When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? Concealed 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 usually linked to a corresponding input card in the PLC.
In this context it can often be confusing, as the day-to-day using the terms ?active?, ?passive?, ?current source?, ?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?, for instance a pressure sensor, which represents the ?passive part?. Sometimes the active section of the interconnection is also referred to as a power source/voltage source and the passive part is referred to as a ?current sink?. In order that a power circuit can function, current must flow in a circuit ? even though an instrument is usually known as lots, the current isn’t consumed by it, rather it only flows from the current or voltage source through the strain and back to the existing source.
This works only when 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) so when does it work actively (current source)?
How does the input card in my own PLC operate?
As a rule of thumb, you can understand that 2-wire sensors usually work passively and thus need a dynamic PLC input card. It is difficult with 4-wire sensors, since, for example, a 4-wire flow sensor includes 2 wires for another voltage supply and 2 wires for a dynamic or passive 0/4 ? 20 mA signal output. Hence, it is imperative to check the datasheets for the sensor and PLC input card used.