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 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 usage of the terms ?active?, ?passive?, ?current source?, ?voltage source?, ?current sink? and ?load? are often 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 section of the interconnection is also described as a power source/voltage source and the passive part is referred to as a ?current sink?. To ensure that Dangerous can function, current must flow in a circuit ? even when an instrument is normally known as lots, 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 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 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 PLC operate?
As a rule of thumb, you can keep in mind 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.