# Npn pnp



## dronai (Apr 11, 2011)

I understand how they work and how to determine what is sink and what is source.

In my work experience, I have yet to work with either of these. I learned that 120V inputs signals are used in older factories. This is all I've worked with. Do you guys that work with this type of sensor, see it in newer plants ?

Also is this usually in 24V DC or less vs any AC voltage ? Can you use this type of sensor with AC ?


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## Big John (May 23, 2010)

Proximity switches are the big ones that I can think of, those are almost always transistor outputs. But many other electronic sensors come standard with those outputs, I know it's not uncommon to see them on photoelectric and pressure sensors. Any application that requires a high frequency pulse rate, or very long life cycle is likely to have solid-state controls.

Where I've also seen them is as an optional output in a lot of controllers, where they'll have a couple outputs that are relays and a couple are solid-state. I just did a rolling-door controller that had them. I'm not sure what the reason is for including them on applications like that; I don't know what the transistor would do that a relay would not.


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## JRaef (Mar 23, 2009)

PNP or NPN refers to transistors, transistors work on DC, not AC. For AC, they might be triacs or diacs, so "sinking or sourcing" becomes irrelevant.

They are primarily on low voltage DC sensors such as 24VDC, but sometimes on proprietary equipment they will use something like 15V or 18VDC just to force you to buy replacements from them.

You will also see them used as I/O on controllers, mostly because they will only handle a low amount of current and can therefore be made to be much higher density than AC I/O, plus they can more easily be (opto)isolated from the controller logic voltage, so it all helps to keep the controller size down.






Big John said:


> ...I don't know what the transistor would do that a relay would not.


Saves them the size and cost of a relay. By using a transistor output, the USER has to absorb the cost of driving an interposing relay, so it makes their controller look smaller and cheaper. If you want a relay controlled by a microprocessor, you have a TTL controlled opto-isolated transistor that drives a relay coil, coil suppression etc., then have traces on the PCB for the relay contacts out to terminals. If you just put terminals on the opto-isolated transistor, you're done!

But also, transistors can be very very much faster than relays, so sometimes that's why they provide them. Depends on the application.


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## dronai (Apr 11, 2011)

JRaef said:


> You will also see them used as I/O on controllers, mostly because they will only handle a low amount of current and can therefore be made to be much higher density than AC I/O, plus they can more easily be (opto)isolated from the controller logic voltage, so it all helps to keep the controller size down.
> 
> 
> 
> ...


 
My experience is only with conveyors, and all I am seeing is 120V sensors:limit, proximity, or a reflector to a photo eye, tied in with an encoder to the logic. 

From the first paragraph, when you say higher density, you mean more positions at the terminals on the cards ? This would also have to be rectified to DC first, does this happen at the MCC typically ?


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## Jlarson (Jun 28, 2009)

Most all our new work is 24vdc sensors and switches and 24v I/O in the PLC racks.


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## drsparky (Nov 13, 2008)

Here are NPN and PNP transiators, what the OP is referring to remains a mystery.


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## dronai (Apr 11, 2011)

drsparky said:


> Here are NPN and PNP transiators, what the OP is referring to remains a mystery.


 
Source and Sink in regards to PLCs inputs


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## sparkywannabee (Jan 29, 2013)

http://www.google.com/url?sa=t&rct=...H9-4B4&usg=AFQjCNFkSGao219S3MOhAPf3_GMpL2i2ag
Section 2-3


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## greenman (Apr 20, 2012)

sink or source ???


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## Frank Mc (Nov 7, 2010)

greenman said:


> sink or source ???


Sinking is NPN.....Your pulling (sinking) the signal generally to ground / ov...when signal is true

Sourcing is PNP.....Your supplying a positive signal ..when signal is true..

Hope that makes some sense...??

Frank


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## bill39 (Sep 4, 2009)

Sinking is switching the - wire (think switched neutral).

