Current loop

<p>In electrical <a href="/facts/Signal_(electrical_engineering)/41YqUJ8c">signalling</a> 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.
</p><p>A major application of current loops is the industry <a href="/facts/De_facto_standard/TRELHVwI">de facto standard</a> 4–20 mA current loop for <a href="/facts/Process_control/tmjQhchm">process control</a> applications, where they are extensively used to carry signals from process <a href="/facts/Instrumentation/znu7yF5t">instrumentation</a> to <a href="/facts/Proportional%E2%80%93integral%E2%80%93derivative_controller/7XcYk0CH">proportional–integral–derivative (PID) controllers</a>, <a href="/facts/Supervisory_control_and_data_acquisition/nKL6Mfd1">supervisory control and data acquisition (SCADA)</a> systems, and <a href="/facts/Programmable_logic_controllers/bqh0GAdb">programmable logic controllers (PLCs)</a>. They are also used to transmit controller outputs to the modulating field devices such as <a href="/facts/Control_valves/7cE7Jt09">control valves</a>. 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" <a href="/facts/Highway_Addressable_Remote_Transducer_Protocol/D2E5UNG7"> Highway Addressable Remote Transducer (HART) Protocol</a> uses the loop for communications between field devices and controllers. Various <a href="/facts/List_of_automation_protocols/PBsNE25a">automation protocols</a> may replace analog current loops, but 4–20 mA is still a principal industrial standard.
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Current loop open-in-new

Current loop