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 further processed. For 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? tend to be 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 an electrical source/voltage source and the passive part is known as a ? Super-human ?. To ensure 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 because of 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, 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) so when does it work actively (current source)?
So how exactly does the input card in my PLC operate?
Generally of thumb, you can keep in mind 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 instance, a 4-wire flow sensor includes 2 wires for a separate voltage supply and 2 wires for a dynamic or passive 0/4 ? 20 mA signal output. Hence, it is imperative to check the datasheets for the sensor and PLC input card used.