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 further processed. For this, the signal output of the pressure sensor is usually connected to a corresponding input card in the PLC.
In this context it can often be confusing, as 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?, like a pressure sensor, which represents the ?passive part?. Sometimes the active section of the interconnection is also described as a power source/voltage source and the passive part is known as a ?current sink?. In order that a power circuit can function, current must flow in a circuit ? even when an instrument is usually 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 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 will not 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)?
So how exactly does the input card in my own PLC operate?
Generally of thumb, one can keep in mind that 2-wire sensors usually work passively and thus need an active PLC input card. It is difficult with 4-wire sensors, since, for instance, a 4-wire flow sensor includes 2 wires for a separate voltage supply and 2 wires for a dynamic or passive 0/4 ? Trade Secret . Hence, it is vital to check the datasheets for the sensor and PLC input card used.