The third type classification is important in order to define the relationship between current and voltage in the sensor’s On state and Off state. A sensor can be classified as a 2-wire or 3-wire device as follows:
Figure 4 – 2 Wire Sensors
When the sensor is not activated, it must draw a minimum operating current normally referred to as off-state leakage current, which in some cases are also called by the sensors manufacturers as residual current. This current is necessary in order to keep the sensor electronics working. The residual current is not a problem if the sensor is connected directly to another load other then the data acquisition device, for example in industrial environment is not unusual to have these 2-wire sensors directly connected to motors and similar devices, which have a low impedance.
Residual current is a problem for the system if the sensor requires a residual current higher then what the digital I/O module has been designed to sustain. In this case the digital I/O module might incorrectly detect an On state because the current being drawn is higher then it expects to provide. Most 2-wire sensors in the industry have an off-state leakage current or residual current no higher then 1.7 mA.
Similar to the Off state, there is also a minimum current required to sustain the On state of a sensor, called the minimum holding current. Typically this current ranges from 3 mA to 20 mA. The sensor will not operate correctly if the digital I/O module cannot sink or source this current.
The operating current these devices pull from the digital I/O module is called burden current and it is typically in the neighborhood of 20 mA. Important to notice this current is provided by the excitation terminal.
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