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

    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

    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


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

    Retired Moderator

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