# 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
12,221
2,701
convert both currents to voltages and sum them in either the analog or the digital domain.

3. ### thingmaker3 Retired Moderator

May 16, 2005
5,073
8
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
5,073
8
I must say PapbaBravo's wording is vastly more eloquent than mine. Not to mention quicker to compose!