Sourcing is when the + wire is switched (think switching the hot wire).

If there is a safety consideration, do not use sinking because if the "switched side" of the device goes to ground then there's no way to shut it off.


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## dronai (Apr 11, 2011)

bill39 said:


> Sinking is switching the - wire (think switched neutral).
> 
> Sourcing is when the + wire is switched (think switching the hot wire).
> 
> If there is a safety consideration, do not use sinking because if the "switched side" of the device goes to ground then there's no way to shut it off.


Which method is used more often and why ?


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## bill39 (Sep 4, 2009)

dronai said:


> Which method is used more often and why ?


On industrial machinery in the US I believe sourcing is used most because of the safety facto I mentioned earlier.

I seems like Japanese designed circuits will often use sinking. This is not a knock on them, just my observation.


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## 123 (Oct 28, 2012)

PNP is more common. 24VDC is a standard voltage.
brown wire is positive, blue wire is zero,
black wire is signal, typically N.O.
white wire can be NPN output, or N.C.
The load on the PNP gets connected to the black wire
and zero, and the black wire is 24V or nothing (not 0V).
In a NPN, the load gets connected between the black and 
+24. The signal on the black wire is zero or nothing.
For troubleshooting it may be harder to see the difference between
zero Volt and nothing.
There are some 120VAC sensors, and many are 2 wire.
new equipment is almost always 24VDC.
Most sensors can’t drive big load, maybe a small relay.
Most are connected to the input of a PLC.
If you build new equipment buy PNP sensors.


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## Michigan Master (Feb 25, 2013)

*Sensors*

Our build spec requires all proximity sensors to be 24VDC PNP (Sourcing). Really, really old equipment had 120VAC sensors; this is no longer acceptable practice due to safety concerns (physical damage presents shock hazards). We use a wide variety of sensors including limit switches, pressure switches, flow switches and float switches.

*Inductive Sensors* are the most widely used proximity sensors. Inductive sensors have four components; the coil, oscillator, detection circuit and output circuit. The oscillator generates a magnetic field around the coil located in the sensor’s face. When metal enters the field, eddy currents build up in the object and magnetically push back and dampen the sensor’s oscillation field. The detection circuit monitors the amplitude of oscillation and then triggers the output circuit.
Typical Applications:
•	Target must be metal
•	Must be able to be sensed at a fairly close range (<1”)
•	Available in shielded (able to be flush mounted), nonshielded and quasi-flush.

*Capacitive Sensors* respond to a change in the dielectric medium surrounding the active face and can therefore be used to sense almost any substance.
Typical Applications:
•	Level control of non-conductive liquids (oil, alcohol, fuel)
•	Level control of granular substances (flour, wheat, sugar)
•	Sensing substances through a protective layer (glass)

*Ultrasonic Sensors* utilize the reflection of high frequency (20KHz) sound waves to detect parts or distances. Usually the best choice for transparent targets as they can detect a transparent plastic film as easily as a wooden pallet.

*Hall Effect* sensors are a transducer that varies its output voltage in response to changes in the magnetic field.

*Reed Switch* is an electrical switch operated by an applied magnetic field.

*Photoelectric Sensors* are used to detect the distance, absence, or presence of an object by using a light transmitter (often an LED in either infrared or visible light spectrum) and a detector (photodiode)
Photo Sensor Modes:
•	_Opposed-mode_ (Thru-beam) - consists of a separate emitter and receiver as objects pass between the two components the beam is interrupted causing the receiver signal to change state
•	_Retro-reflective_ - arrangement places the emitter and receiver in the same housing and then uses a reflector to bounce the light beam back from the transmitter to the receiver 
•	_Diffuse-mode_ (Proximity-Sensing) - photoelectric sensors detect the light reflected back by the target itself. Some photoeyes have two different operational types, light operate and dark operate. Light operate photoeyes become operational when the receiver "receives" the transmitter signal. Dark operate photoeyes become operational when the receiver "does not receive" the transmitter signal. 

