Electrical circuit for pressure sensors: when is really a sensor active, and when passive?

When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? เกจอาร์กอน are frequently 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 connected 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? tend to be wildly mixed together. Any electrical signal processing always takes a voltage supply (an ?active part?) and a ?load?, such as a pressure sensor, which represents the ?passive part?. Sometimes the active portion of the 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 normally known as lots, the current isn’t 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 if an ?energy gap? exists between current source and current sink, so 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 will not 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 PLC operate?
As a rule of thumb, you can remember 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 another voltage supply and 2 wires for an active or passive 0/4 ? 20 mA signal output. Hence, it is imperative to check the datasheets for the sensor and PLC input card used.

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