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 frequently chosen in order for the sensor signals to be evaluated and additional processed. Because of this, the signal output of the pressure sensor is usually linked to a corresponding input card in the PLC.
In this context it can often be confusing, because the day-to-day using the terms ?active?, ?passive?, ?current source?, ?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?, such as 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?. In order that an electrical circuit can function, current must flow in a circuit ? even though an instrument is usually known as a load, the current isn’t consumed by 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 won’t 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)?
So how exactly does the input card in my PLC operate?
Generally of thumb, one can take into account that 2-wire sensors usually work passively and therefore need a dynamic PLC input card. It really 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, Unsurpassed is vital to check the datasheets for the sensor and PLC input card used.

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