PNP vs NPN: What Makes These Outputs Different?

There are two primary types of discrete outputs: electromechanical (EM) relay outputs and solid-state outputs. Both are discrete, which communicate something being one state or another, such as on and off, true or false.

Electromechanical relay outputs use a mechanical switch to change the state of signal flow. When an electromagnet turns on in an EM relay output, the electromagnet pulls an armature down to close a contact. This completes a circuit and allows the current to flow. When the electromagnet turns off, a spring pulls back the armature, opening the contact and interrupting the current flow. These switches are better at switching high electrical loads than solid-state outputs, but the moving parts limit their lifespan and have a slightly slower response speed compared to solid-state outputs.

The most common discrete outputs are solid-state. They are transistor-driven, an electronic switch that turns current on or off. You can think of solid-state outputs as being like an EM relay, but without any moving parts. They offer fast switching speeds and are tolerant of shock and vibration. Banner offers sensors with both AC and DC solid-state outputs. However, PNP or NPN solid-state outputs are available on DC devices only. 

Though it’s common for people to refer to sensors as PNP or NPN, the abbreviations actually refer to the type of transistor used in the device. The difference between PNP and NPN is the construction of the semiconducting material found inside the transistor. In a PNP transistor, the semiconducting material is made of three layers: a negative (N) layer between two positive (P) layers: Positive-Negative-Positive, or PNP. Likewise, an NPN transistor has a positive layer sandwiched between two negative layers: Negative-Positive-Negative, or NPN. Despite their different construction, both forms have positive and negative power leads and are connected to a device called the load, such as an attached indicator light, hub or IO-Link master, PLC, etc. 

Also referred to as a sourcing output, a PNP output provides the current to the attached load. The electrical load is connected between the sensor output and the negative (common) side of the power supply. The output voltage is the same as the supply voltage.

Because they provide the ground to the circuit, NPN outputs are also known as sinking outputs. In this case, the electrical load is connected between the sensor output and the positive side of the power supply. The output voltage is a ground signal. In NPN outputs, the current flows opposite that of PNP outputs.

Different sensors will have different types of outputs. Bipolar sensors contain both PNP and NPN transistors in a dual output configuration. If you’re not sure which output type you need, you can connect one or the other. Only one output, however, is typically connected to the PLC.

Sensors with complementary outputs will be either PNP or NPN. These devices have two output wires, one being open and the other being closed.

A push-pull output contains both PNP and NPN output transistors on the same circuit. If the circuit is on and sending a PNP signal, it will send an NPN signal when turned off (and vice-versa). Sensors with a push-pull output have a polarity setting that allows the user to choose which output is active when the sensor actuates.

Depending on what part of the world you’re in often dictates what type of discrete output you’ll use. In Asia, roughly 90 percent of outputs use NPN. In Europe, about 90 percent of outputs are PNP. In the United States, there is about a 60/40 NPN to PNP split.

A PLC will be looking for either a PNP or NPN signal, so make sure you are using the correct sensor output to match the particular signal the PLC requires for your specific application. This information will be in the PLC documentation and wiring diagrams.

When working on your next application, contact the experts at Banner Engineering for help determining your specific PNP or NPN output needs.