0-20mA isn't used because you wouldn't be able to discern between zero output and cut wires. 4-20mA is a good method to provide supervised analogs over long distances in a noisy environment. The wiring resistance doesn't even have to be constant as long as the full 20mA can flow through the loop within the source's supply capabilities.
Let's say that I have a 0-60PSI ---> 4-20mA pressure sender but no cut sheet. I do know that it works off of a 24VDC supply. If I hook this thing up to my bench supply with no load,(output leads open), there will probably be close to 24 volts on those leads. Let's say that I measure 23.0 volts with a 24.0 volt supply. 23V / .020A = 1150 ohms. This is the highest load (loop) resistance that would allow a full scale output current. It would be 1000 ohms max. with a 20 volt supply.
My problem is not loop resistance. My issue is that my 2 SCADA (Supervisory Control And Data Acquisition) analog input cards use 0-1mA with 1000 and 5000 ohm load resistances. 1mA will develop either 1V or 5V across each of my analog input points with a full-scale output. So in my case, I need to shunt 19mA, at either 1 or 5 volts, around my input. 1V / .019 A = 52.6+ ohms. 5V / .019 A = 263+ ohms. Now the transducer outputs 4-20mA, but my SCADA sees .2-1mA through its input resistance. Master computer scaling and offset numbers are programmed in on the other end.
Finally, if I were crazy enough to want to do it, I could probably series my 4-20mA sensor output through several different analog inputs, and get accurate readings on every one. But again, the cumulative full-scale current voltage drops plus drop across the wiring, can't exceed the available supply. This strange connection would also require isolation on all of the inputs. This has never been a problem for me since the more I talk theory, the more isolated i seem to end up.
Joe
[This message has been edited by JoeTestingEngr (edited 02-07-2007).]
