Electrical circuit for pressure sensors: when is really a sensor active, and 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. Because of this, the signal output of the pressure sensor is normally connected to a corresponding input card in the PLC.
In this context it can often be confusing, because the day-to-day usage of the terms ?active?, ?passive?, ?current source?, ?voltage source?, ?current sink? and ?load? tend to be wildly mixed together. Any electrical signal processing always requires a voltage supply (an ?active part?) and a ?load?, for ไดอะแฟรม , 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 referred to as a ?current sink?. To ensure that a power circuit can function, current must flow in a circuit ? even though an instrument is usually known as lots, the current is not consumed by it, rather it only flows from the current or voltage source through the load and back to the current source.
ไดอะแฟรม works only when an ?energy gap? exists between current source and current sink, so the power source operates actively (= sending out current) and the existing sink passively (= current flows through it) . Therefore, an interconnection of two current sources or two current sinks won’t operate normally. This situation is complicated in day-to-day application:
When does a pressure sensor work passively (current sink) so when does it work actively (current source)?
How does the input card in my PLC operate?
Generally of thumb, you can understand that 2-wire sensors usually work passively and therefore need a dynamic PLC input card. It is difficult with 4-wire sensors, since, for example, 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 imperative to check the datasheets for the sensor and PLC input card used.

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