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4-20mA Analouge input

Discussion in 'Electronic Design' started by AJ, Feb 18, 2007.

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  1. AJ

    AJ Guest

    Hi,

    I have been asked to make a 4-20mA input and im not 100% how to go about
    this. Im sure there is a standard im meant to be following but im not sure
    what it is. Can anyone help point me in the right direction? Im imagining
    a sense resistor and an amplifier???

    Best regards


    AJ
     
  2. Lefty

    Lefty Guest

    4-20 dc ma is a standard industrial process control interface signal
    between field sensors (pressure,temp,flow,etc) and process control
    electronics in the control house. This 'loop' is generally powered
    from a 24vdc power supply. The advantage, of course, is that there is
    no error induced because the length (resistance) of the signal wire
    pair that can be hundreds to a few thousands feet long if needed. The
    loop voltage also 'powers' the field sensor device. Most input
    circuits just wire the 4-20 in series with a 250 ohm resistor to
    measure the 1-5vdc voltage drop using a diff input voltage amplifier
    to further process or convert it with a A/D convertor. The reason the
    4ma = 0% level (live zero) is to have a means of detecting an open
    loop condition and of course to have a small amount of power for the
    field device to consume independent of the measurement value. Works
    very well and has been in use for decades.

    Lefty
     
  3. Jamie

    Jamie Guest

    Yes, you supply a 24 volt current source from your input to the
    transmitting device.
    Normally, you do that via a 24 volt supply through a resistor
    where at which point, you measure the return voltage and do the
    math./.
    just subtract what ever is coming back and use that to do the
    basic ohms law of the source resistor you're using.
    just put this into a op-amp as a voltage follower.
     
  4. Jamie

    Jamie Guest

    and remember, that in cases where you have multiple devices connected
    in series, each device will attempt to maintain 20 ma on the TX in the
    loop as an off condition so that the other devices in the loop can lift
    the loop to 4 ma so that a change in condition can be seen at the
    receiver. This is mostly for SERIAL communications.
    Just thought i would through that in!
     
  5. Lefty

    Lefty Guest


    I'm not sure at all what you are describing but it's not the
    industrial standard instrumentation 4-20ma interface. This is strickly
    a DC analog measurement standard and not a digital communication
    interface. There can be digital communications superimposed by
    injecting a small ac fsk signal onto the DC measurement (HART
    protocol) but the basic 4-20ma standard is to support one field device
    hardwired to a central control point and was in use well before
    digital electronics came onto the industrial control world.

    Lefty
     
  6. You need to know a little more about what you are connecting to.
    4-20mA, 'does what it says on the tin', delivering a varying current,
    dependant on some signal that wants to be transmitted. Now it is common on
    some stuff, for the source to provide isolation, and to power the loop.
    However on other areas using the same interface, the receiver powers the
    loop. Even worse, there are some sensors, which require receiver
    isolation...
    So you need to know, which end of the loop is going to supply power, and
    if the other end is isolated or not (if not, then you should provide
    isolation). Some applications will require both ends to provide isolation.
    AN54, from Siemens, shows how to use their IL300 analog opto-coupler, to
    provide a line powered isolated interface. TI (Burr Brown), do an op-amp,
    which provides an isolation barrier in the amplifier, as an alternative
    way of doing this.

    Best Wishes
     
  7. AJ

    AJ Guest


    Thanks heaps Lefty, Jamie and Roger, Been a great help.

    Best regards


    AJ
     
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