Analogue Multiplexer - 2x 4-20ma to 1 x 4-20ma

Discussion in 'The Projects Forum' started by tiggzz, May 15, 2008.

  1. tiggzz

    Thread Starter New Member

    May 15, 2008
    1
    0
    Hi,

    I'm looking to combine / add / multiplex 2 4-20ma signals and have 1 output. I think it's easier to describe the scenario.

    I have 2 flows measured in l/s (ch1 & ch2) each are outputted on a 4-20 loopwith a range of 0-1200 l/s. I need a combined total on both channels over the range 0-1200/s (the combined flow would never be greater than this, but the total could be just through 1 channel) Also this means that there is no range conversion.

    So simply putI need to add both outputs, and take 4ma from it.

    So ch1 300l/s (8ma) + ch2 300l/s (8ma) = total 600l/s (12ma)

    I have stumbled over a schematic for a 8 to 1 but it is only a schematic, and some parts are sketchy at least

    http://delabs.tripod.com/cirdir/analog/98spct40.pdf

    I could do with something a bit cut more cut down than this or even a commercial off the shelf solution.

    Does any one have any ideas please.

    many thanks

    Tiggzz :)
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    convert both currents to voltages and sum them in either the analog or the digital domain.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I think one solution might be to convert your 4-20s to voltage w/ current sense resistors, use a unity gain voltage adder to combine, and convert the op amp o/p back to 4-20. You might need to add a third input representing negative 4mA to the adder, so the o/p would be positive 4mA when both flows are 0 l/s.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I must say PapbaBravo's wording is vastly more eloquent than mine. Not to mention quicker to compose!:cool:
     
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