What’s your take on the legacy background of NPN vs.Most bipolar junction transistors and field-effect transistors (FETs) have three terminals. We’d like to hear from those of you out there who are experienced industrial controls experts. PNP is more popular in Europe and much of North America, but note that in North America NPN is quite prevalent in Asian transplant facilities and is often found in the metal stamping industry. That is, the use of NPN sensors leads to the continued use of more NPN sensors. 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. 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 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. 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. An unprotected, unfused sensor may also have its output blown. 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. I’ve heard some theories about the ease of trouble-shooting PNP vs. 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. So early solid-state sensors were more likely to use NPN transistors in their switched outputs, so they were load-sinking.Ģ) The most basic type of “sensor” was once an ungrounded hot “whisker” wire energized by a load such as a relay. I’ve heard two arguments advanced to explain the existence of NPN sensors.ġ) Early solid-state bipolar junction transistors were more commonly available as NPN, which were load-sinking. Industrial automation is a field that, for all its sophistication, is sometimes slow to change and legacy methods sometimes die hard. The type of transistor determines the direction of current flow.įor additional information about NPN-PNP devices, connections, and working with PLC’s check out the following from the Industrial Electrical and Maintenance forum. NPN and PNP are technical terms for the type of transistor used to switch the output. The only drawback is, that you have to know the direction of current flow. 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. Most sensors use only 3 wires by having one wire do double duty, carrying both power and output signals. The drawing below shows 2 wires for sensor power and 2 for the switch. Types of Discrete Electrical Outputs – The majority of sensors used today use Solid-State outputs, not mechanical relays The load could be a lamp, pneumatic valve, relay or PLC input. The term “load” identifies the device the sensor powers. NPN sensors are sometime called “ sinking sensors” because they sink ground to the output. PNP sensors are sometimes called “ sourcing sensors” because they source positive power to the output. The next question to ask is, what direction do you want the current to flow? When it comes to wiring a sensor, you can think of the “N” as standing for “Negative” and the “P” as standing for “Positive”. 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. In the context of this post, they refer to the construction of a sensor’s transistor and whether it has a p-type or n-type semiconductor. What’s the difference and why should anyone care? If you’re confused by the terms PNP and NPN, then hopefully this post will shed some light on the differences between the two.
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