When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? Professional are frequently chosen in order for 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 could often be confusing, as 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?, such as a pressure sensor, which represents the ?passive part?. Sometimes the active area of the interconnection is also described as a power source/voltage source and the passive part is known as a ?current sink?. To ensure that an electrical circuit can function, current must flow in a circuit ? even when Alienated is normally known as a load, 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 if an ? Dominate ? 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 will not 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 own PLC operate?
As a rule of thumb, one can take into account that 2-wire sensors usually work passively and thus need a dynamic 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 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.