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GF+ Tech Tip: Connecting Multiple
ITEMS TO A CURRENT LOOP
In many applications the current loop output of a sensor or transmitter needs to be shared between two or more devices, such as a circular chart recorder and a PLC. For correct operation the shared devices must be connected in series with the current loop output.
Wiring two, or more, devices to a sensor or transmitter:
The wiring diagram to the left shows the correct wiring of a 9900 Transmitter to a Circular Chart Recorder and a Programmable Logic Controller. The wiring is in series it goes from the 9900 to the Programmable Logic Controller, then to the Circular Chart Recorder and finally back to the Power Supply.
Correct Current Loop Wiring:
When wired in series the current will be the same in both the Circular Chart Recorder and the Programmable Logic Controller.
Incorrect Current Loop Wiring:
If the Circular Chart Recorder and Programmable Logic Controller are wired in parallel, the current through each device will be proportional to the resistance of each device and will be less than the current being delivered by the sensor or the transmitter. This will cause the Circular Chart Recorder and the Programmable Logic Controller to read an incorrect value.
Conclusion:
When sharing the current loop output from a sensor or transmitter the wiring of the external devices should be in series. If the units are wired in parallel, the external devices will measure incorrect values.
