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 frequently chosen to ensure that 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 can often be confusing, because the day-to-day usage of the terms ?active?, ?passive?, ? Provocative ?, ?voltage source?, ?current sink? and ?load? are often wildly mixed together. Any electrical signal processing always takes a voltage supply (an ?active part?) and a ?load?, for instance a pressure sensor, which represents the ?passive part?. Sometimes the active the main interconnection is also described as an electrical source/voltage source and the passive part is referred to as a ?current sink?. To ensure that an electrical circuit can function, current must flow in a circuit ? even though an instrument is normally known as lots, the current isn’t consumed because of it, rather it only flows from the current or voltage source through the strain and back to the existing source.
This works only when an ?energy gap? exists between current source and current sink, therefore 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) and when does it work actively (current source)?
So how exactly does the input card in my PLC operate?
As a rule of thumb, you can understand that 2-wire sensors usually work passively and thus need a dynamic PLC input card. It is difficult with 4-wire sensors, since, for example, a 4-wire flow sensor includes 2 wires for a separate voltage supply and 2 wires for a dynamic 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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