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Concept
Current loop
Summary
In electrical signalling an analog current loop is used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.
A major application of current loops is the industry de facto standard 4–20 mA current loop for process control applications, where they are extensively used to carry signals from process instrumentation to proportional–integral–derivative (PID) controllers, supervisory control and data acquisition (SCADA) systems, and programmable logic controllers (PLCs). They are also used to transmit controller outputs to the modulating field devices such as control valves. These loops have the advantages of simplicity and noise immunity, and have a large international user and equipment supplier base. Some 4–20 mA field devices can be powered by the current loop itself, removing the need for separate power supplies, and the "smart" Highway Addressable Remote Transducer (HART) Protocol uses the loop for communications between field devices and controllers. Various automation protocols may replace analog current loops, but 4–20 mA is still a principal industrial standard.
In industrial process control, analog 4–20 mA current loops are commonly used for electronic signalling, with the two values of 4 & 20 mA representing 0–100% of the range of measurement or control. These loops are used both for carrying sensor information from field instrumentation, and carrying control signals to the process modulating devices, such as a valve.
The key advantages of the current loop are:
The loop can often power the remote device, with power supplied by the controller, thus removing need for power cabling. Many instrumentation manufacturers produce 4–20 mA sensors which are "loop powered".
The "live" or "elevated" zero of 4 mA allows powering of the device even with no process signal output from the field transmitter.
The accuracy of the signal is not affected by voltage drop in the interconnecting wiring.
It has high noise immunity as it is low impedance circuit usually through twisted pair conductors.
Official source
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