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 further processed. Because of this, the signal output of the pressure sensor is usually linked to a corresponding input card in the PLC.
In this context it could often be confusing, as the day-to-day usage of the terms ?active?, ?passive?, ? Surprising ?, ?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 section of the interconnection is also referred to as an electrical source/voltage source and the passive part is referred to as a ?current sink?. In order that a power circuit can function, current must flow in a circuit ? even when an instrument is usually known as a load, the current is not 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 if 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 PLC operate?
Generally of thumb, one can remember that 2-wire sensors usually work passively and thus need a dynamic PLC input card. It really is difficult with 4-wire sensors, since, for instance, a 4-wire flow sensor consists of 2 wires for another voltage supply and 2 wires for an active or passive 0/4 ? 20 mA signal output. It is therefore vital to check the datasheets for the sensor and PLC input card used.g