Electrical circuit for pressure sensors: when is really a sensor active, so when passive?

When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are generally chosen in order for the sensor signals to be evaluated and additional processed. For this, the signal output of the pressure sensor is normally connected to a corresponding input card in the PLC.
In this context it could often be confusing, as the day-to-day usage of the terms ?active?, ?passive?, ?current source?, ?voltage source?, ?current sink? and ?load? Bona to be wildly mixed together. Any electrical signal processing always requires a voltage supply (an ?active part?) and a ?load?, like a pressure sensor, which represents the ?passive part?. Sometimes the active section of the interconnection is also referred to as an electrical source/voltage source and the passive part is known as a ?current sink?. To ensure that a power circuit can function, current must flow in a circuit ? even when an instrument is normally known as a load, the current is not consumed because of it, rather it only flows from the current or voltage source through the strain and back to the current source.
This works only when an ?energy gap? exists between current source and current sink, so the power source operates actively (= sending out current) and the current sink passively (= current flows through it) . Therefore, an interconnection of two current sources or two current sinks will not operate normally. This example is complicated in day-to-day application:
When does a pressure sensor work passively (current sink) and when does it work actively (current source)?
How does the input card in my PLC operate?
As a rule of thumb, one can understand that 2-wire sensors usually work passively and therefore need an active PLC input card. It is difficult with 4-wire sensors, since, for instance, a 4-wire flow sensor consists of 2 wires for another voltage supply and 2 wires for an active or passive 0/4 ? 20 mA signal output. Hence, it is vital to check the datasheets for the sensor and PLC input card used.

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