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 normally connected to a corresponding input card in the PLC.
In this context it could 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 requires a voltage supply (an ?active part?) and a ?load?, for instance a pressure sensor, which represents the ?passive part?. Sometimes the active part of the interconnection is also described as an electrical source/voltage source and the passive part is known as a ?current sink?. To ensure that a power circuit can function, current must flow in a circuit ? even though an instrument is normally 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 existing 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 current 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?
As a rule of thumb, one can understand that 2-wire sensors usually work passively and thus need a dynamic PLC input card. It is difficult with 4-wire sensors, since, for example, 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. pressure gauge 10 bar is therefore imperative to check the datasheets for the sensor and PLC input card used.