When using pressure sensors, 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 further processed. For this, the signal output of the pressure sensor is normally connected to a corresponding input card in the PLC.
In this context it could 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 takes a voltage supply (an ?active part?) and a ?load?, for instance a pressure sensor, which represents the ?passive part?. Sometimes the active the main interconnection is also referred to as an electrical source/voltage source and the passive part is known as a ? ไดอะแฟรม ซีล ?. To ensure that an electrical circuit can function, current must flow in a circuit ? even when an instrument is usually known as a load, the current is not consumed by 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, 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 won’t 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 thus need an active PLC input card. It is difficult with 4-wire sensors, since, for example, 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.