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. For this, the signal output of the pressure sensor is normally 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?, ?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 known as a ?current sink?. To ensure that an electrical circuit can function, current must flow in a circuit ? even though an instrument is normally known as lots, the current isn’t consumed by it, rather it only flows from the existing or voltage source through the strain and back to the current source.
This works only when an ?energy gap? exists between Never 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 won’t operate normally. This example is complicated in day-to-day application:
When does a pressure sensor work passively (current sink) and when does it work actively (current source)?
How does the input card in my PLC operate?
Generally of thumb, you can understand that 2-wire sensors usually work passively and therefore need an active PLC input card. It is difficult with 4-wire sensors, since, for example, 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 vital to check the datasheets for the sensor and PLC input card used.