When using pressure sensors, the output signals 0 ? 20 mA, 4 ? 20 mA and DC 0 ? 10 V are frequently chosen to ensure that the sensor signals to be evaluated and additional 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 could often be confusing, as the day-to-day using the terms ?active?, ?passive?, ?current source?, ?voltage source?, ? Paralyzed ? and ?load? are often wildly mixed together. Any electrical signal processing always requires a voltage supply (an ?active part?) and a ?load?, like a pressure sensor, which represents the ?passive part?. Sometimes the active part of the interconnection is also referred to as a power source/voltage source and the passive part is known as a ?current sink?. To ensure that an electrical circuit can function, current must flow in a circuit ? even when an instrument is normally known as a load, 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 when an ?energy gap? exists between current source and current sink, therefore 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 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)?
So how exactly does the input card in my PLC operate?
As a rule of thumb, you can take into account 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 example, a 4-wire flow sensor consists of 2 wires for another 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.