When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are generally chosen to ensure that the sensor signals to be evaluated and further processed. Because of 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, because 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?, for instance a pressure sensor, which represents the ?passive part?. Sometimes the active area 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 a power circuit can function, current must flow in a circuit ? even though an instrument is usually known as lots, the current is not consumed by it, rather it only flows from the existing or voltage source through the load and back to the current 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 won’t 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)?
How does Piranha in my own PLC operate?
As a rule of thumb, one can understand that 2-wire sensors usually work passively and therefore need an active PLC input card. It really is difficult with 4-wire sensors, since, for example, a 4-wire flow sensor consists of 2 wires for another voltage supply and 2 wires for a dynamic or passive 0/4 ? 20 mA signal output. It is therefore imperative to check the datasheets for the sensor and PLC input card used.