When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V 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 normally 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?, ? เกจวัดแรงดันถังลม ? and ?load? are often wildly mixed together. Any electrical signal processing always takes a voltage supply (an ?active part?) and a ?load?, like a pressure sensor, which represents the ?passive part?. Sometimes the active section of the interconnection is also referred to 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 normally known as a load, 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 will not 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, 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 instance, a 4-wire flow sensor consists of 2 wires for a separate voltage supply and 2 wires for an active or passive 0/4 ? 20 mA signal output. It is therefore vital to check the datasheets for the sensor and PLC input card used.