Most engineers and technicians envision sensors as being the device that sends process information into a control system such as a PLC, VFDor similar.
That thought process may be correct, but we should not get too locked into thinking about information flow and current flow in the same direction. When faced with the task of installing a new sensor or replacing one that has failed, a technician or engineer faces a decision. The PNP variety is usually easier to understand because it follows the conventional directional flow of current.
Although NPN sensors still follow electrical principles, they can be more confusing. Sensors are used in control systems to provide information to the controller, such as a PLC or motor drive.
This input information is used to determine whether an input condition is true or false, or in other words, detection of current flowing or not flowing.
To briefly summarize the operation of an NPN polarity sensor, when de-energized, current cannot flow through the load wire. But when energized by whatever sensing method is used inductive, optical, etccurrent flows through the load wire into the sensor which returns the current to ground. In order for an internal circuit to be completed, the current must be sent out from the control module like the PLC input module and into the sensor. The concept of current flowing out of an input module is the main reason why NPN sensors seem to oppose the conventional directions of electrical flow.
A quick indicator as to whether an NPN sensor would be required is the common voltage to the input module. In order for a sensor to be compatible with a sourcing input module, that sensor must allow current to flow in through the load wire and return the current to the ground.
In some device construction, the module will be fixed as either a sinking or sourcing mode, and cannot be changed. Other times, changing the destination of the common wire is the only change required to toggle between source and sink.
Industrial Sensing Fundamentals – Back to the Basics: NPN vs PNP
This load wire was attached to the rectangular symbol of a load device, which commonly would be the PLC input terminal. The opposite side of this load rectangle will connect either to power or to ground.
The current can flow through the completed circuit and the input module detects the energizing of the sensor. To test the polarity of an NPN sensor, remove the load wire from the input module. With the voltmeter measuring the open load wire observe the voltage as the sensor energizes.
A PNP sensor, in contrast, will toggle between 0 and 24 volts. The internal construction of an NPN sensor uses the transistor with the same name, based on the N-P-N layer construction of the semiconductor. Inside this transistor, when the base pin is given a small current, such as from the actual sensor transducer, a larger amount of current flows from the Collector to the Emitter. The load wire going to the PLC is connected to the base.
When energized, current will then flow from the PLC into the Collector, and return to ground through the sensor.
In this way, the current enters the sensor from the load wire and returns to ground, the characteristic operation of the NPN sensor. NPN Sinking sensors are necessary when the interacting input module is of the sourcing polarity, that is, voltage is continually supplied to the input terminals. When the sensor is energized, current flows from the module to the sensor and returns to ground.
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How to Identify and Use NPN Sensors
I - Fundamentals Vol. II - Instrumentation Vol. III - Measurement Vol. IV - Control Vol.These signal levels sourcing input module with sinking sensor vs.
We need to make some decisions on NPN vs. PNP transistors. 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.
PNP or other signal issues? 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.
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. 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.
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.
Control Design. Menu Newsletters Subscribe. PNP sensors for the U. This is just one of the answers we received from industry experts. Servo-motor applications and temperature-control applications often need training after the…. Calculations, advice and formulas for ensuring you have the right motor for your application.By martinmin Electronicsjuly 07, From time to time, Yoctopuce support receives questions from customers who do not quite know what to do with the three wires coming out of a PNP or NPN sensor.
It's used for example in proximity sensors, but you can also find them in more exotic sensors such as bubble sensors.
You therefore find them mostly in detectors rather than in measuring devices. These outputs have three wires with standard, although counter-intuitive, colors:. How this type of output works is relatively simple: the sensor acts as a simple switch.
When a sensor with a PNP output is activated, it puts into contact the signal wire with the power supply wire. The voltage on the signal wire therefore changes to the value of the power supply voltage. It has the advantage of being very simple, but you can't really choose the signal output voltage.What is NPN and PNP Sensor Output?
How PNP outputs work. NPN outputs work the other way around. When the sensor is active, it puts into contact the output wire with ground. Which means that it doesn't inject current in the output wire.
Therefore, for the change to be detected, someone has to put some voltage on the wire. It's the job of a pull-up. How NPN outputs work.
