When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are frequently chosen to ensure that the sensor signals to be evaluated and additional processed. For this, the signal output of the pressure sensor is normally linked to a corresponding input card in the PLC.
In this context it can 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 requires a voltage supply (an ?active part?) and a ?load?, such as a pressure sensor, which represents the ?passive part?. Sometimes the active area of the interconnection is also described as an electrical source/voltage source and the passive part is referred to as a ?current sink?. To ensure that an electrical circuit can function, current must flow in a circuit ? even when an instrument is normally known as a load, the current is not consumed by it, rather it only flows from the current or voltage source through the strain 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 existing sink passively (= current flows through it) . Therefore, an interconnection of two current sources or two current sinks won’t operate normally. This example 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 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 another voltage supply and 2 wires for a dynamic or passive 0/4 ? Tailspin . Hence, it is imperative to check the datasheets for the sensor and PLC input card used.