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    Controls Engineering 101: Why 4-20mA?

    by Scott Morris

    Wouldn't life just be easier if we could just buy voltage analog cards and set our meters to VDC and see our levels? Why do we horse around with milliamp signals?

    The easy answer is this: noise immunity. Because a voltage signal only contains enough power to drive a 1Mohm to 10Mohm meter or analog input, the current draw of the signal is very low, making it susceptible to noise. On the other hand, a milliamp signal has higher power overall and less susceptible to RFI. Since it is current being monitored and not voltage, there is not as much signal loss over long distance runs as voltage signals. That's the easy answer.

    The history of 4-20mA is more interesting. In the good old days (before PLC and DCS systems), more than one device could be put in series to read the same signal. As long as the  overall resistance did not exceed a maximum, several devices could read the same signal. For example, a readout could show a process variable in one physical location and a PID controller could use it as feedback in another.

    The other piece of magic provided by 4-20mA is the ability to detect the difference between a broken wire and a zero value process reading. With a broken wire, a reading of -25% of the monitored value is displayed instead of 0.

    Even though we live in the wonderful land of PLC's and PAC's, it is important to remember that there are rules to sharing a 4-20mA process signal between more than one monitoring device. I am referring to loop isolation and grounding issues. If you are using a single ended (shared common or non isolated) analog input card, this means that all of the common wires for all of the channels on the card are wired together and probably wired to 24V common. This means that running a wire from the minus of a channel to the plus of another device in a loop will not only fail but also provide a potentially undesireable short circuit. If you plan to use a common device as an input to two or more analog inputs, be sure BOTH analog inputs are differential (isolated common) channels. If you need to use a PLC that does not sell an isolated common analog input card, be sure to search online for “loop isolator” and use isolation in accordance with the directions of the device you select. 

     

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