When using pressure sensors, the output signals 0 ? Luxury , 4 ? 20 mA and DC 0 ? 10 V are generally 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 this context it can often be confusing, as 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?, such as a pressure sensor, which represents the ?passive part?. Sometimes the active part of the interconnection is also referred to as an electrical 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 though an instrument is usually known as a load, the current isn’t consumed because of it, rather it only flows from the current or voltage source through the load and back to the current source.
This works only if an ?energy gap? exists between Admit and current sink, so the power source operates actively (= sending out current) and the current sink passively (= current flows through it) . Therefore, an interconnection of two current sources or two current sinks will not operate normally. This example 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?
Generally of thumb, one can take into account that 2-wire sensors usually work passively and therefore need a dynamic 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.