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How to Read a Sensor Connection Diagram

Posted by Tracy Molnar on Tue, Nov 05, 2013

Look before wiring!

Let’s talk about connection diagrams.  Although sensor hookup is typically simple – just a few wires – it’s always a good idea to consult the connection diagram before wiring to be sure you’ll end up with the outcome you expect. 

How to Read a Sensor Connection Diagram

Most diagrams follow a pattern where the industrial sensor connections are shown on the left, and the load and power supply to be connected are indicated to the right.  If no load is shown (it usually appears as a small unlabeled box), one should be assumed.  This is the component or circuit that will see the switching or analog output of the sensor – common examples are a relay coil or PLC input.

Sensor connection diagram

Power supply connection

The majority of electronic sensors require power for operation.  Two-wire sensors have only two connection points, so the output is wired in series with the power supply.  For other configurations, power is supplied to the sensor using two connection points, and the output is on a third wire.  The supply voltage range varies depending on the sensor specifications, and may be DC, AC, or AC/DC.  Power supply connection points are pictured on a diagram using a variety of symbols as follows:

  • For AC power supplies: ~, L1 and L2, L and N
  • For DC power supplies: +UB and 0V, +Ub and -Ub, L+ and L-
Power supply connection points

Sensors with switching output

The main function of many industrial sensors is to act as an electronic switch.  If a sensor has switching output, this will be pictured on a connection diagram in different ways depending on the type of sensor you’re working with.

Inductive, capacitive, or magnetic sensors

For these three types of sensors, the switching output is usually shown inside the box that represents the sensor, and appears as an open or closed contact.  This is the normal state of the switch, which is when no activating target is in the sensor’s detection field.  Connection diagrams for 3- or 4-wire sensors will also show the required wiring configuration for their type of output:

Current-sourcing (PNP) outputs are internally connected to the power supply voltage, and so should be wired externally to a load that has a connection to DC ground.

PNP outputs

Current-sinking (NPN) outputs are internally connected to DC ground, and so should be wired externally to a load that has a connection to the power supply voltage.

NPN outputs

Ultrasonic or photoelectric sensors

These sensors are more likely to have multiple switching outputs, or outputs that are programmable, and a contact is generally not shown.  Instead, outputs are marked by symbols such as Q1 and Q2, or light ON and/or dark ON circles, to show different modes of operation. 

If a switching output is strictly PNP or NPN, the external connection through the load may be shown as described above.  In the case of a push-pull output, this will not be part of the connection diagram, since sensors with this output can operate using either type of external wiring configuration.

Sensors with analog output

Some sensors feature analog output, which changes in a linear fashion according to some distance traveled by the target in relation to the sensor’s detection field.  Analog output can be shown on connection diagrams using the symbols or labels shown below.

Analog output

Other connection points

Sometimes there will be an input connection on the sensor, which is used for Teach-in, mode selection, or another programming function.  These are described in the sensor data sheet and labeled accordingly on the connection diagram; some common labels are IN, XI, Test, Sync, or ET.

Sensor connections

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Topics: Inductive Sensors, Capacitive Sensors, Ultrasonic Sensors, Photoelectric Sensors, Magnetic Sensors

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