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4-20mA transmitter and reciever

Discussion in 'Electronic Design' started by Mook Johnson, Nov 30, 2004.

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  1. Mook Johnson

    Mook Johnson Guest

    I'm having some problems specing out a data acquisition box that needs 4
    4-20mA analong outputs and 4 4-20mA inputs. This is a custom box and so the
    circuit design will be at the IC level.

    Questions.

    Receiver:
    1) should it always provide power for the loop (13 - 40VDC)? nominal 30
    VDC.
    2) Should it sense current on the positive output of the powersupply or the
    return (I've seen both ways)


    Transmitter:
    1) Always powered by the loop?
    2) Common positive or ground? (or isolated)

    The TX and RX are connected to a the same ground (all minus leads tied
    together) that is electrically floating with reference to earth.

    The issue is that I've seen implementations that go either way for which
    side to sense and which current to control (positive draw with common return
    or return current with common positive).

    Currently the transmitters are simple darlington tranny pairs with an
    emitter resistors and a opamp regulating the voltage drop across the
    resistor to match its input voltage. (textbook ckt) the ground side of all
    four emitter resistors is tied to the same board ground the input signal
    (quad DAC) is referenced to.


    This works well for a system designed for this operation but fall on it a$$
    if the positives are tied together and the current is sensed on the return
    lines (they all read the same due to current sharing).

    The receiver is a simple 30V powersupply (referenced to board ground) with
    four 100 ohm resistors fanned off the positive and difference amplifiers
    measuring the drop from each one.

    Again this does not work if the transmitters tie positives together and
    attempt to regulate the retuen current. (current sharing again)

    What do you guys recommend?
     
  2. On the last 4-20mA job I worked on everything was decided
    by the requirements of the PLCs. Their '4-20mA' input
    was no more than a burden resistor into a 5v ADC, all
    ADCs being ground-referenced.

    It meant the all generated 4-20mA had to be positive output
    currents off the +24v rails, and everyone else's 4-20mA
    input amplifiers had to have the full common-mode voltage
    capability.
     
  3. YD

    YD Guest

    Depends on whether the field transmitter is two-wire or has a separate
    supply terminal. The receiver itself doesn't supply any power, you use
    an internal bus or external power supply.
    Normally it senses the return if two-wire. Otherwise just tie positive
    to positive, negative is common for all signals.
    Not necessarily but if you mean your own analog outputs they'll of
    course be internally powered.

    Positive common, source.
    Short answer: Make outputs sources (positive common) and inputs sinks
    (negative common).
    You'll need to redesign the sources. No biggie but I'm too lazy to get
    off the chair and look up the exact circuit.
    Reference them to ground and use 250 ohms.
    By your description I'm not sure what you really are up to. Anyway,
    below are some standard instrument loop wirings.

    ......................
    Internal power .+
    30V-----------------o---------------+---------------------+
    . o + |
    .+ ( ) [1] |
    --------------+-----o---------------o - out o +
    | .Analog input 1 +--o( ) [2]
    - . | o -
    Internal | |etc o--------------------------------+ |
    circuitry | | .input 2 |
    - . |
    | . |
    0V------------+-----o-------------------------------------+
    ......................



    ............... [1]
    . - +
    o-----------------o( )o--+-----------------+
    . input 1 + | |
    . +----+ o +
    . out o + -----
    o-----------------o( ) [2] | |
    .input 2 + o - External | |
    . | power -----
    . | supply o -
    ------o-------------------+----------------------+
    . input common
    ............... [1] two-wire instrument
    [2] three-wire instrument

    I won't bother drawing the current output, suffice it to say it's
    usually a source, the receiver goes between the output and the
    negative common.

    - YD.
     
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