An Easy Way to Remember PNP and NPN Sensor Wiring
A pull-up consists simply in connecting a signal line to any voltage with the help of a resistance. In our case, when the sensor is idle, the signal line is at the pull-up voltage and when the sensor is active, voltage goes to zero.
Note, a resistance is compulsory. If you directly connect the signal line to the voltage source, there is going to be a short-circuit as soon as the sensor puts together the signal wire and ground. The typical value of a pull-up resistance is 4. In the case of Yoctopuce modules, you don't need to add an external resistance: pull-ups are embedded in the modules.
The pull-up principle. The advantage of NPN outputs is the possibility to choose an arbitrary signal voltage. However, as these modules cannot provide more than 5V, you'll probably need an external power supply to power your sensor. What you have to do is:. It even has a pulse counter enabling to precisely count the number of transitions happening in a given time frame.
It can count tens of thousands transitions par second. That's why one can use it to count objects passing ahead of a sensor, even if they are very fast, but one can't use it to know exactly when an object is front of the sensor. As with the Yocto-IOyou need an external power supply for your sensor. There as well, you need an external power supply.
You must therefore ensure that the external power supply of the sensor and that of the Yocto-Knob can share the same ground and that they are powered from the same phase. Otherwise, you have a high risk of blowing a few components. As a conclusion, here is a little table explaining the main differences between all four device when used for interfacing NPN ou PNP outputs. Here you are, you know everything.
PNP outputs When a sensor with a PNP output is activated, it puts into contact the signal wire with the power supply wire. How NPN outputs work A pull-up consists simply in connecting a signal line to any voltage with the help of a resistance.Since so many typical industrial automation sensors operate at 24 Vdc, it is important to understand two of the main variations of these solid-state devices. In some industries, these signals are operated at Vac.
A safer and more common option is to use 24 Vdc, and many end users choose devices with plug-and-cord connectors for easy installation and servicing. As it turns out, a little planning is necessary to ensure 24 Vdc sensors and PLC discrete input DI modules are connected properly.
These must be correctly matched with sinking and sourcing DI modules in order to function. They amplify a very small signal, such as the position sensing portion of a proximity switch, to turn a larger signal on or off.
This larger signal can go to a DI point or an indicating light, or to any other device with an acceptable current rating. Transistors have connections termed as the base, collector and emitter. Solid-state devices are active as opposed to passive, and therefore usually require some small amount of operating power.
They are typically designed as three-wire devices with leads or connections for:. Many of these three-wire sensors use standard M12 connectors and cordsets. Sometimes more wires are included for extra functions, but always review data sheets for the exact connections.
Leakage current is a term indicating how much current might leak out on the switched lead even if the switch is off. This is usually more of an issue for AC devices than DC devices. However, be aware that a field device with an unusually high leakage current might falsely turn on a DI module with a low input current switching threshold.
A diagram may state that a DI point is compatible with sourcing field devices, which means the DI point is effectively sinking. If a PNP cable is damaged there is a chance the signal could short to ground and damage the sensor. If an NPN cable is damaged there is the chance the signal could short to ground and cause a false true signal, but there would be no damage to the circuit. Designers should look to the readily available products from their distributors.
However, it is important to be aware of both product types because end users could find themselves interfacing with or repairing equipment using NPN devices.
One final caution is to carefully check the device part numbers. Certain devices may have both capabilities built into them, so they will need to be connected and configured carefully. The PNP versus NPN discussion is somewhat related to, but not to be confused with, the discussion of normally open versus normally closed contact configurations and control circuits, a topic that will be covered in another article. Shop Sensors. Save my name, email, and website in this browser for the next time I comment.
My Account My Cart. PNP versus NPN Switching Solid-state devices are active as opposed to passive, and therefore usually require some small amount of operating power. Check the Part Numbers One final caution is to carefully check the device part numbers. How helpful was this article?
Click a star to rate. Average rating of 5 ].With respect to sensors, an NPN device is one that can switch the negative side of the circuit while a PNP device switches the positive side. The load could be a lamp, pneumatic valve, relay or PLC input.
Types of Discrete Electrical Outputs — The majority of sensors used today use Solid-State outputs, not mechanical relays. The drawing below shows 2 wires for sensor power and 2 for the switch.
