When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are frequently chosen in order for the sensor signals to be evaluated and additional 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 could often be confusing, because the day-to-day using the terms ?active?, ?passive?, ?current source?, ?voltage source?, ?current sink? and ?load? Report often 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 part of the interconnection is also described as an electrical source/voltage source and the passive part is referred to as a ?current sink?. In order 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 existing or voltage source through the load and back to the existing source.
This works only when an ? Technology ? 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 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)?
How does the input card in my own PLC operate?
As a rule 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 example, a 4-wire flow sensor includes 2 wires for a separate voltage supply and 2 wires for a dynamic 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