Forms of Passive Signal Isolators

To eliminate mutual interference caused by grounding among various instrumentation devices, it is common to employ Isolator To address this, isolators are used to achieve complete electrical isolation between instrumentation devices. In other words, there is no common ground between the input and output, thereby preventing any interference with the normal transmission of the signal.
Isolators can be classified as Active isolator and Passive Isolator . The following introduces the types of passive isolators:
1. Input-side circuit power-supply isolation type ( PA-0133)
In this isolation technology, all the electrical power required by the input/output circuitry is supplied from the input side via a loop-powered supply. The loop current typically carries a standard 4–20 mA signal, which serves as the input signal (e.g., from a PLC output card). After isolation, the input signal is transmitted through the output side to drive actuators or controllers. Loop-powered isolators eliminate the need for an external auxiliary power supply, making them convenient to use.
2. Output-side circuit power-isolated type ( PA-0371)
In this isolation technology, all the electrical power required by the input/output circuitry is supplied from the output side, which receives power via a loop-powered configuration (e.g., a PLC input card). The loop current typically conforms to the standard 4–20 mA signal, which serves as the output signal.
The input can be signals from various sensors or standard 4–20 mA signals. An important feature of this technology is that the signal current flowing to the output not only powers the isolation unit but also supplies the load of the isolator (e.g., the input resistance of a PLC input card).
3. The output circuit provides power isolation, and in the case of input-distribution-type configurations, all the electrical energy required by the input and output circuitry is supplied from the output side. The output side obtains its power through loop-powered supply (e.g., a PLC input card), with the loop current typically being a standard 4–20 mA signal, which serves as the output signal.
In addition, the input side must also obtain electrical power from the output side as a auxiliary power supply, which is the distribution voltage; this voltage powers various Transmitter When in use, the transmitter’s output current serves as the isolator’s input current.
Passive signal isolators are conveniently used to isolate transmitters from PLCs or DCS systems. When using loop-powered PLC or DCS input cards, no additional auxiliary power supply is required.