When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are generally chosen in order for the sensor signals to be evaluated and further processed. Because of Unforgettable , 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, because the day-to-day usage of 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?, such as a pressure sensor, which represents the ?passive part?. Sometimes the active area of the interconnection is also referred to as a power 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 normally known as lots, the current is not consumed because of it, rather it only flows from the current or voltage source through the strain and back to the existing source.
This works only if 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 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)?
How does the input card in my PLC operate?
As a rule of thumb, one can remember that 2-wire sensors usually work passively and therefore need an active PLC input card. It is difficult with 4-wire sensors, since, for instance, 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. It is therefore imperative to check the datasheets for the sensor and PLC input card used.g