Most sensors use only 3 wires by having one wire do double duty, carrying both power and output signals. This is similar to a bathroom with only one line carrying water to both the sink and toilet as opposed to individual lines for each. Solid-state discrete sensors functions similar to a switch, but current only flows in one direction. Solid-state devices are reliable, economical, small and fast. The type of transistor determines the direction of current flow. Add explanation of push-pull output would be a bonus to the reader.
Thanks for the positive feedback, tlane. Thank you for the explanation, I was thinking the NPN sensors were normally used when the sensor and the PLC do not work on the same voltage. Also, in dangerous environments is it probably better to have a 0VDC to activate something in case of a short circuit?
The switching logic PNP or NPN are not related to the supply voltage of the sensor or the operating voltage of the input. Referring to the NPN wiring diagram above, note that the sensor supply voltage and the high side of the load are connected to the same point and are therefore at the same voltage.
In hazardous or explosive atmospheres, special care must be exercised to determine the classification of the hazard and to employ components with an approved protection method certified for the classification at hand. Refer to this previous post for more information.
Informative better exploration on diagrams. Nice explanation, however could someone explain the pros and cons of using one over the other? I typically see Japanese controls that require NPN sensors. Industrial automation is a field that, for all its sophistication, is sometimes slow to change and legacy methods sometimes die hard.
So early solid-state sensors were more likely to use NPN transistors in their switched outputs, so they were load-sinking. The machine bed was connected to ground. When a metal part, such as a piece of sheet steel or a stamped metal part, made contact with the whisker wire, the circuit became grounded and the relay pulled in. This was a load-sinking connection. NPN control circuits. With PNP, if a load wire shorts to ground, the short can be found relatively easily because there may be visible damage due to the higher-than-normal current that would flow in the absence of a load impedance.
An unprotected, unfused sensor may also have its output blown. If the PNP sensor has integrated thermal, latching, or pulsing short-circuit protection, the existence of a short circuit condition on the load wire will be apparent at the sensor. In the case of a short to ground in an NPN circuit, there is no abnormal current flow; the load is simply pulled in regardless of the sensor state.
This could prove to be somewhat difficult to detect, because if the sensor associated with the grounded load wire is examined, it will be found to have no fault. In the modern industrial world, NPN is considered more common and popular in the Asia-Pacific region simply because of legacy practice and the desire for backward-compatibility.
Nice website and good explanaition, however I still dont know in which situation should I use each other.Posted by Sam Horn on Tue, Sep 18, Connecting a proximity sensor to a PLC can be a daunting task at first glance.
For one thing, there is no universal way to connect proximity sensors. The process varies depending on the sensor and input module used in the configuration. The first step is understanding how PLC networks are set up. The four major components of a network include the following:. It is important to read the wiring diagram of an input module to understand its operation.
Terms like load, sinking, sourcing, positive or common, PNP, NPN, 2- 3- and 4-wire will be used throughout this discussion. To get familiar with some of these terms, read our blog post How to Read a Sensor Connection Diagram. A sourcing PNP sensor must run into a sinking input card, while a sinking NPN sensor must run into a sourcing input card. This is true because the network can be modeled as a simple circuit, and in order to complete the circuit, current must flow in a complete loop.
A common wire is used to reduce the number of terminal connections made between sensors and a PLC. This means that multiple sensors can be connected to an input card with all sensor negative wires to one common - wire. NPN sensor outputs switch in a negative - fashion. Hardwire Connections. Sensors with 3 and 4 wires have dedicated output wires. Normally open or normally closed sensor logic does not affect physical connections between a sensor and a PLC.
Internal Loads. Sinking cards have an internal resistive load connected between its inputs and V- also called ground or 0 VDC. Why is this true? System Layout. Sensor datasheets show the following:.
In a PNP configuration, the sensor output is internally connected to the positive voltage polarity. To make a complete circuit, the load PLC input must be externally connected to a negative common voltage polarity. In an NPN configuration, the sensor output is internally connected to the negative voltage polarity. To make a complete circuit, the load must be externally wired to a positive common voltage polarity:.Thank you for your ongoing business and the 5 star rating.
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