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Pepperl+Fuchs Blog

Electronic Sensor Outputs and Wiring Configurations

Posted by Tracy Molnar on Thu, Jul 18, 2013

Electronic sensors with switching output are commonly used to energize loads such as relay coils or PLC inputs, and a variety of sensor output configurations are available to operate in conjunction with these loads.  In this post, we’ll look at the most common standard output types, their wiring, and the advantages and disadvantages of each.

Common electronic sensor output configurations 

How electronic switching is accomplished

Standard electronic sensors require connection to a power supply in order to operate and include an output circuit which contains a solid-state switching element, such as a transistor or thyristor.  When this output circuit is triggered, the normal state of the switch changes: normally open switches close, and normally closed switches open.  Because the switching element is solid-state rather than dry contacts, there is always a voltage drop when it is closed, and a leakage current (also known as off-state current) when it is open.  The value of these differs depending on output circuit design, and are listed on the sensor’s datasheet.

Issues to consider when choosing an output type

  • Available power supply – Most standard DC sensors operate in the range of 10 VDC-30 VDC, while AC sensors operate from 20 VAC-250 VAC.  Some models are AC/DC and will operate on either type of power over a wide range, such as 20 VAC/VDC-250 VAC/VDC.
  • Load characteristics – A sensor’s maximum leakage current must be lower than the load’s off-state threshold, or else the load will be energized even when the sensor’s switching element is open.  In addition, the load current with the switch closed must be greater than the minimum on-state current (also known as lowest operating current) of the switch. This information is listed on the sensor datasheet.

Standard sensor output types

3-wire DC – With this type of output, the sensor is connected to the power supply using two wires, and the switching output is on a third wire.  Because of this separation, when the sensor is properly connected to the load, the power supply current does not pass through the load, and leakage current is minimized.  The 3-wire outputs are usually offered with either PNP- or NPN-type switching elements.  A PNP output sources current (switches the load to DC positive), and is required when the other side of the load is connected to DC negative.  An NPN output, which sinks current (switches the load to DC negative), is used when the other side of the load is connected to DC positive.  These two types of connection are illustrated below, where the large square to the left is the sensor, the power supply connections are to the right, and the small rectangle represents the load.

3-Wire DC Sensor Outputs

Advantages of 3-wire DC output

  • Leakage current is very low
  • Typically short-circuit protected
  • Simple testing, no load required, only power supply and sensor output LED
  • Common, readily available, offered in many configurations
  • Also come in 4-wire variety, which include both N.O. and N.C. outputs

Disadvantages of 3-wire DC output

  • Connection to load must be done according to PNP or NPN configuration
  • More wires to deal with

2-wire DC – In a 2-wire configuration, the output and load are connected in series with the power supply for the sensor, and the leakage current is higher so that the sensor can operate even when the switching element is in the open state.  Connection to the load can be either through the DC positive or negative, which makes the 2-wire output more versatile.  The current-sourcing configuration is shown below.

2-Wire DC Sensor Output

Advantages of 2-wire DC output

  • Simple 2-wire connection
  • Typically short-circuit protected
  • Will either sink or source current depending on load placement

Disadvantages of 2-wire DC output

  • Higher voltage drop and leakage current
  • A load must be in place for proper operation or testing
  • Fewer model choices available compared to 3-wire DC devices

2-wire AC – This type of connection is very similar to 2-wire DC, with a voltage drop and leakage current to consider when determining compatibility of the sensor output with the load characteristics.  It should be noted that most 2-wire AC sensors do not include short-circuit protection, and should always be operated with a load connected.  Applying power to the sensor without a load can damage it.  Another point to make is that 2-wire AC sensors with metal housings usually have three wires, but this should not be confused with 3-wire operation – only two of the wires are used in connecting the load, with the third wire being available to ground the housing if this is desired (see example connection diagram below).

2-Wire AC Sensor Output

Advantages of 2-wire AC output

  • Simple 2-wire connection
  • Load placement can generally be on either the hot or neutral side

Disadvantages of 2-wire AC output

  • Typically not short-circuit protected
  • A load is required for sensor operation or testing – damage to the switch is possible if power is connected with no load in place
  • Fewer model choices available compared to 3-wire DC devices

2-wire AC/DC – A specialized circuit design enables 2-wire AC/DC sensors to operate using either AC or DC power supply and loads, and short-circuit protection is usually included as well.  Their voltage drop and leakage current specifications are typically higher, but otherwise their connection and operation is very similar to either 2-wire AC or 2-wire DC devices.

2-Wire AC/DC Sensor Output

Advantages of 2-wire AC/DC output

  • Simple 2-wire connection
  • Universal wide-range operating voltage
  • Typically short-circuit protected
  • Load placement can generally be on either side of the sensor’s power supply connections

Disadvantages of 2-wire AC/DC output

  • Higher voltage drop and leakage current
  • A load must be in place for proper operation or testing
  • Fewer model choices available compared to 3-wire DC devices
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Topics: Inductive Sensors, Capacitive Sensors

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