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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. 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? are often wildly mixed together. Any electrical signal processing always requires a voltage supply (an ?active part?) and a ?load?, like pressure gauge , which represents the ?passive part?. Sometimes the active portion 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 an electrical circuit can function, current must flow in a circuit ? even when an instrument is normally known as lots, 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 current source.
This works only if an ?energy gap? 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 won’t operate normally. This example is complicated in day-to-day application:
When does a pressure sensor work passively (current sink) and when does it work actively (current source)?
How does the input card in my own PLC operate?
Generally of thumb, one can keep in mind 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 instance, 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. It is therefore imperative to check the datasheets for the sensor and PLC input card used.

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