3051 transmitter, the power supply used, and circuitry common to the
    transmitters, should not have more than 20 ohms of impedance at 1200 Hz.

    Figure 2-19. Load Limitation
    Maximum Loop Resistance = 43.5 * (Power Supply Voltage – 10.5)
    The Field Communicator requires a minimum loop resistance of 250Ω for communication

    The total resistance load is the sum of the resistance of the signal leads and
    the load resistance of the controller, indicator, and related pieces. Note that
    the resistance of intrinsic safety barriers, if used, must be included.
    If this is a new installation, recommed going through the entire commissioning and calibration procedure, Sections 3 and 4. Link to manual...

    http://www2.emersonprocess.com/sitea...-0100-4001.pdf
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  8. #8
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    Quote Originally Posted by mike_kilroy View Post
    put ammeter in series with 4-20ma line and measure it over full range?

    just scale the plc input to match?

    4-20ma circuits often have scaling adjustments called span & elevation or similar names.
    While that may work, that's not the proper way to do it, and should only be used as a last resort remedy to keep things running.
    I'll never get there. No matter where I go, I'm always here.

  9. #9
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    if wire sizing is not proper. what will be the effect. for 24 v dc and 4-20 ma, our selected cable size is 1.5 sq.mm and 2 core. is it reason for voltage drop

  10. #10
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    130526-0829 EDT

    srinivasan:

    If your open circuit voltage is a regulated 24 VDC, and there is a 5 V drop at 20 mA in the receiver, then your wire loop resistance drop is only 24 - 5 - 16.5 = 2.5 V. That is not much drop for the wiring. Find out the minimum voltage that the transmitter will operate at over its full range. In other words at 20 mA.

    Pay attention to Smart $'s information.

    .

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