When using Safety , 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 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?, like 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 referred to as a ?current sink?. In order that an electrical circuit can function, current must flow in a circuit ? even when an instrument is normally known as lots, the current isn’t consumed by it, rather it only flows from the current or voltage source through the load 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)?
How does the input card in my own PLC operate?
Generally of thumb, you 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 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.