When using pressure gauge 10 bar , 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 additional 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 can 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 requires a voltage supply (an ?active part?) and a ?load?, for instance a pressure sensor, which represents the ?passive part?. Sometimes the active portion of the interconnection is also described as a power source/voltage source and the passive part is referred to as a ?current sink?. To ensure that a power circuit can function, current must flow in a circuit ? even though an instrument is normally known as a load, the current isn’t consumed because of it, rather it only flows from the current or voltage source through the load and back to the existing source.
This works only when an ? diaphragm seal ? exists between current source and current sink, therefore 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 will not 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 the input card in my own PLC operate?
As a rule of thumb, you can take into account that 2-wire sensors usually work passively and therefore need a dynamic PLC input card. It is difficult with 4-wire sensors, since, for instance, 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.