*Laser Sensors* can be used to sense the presence of objects using an analog output (ON/OFF) and offers the alignment advantage of a visible beam. 
Typical Applications:
•	Precise position is required
•	Multiple outputs over a very tiny range
•	Distance measurements
•	Changes in material thickness


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## Frank Mc (Nov 7, 2010)

As Bill stated the jap,s tend to use NPN a lot....Most equipment i work on is PNP...I think the NPN switching may be something that was carried on from electronic design were microprocessor control signals were active low....

HTH
Frank


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## oliquir (Jan 13, 2011)

there are also universal sensor 2 wire dc. they can be connected as npn or pnp. the only negative point is they need some current (over 5 ma) to work correctly. some plc inputs have too high impedance to make them work ok, in this case we need to add an extra resistor for load


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## dronai (Apr 11, 2011)

oliquir said:


> there are also universal sensor 2 wire dc. they can be connected as npn or pnp. the only negative point is they need some current (over 5 ma) to work correctly. some plc inputs have too high impedance to make them work ok, in this case we need to add an extra resistor for load


Are you using these in high speed applications ? (2, and 3 wire npn pnp)


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## oliquir (Jan 13, 2011)

2 wire sensor can't go as fast as normal npn or pnp but can be a lot faster than ac sensors. 
i use them on upgarde of old machines that used 120v sensor (prox or limit switch) that only have 2 wires cable.


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## triden (Jun 13, 2012)

There are various reasons why you would want to use an NPN or a PNP output driver and I'll go over a few of them here.

First of all, the primary choice will come down to safety. 
When you have an NPN topology output with the load connected between the voltage source and the I/O card, the output just grounds the device to turn it on. Now if for some reason the output wires on your load get shorted to ground, the device will energize because it always has a voltage supplied to it. A PNP on the other hand sources the current to the load (supplies positive voltage if you will) because the load is always grounded. This is a much safer topology because if the lines to the load were to ever short to ground, the load will not turn itself on like with an NPN.

This is also where you get the terms high-side and low-side drivers. NPN being the low-side and PNP being the high-side driver.

The second concern when choosing between NPN and PNP is more of a design concern, and not a concern that any integrator will have to worry about. To elaborate though, the silicon channel in an NPN device has a MUCH lower on resistance than an equivalent PNP device. This means that an NPN can drive higher current loads without voltage drop and heating issues. PNP's are generally limited to about the half the current sourcing abilities of an NPN because of their internal construction.

I'll draw a picture to help you understand the safety and connection differences between high-side and low-side output drivers.



PNP outputs are much safer, although not as efficient.


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## dronai (Apr 11, 2011)

triden said:


> There are various reasons why you would want to use an NPN or a PNP output driver and I'll go over a few of them here.
> 
> First of all, the primary choice will come down to safety.
> When you have an NPN topology output with the load connected between the voltage source and the I/O card, the output just grounds the device to turn it on. Now if for some reason the output wires on your load get shorted to ground, the device will energize because it always has a voltage supplied to it. A PNP on the other hand sources the current to the load (supplies positive voltage if you will) because the load is always grounded. This is a much safer topology because if the lines to the load were to ever short to ground, the load will not turn itself on like with an NPN.
> ...


What applications are you using these sensors on vs 120V ?


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## triden (Jun 13, 2012)

24 volts is much nicer to work with and I think more things are going towards it. A lot of instrumentation devices run of 24vdc, so it makes more sense to use it for everything. I use it to power contactors, valves, and sensors. Also with 24V you can keep panels classified as low voltage and outside of the NFPA 70E arc flash standard.


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## Semi-Ret Electrician (Nov 10, 2011)

Triac outputs are a "third party" designers nightmare.

An Original Equipment Manufacture (OEM) can select the loads and specify the failure mode and go w/ relay output, if need be.

I always selected relay output, just to sleep well.


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