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. For this, the signal output of the pressure sensor is usually connected to a corresponding input card in the PLC.
In this context it could often be confusing, because the day-to-day using the terms ?active?, ?passive?, ? Tense ?, ?voltage source?, ?current sink? and ?load? are often 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 the main interconnection is also described as a power 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 though an instrument is usually known as a load, 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, 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) and when does it work actively (current source)?
So how exactly does the input card in my PLC operate?
Generally of thumb, one can remember 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 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.