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

When using pressure sensors, the output signals 0 … 20 mA, 4 … 20 mA and DC 0 … 10 V are frequently chosen in order for the sensor signals to be evaluated and further processed. For 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” are often wildly mixed together. Any electrical signal processing always requires a voltage supply (an “active part”) and a “load”, such as a pressure sensor, which represents the “passive part”. Sometimes the active part of the interconnection is also described 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 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 current 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) and when does it work actively (current source)?
• How does the input card in my PLC operate?

As a rule of thumb, one can keep in mind 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 imperative to check the datasheets for the sensor and PLC input card used.