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Industrial Sensors - Understanding the Push-Pull Output

Posted by Zach Steck on Thu, Oct 03, 2013

When you are browsing through various industrial sensors in a catalog or online, you may encounter a signal output listed as push-pull. A push-pull output is a 4-in-1, meaning that one sensor can be wired to provide four different output signals.

Understanding push-pull outputsA push-pull output offers light ON or dark ON operation, automatically detects the load, and sinks or sources the output accordingly. This single sensor can be wired as:

  1. NPN light ON
  2. NPN dark ON
  3. PNP light ON
  4. PNP dark ON

Light ON and dark ON are terms that describe the outputs of a sensor. These terms are very similar to normally open and normally closed. For example, a diffuse photoelectric sensor in light ON mode triggers the output when a target is present (low to high). In dark ON mode, the output is triggered when no target is present and drops out once the object is detected (high to low). These two modes allow you to choose the configuration options that best suit your application needs.

There are several advantages to selecting a push-pull output for industrial sensor applications. A push-pull output reduces inventory costs since you only have to stock one model instead of four different models to cover the same outputs. Cutting down the number of sensors that you need to carry saves you space in the stockroom. By design, a push-pull output gives these sensors outstanding EMC resistance while featuring short-circuit and reverse polarity protection. A fairly simple wiring diagram means that this sensor output is easy to use. This simplicity can reduce the wiring time, making sensor installation much quicker.

With all of the advantages, there is one distinct disadvantage to the push-pull output. You cannot connect two or more of these sensors together because if one sensor pushes and the other pulls, the transistor can become damaged. So in applications that use sensors with a push-pull output, you must wire each sensor individually.

Push-pull outputs are commonly found in the following product groups:

  • Photoelectrics
  • Ultrasonics
  • Rotary Encoders - In encoders, the push-pull output is a combination of NPN and PNP outputs that create an improved square wave and have increased interference resistance. No external wiring is needed. Inverted channels are also available.

Let’s take a look at a few push-pull output wiring diagrams for common photoelectric sensors:

Light ON/Dark ON potentiometerThis diagram represents a push-pull output that has a potentiometer for selecting light ON or dark ON operation. Depending on where the load is placed, the sensor will automatically detect PNP or NPN. You can adjust the light ON or dark ON settings with a simple turn of the potentiometer.

Light ON/Dark On electrically switchableThis wiring diagram represents a push-pull output where light ON/dark ON is switched electrically. For these sensors, the white wire (I/O) is tied into either the brown (+UB) or blue (0 V) to determine light ON or dark ON operation.  

See below for examples:

Push-pull outputsPush-pull output is also an excellent choice when you are not sure which operation function is accepted by the PLC input card. This sensor covers both sinking and sourcing, ensuring that you have a DC sensor which will work no matter what. The flexibility of the push-pull output has made this a very popular option in the industrial sensor industry. Hopefully this post has helped you better understand the push-pull output and how it can benefit you in your next sensor application design.

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Topics: Rotary Encoders, Ultrasonic Sensors, Photoelectric Sensors

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