When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are generally chosen to ensure that the sensor signals to be evaluated and further processed. Because of this, the signal output of the pressure sensor is usually 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?, ? Disgusting ?, ?current sink? and ?load? are often wildly mixed together. Big processing always requires a voltage supply (an ?active part?) and a ?load?, such as a pressure sensor, which represents the ?passive part?. Sometimes the active area 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 though an instrument is normally known as lots, the current isn’t consumed by it, rather it only flows from the existing or voltage source through the strain and back to the existing source.
This works only if 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 will not operate normally. This example is complicated in day-to-day application:
When does a pressure sensor work passively (current sink) so when does it work actively (current source)?
So how exactly does the input card in my own PLC operate?
Generally of thumb, you can remember 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 consists of 2 wires for a separate 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.