How to decide between PNP and NPN

What signal level sensors and I/O modules should you standardize on?

By Mike Bacidore, editor in chief

Control Design reader writes: My question arises due to machines we received at our manufacturing plant in Arizona from an OEM in Asia. All of the PLC 24 Vdc digital input modules were NPN (sourcing 24 V dc) and our plant’s standard is PNP inputs (sinking 24 Vdc inputs). These signal levels (sourcing input module with sinking sensor vs. sinking input with sourcing sensor) seem like they will double the number of input modules and sensor spare parts we stock. We need to make some decisions on NPN vs. PNP transistors.

I’ve been asked to clearly document the electrical standards related to NPN vs. PNP and other I/O signal levels, in a machine standard. The question is what signal level sensors and I/O modules should I standardize on—NPN or PNP—in our U.S. plant, and why do the signal levels seem to be different (negative vs. positive type) on the equipment from Asia? Also, do I have the terminology correct, such as a sourcing sensor connects to a sinking input is a PNP configuration?

In the standards, I also wanted to document when to use a pull-up resistor. Is that a way to convert an NPN to a PNP signal level, or am I confusing that with an open collector input? Or is a better solution to install an opto-isolator or just replace the I/O module and sensors? Finally, are there other signal voltage levels I should document such as 5 Vdc and 120 Vac that would have NPN vs. PNP or other signal issues?

PNP sensors for the U.S. plant

Thomas Kolleck, sensor technology engineer, TurckFor the U.S. market, standardizing on PNP sensors would be the better option as they are more popular and easier to find than their NPN counterparts. PNP sensors are also easier to understand and troubleshoot by technicians, since the sensor will give a high-level voltage signal when the output is active. However, it almost always comes down to personal preference and which sensors are easier for you to source.

Historically, Asia standardized early on NPN sensors, while in Europe the popular choice is PNP.

Generally speaking, the sourcing sensor and sinking input terminology is correct. However, some manufacturers will label an input for the corresponding output device, rather than the actual function of the input itself. In the above example you would use a PNP sensor to connect to a PNP input, even though it is technically a sinking (NPN) input.

An external pull-up resistor could be used to convert an NPN sensor to work with a sinking input. However, care must be taken, as this will invert the logic of the sensor. When the output of the NPN sensor is off, the sinking input will be pulled high. Alternatively, a pull-down resistor can be used to convert from PNP to NPN.

For dc sensors, in addition to NPN or PNP, you might also come across two-wire sensors, which could be wired into a PNP or NPN input in either configuration. However, you should always consider the off-state operational current of the sensor in those cases, as it can sometimes cause problems depending on the voltagelevel requirements of the input.

 

This is just one of the answers we received from industry experts. 

We've compiled the rest of the suggestions into an expanded report you can download for free for easy reference. 

→ Our report, "How to decide between PNP and NPN," can be downloaded here.

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  • The whitepaper states: "In an NPN circuit, if a wire breaks and contacts ground, the PLC input is true. This can potentially result in undesirable machine behavior (for example, start push button input turning on). When a wire in a PNP circuit shorts to ground, the PLC input is false." This is only true if the negative side of the power supply is grounded. If, on the other hand, the positive side of the power supply is instead grounded, then the opposite is true; i.e.: in a positive ground system, with a PNP "...circuit, if a wire breaks and contacts ground, the PLC input is true and can result in undesirable machine behavior." I know a positive ground is not usually the standard, but it is just as valid as grounding the negative side; there is nothing that intrinsically favors grounding the positive or negative side of the power supply, it's just that negatively grounded systems are more common (probably due to NPN transistors being more standard in the earlier days of transistors). I may be dating myself, but years ago, our family's 1955 Ford had a positive 'ground' system as did the next door neighbor's 1955 Mercury. For some history please see: THE ARGUMENT FOR POSITIVE GROUND at the following link: http://lajagclub.com/the-argument-for-positive-ground/ The whitepaper also states: "Troubleshooting an electrical problem is less confusing with PNP wiring because when a meter reads 24 Vdc it is clear that an input is true" If this is an issue for someone when troubleshooting an NPN system, then just measure between the positive side of the of the power supply and the input instead of measuring between the negative side of the power supply and the input. This will work whether or not the system is grounded, and if grounded, whether the negative or positive side is grounded; if it is a positive ground system with NPN inputs, then it would be quite natural to measure between the positive side of the of the power supply and the input.

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