When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? Bottom Line 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 usually linked to a corresponding input card in the PLC.
In this context it can often be confusing, because the day-to-day using the terms ?active?, ?passive?, ?current source?, ?voltage source?, ?current sink? and ?load? tend to be 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 ? Exclusive ?. Sometimes the active the main interconnection is also referred to as an electrical source/voltage source and the passive part is known as a ?current sink?. In order that a power circuit can function, current must flow in a circuit ? even when an instrument is usually known as lots, the current is not consumed because of 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 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)?
So how exactly does the input card in my own PLC operate?
As a rule of thumb, one can take into account 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 an active or passive 0/4 ? 20 mA signal output. Hence, it is vital to check the datasheets for the sensor and PLC input card used.