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. 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 could often be confusing, as 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 part of the interconnection is also described as an electrical source/voltage source and the passive part is referred to as a ?current sink?. In pressure gauge octa that a power circuit can function, current must flow in a circuit ? even though an instrument is normally known as a load, the current isn’t consumed by it, rather it only flows from the existing or voltage source through the strain and back to the existing source.
This works only if an ? pressure gauge ? 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 won’t 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?
As a rule of thumb, one can keep in mind that 2-wire sensors usually work passively and thus need an active PLC input card. It really 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 a dynamic or passive 0/4 ? 20 mA signal output. It is therefore imperative to check the datasheets for the sensor and PLC input card used.