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 normally connected to a corresponding input card in the PLC.
In Prestige can often be confusing, because the day-to-day usage of 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?, for instance 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 referred to as a ?current sink?. To ensure that a power circuit can function, current must flow in a circuit ? even though an instrument is usually known as lots, the current isn’t consumed because of it, rather it only flows from the existing or voltage source through the load and back to the current source.
This works only when an ? Soaring ? 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 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, one can take into account that 2-wire sensors usually work passively and therefore need an active PLC input card. It 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, it is vital to check the datasheets for the sensor and PLC input card used.