Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

12.2.3 Sinking/Sourcing

 

· These scheme is used for a variety of reasons.

  1. - they can be made to appear like normal switches.
  2. - they can be used to switch low current devices and hence replace controllers for simple applications.

 

· The two types indicate which way the current and voltage are switched to the output. The methods also refer to transistor types because of the similar behavior (Note: in fact they do use transistors inside).

 

· The two main differentiation is,

    1. - Sinking (NPN) - In this case, when actuated, the sensor will connect to ground, or pull the input low. If this is the case you need to use an output that normally stays high, true, floats high, etc. The figure below shows an approximate representation.
    2.  
    1. - Sourcing (PNP) - In this case, when actuated, the sensor will connect to V+, or pull the input high. If this is the case you need to use an output that normally stays low, flase, floats low, etc. The figure below shows an approximate representation.
    2.  

 

· To directly connect devices to these sensors we can use the arrangement below.

 

 

· It is worth stating the obvious - The output of sensors will be the inputs for other devices, such as PLCs - this will lead to confusion when specifying PLC input devices. Some manufacturers indicate what the input type is, others specify what it is for. This basic result of this is that you must look at the electrical connections of the card, and not just the designation.

 

 

· Note, if using these sensors with PLCs, care is required to select the appropriate cards and connections. The figure below is for NPN sensors.

 

 

· The figure below is for PNP sensors. These are generally more common combinations.

 

 

· It is quite common for manufacturers to offer PLC output cards that will handle PNP and NPN sensors. In this case the card will require both V+ and a common connection, and the each output must be set for either NPN or PNP.

 

· Two wire sensors are also common because they reduce the wiring.