NPN vs. PNP

[cmoore73 2]

I have been working in the automation field for the past 11 years and have heard MANY arguments both ways. I see chakorules put a post in the GE forum about using input modules: "We have our commons (for the inputs, which is NPN) ack! connected to:" Which is better and why?????? Edited 25 Oct 2006 by cmoore73

[Big Country 3]

I like to use NPN not to say that PNP is wrong, it is just something that I was taught an stuck with it. But I look at it like this if I have limit switch that has a broken or exsposed wire it will not take out my power supply or trip a breaker as would a PNP type input that has voltage on it.

[panic mode 246]

depends what you need PNP or NPN for and where you are. NPNs traditionally can sink more current and are faster but we are talking about sensor outputs, not switching power supplies or mobile phone transmitters so this doesn't apply. NPNs are handy to interface I/O that has same polarity, different voltage but share negative as common reference (embeded systems etc.). while this is fine and it is used in many devices, it is something to take on carefully. NPNs are fine but one has to consider machine safety. it's not the same thing to have an extra page printed by mistake by that little inkjet as being pinned by hydraulic or pneumatic actuators. wiring problems do happen, usually later when for example forklift pushes pallet against EMT, someone wanting to replace dead bulb in a stacklight leans ladder on the machine and who knows what not. if accidental short to ground will not pull down power supply or trip breaker, that breaker or input are not wired correctly. it is amazing how many people are scared to see tripped breaker. if there is an accidental short, let the fuse blow and breaker trip, this is why they are used in the first place, not to make them look pretty or to throw money out of window. maybe this little article helps to clarify this: http://www.mrplc.com/kb/index.php?page=ind...;id=44&c=38 Europe and America use PNPs far more often than NPN, as a result devices with PNP outputs are often cheaper, you get better variety and less confussion for new guys troubleshooting. NPN sensors are common in Japan, some companies here use them too (Honda, Toyota etc.) and there are even safety light curtains with NPN outputs but... you don't use any of this on a 24V system that has negative side of the power supply grounded.

[Sleepy Wombat 27]

Panic, why is the positive grounded in the diagram ?

[Crossbow 295]

I had a customer using level probes and only wiring the probe back to the PLC. He tied the + to common and the - to ground, and when the probes contacted water, they grounded to the machine, completing the circuit. NPN is safer because there is no voltage on the external devices, the only voltage it between the power supply and the common terminal. It's just switching ground. But as mentioned by others, I haven't seen many people using NPN on this side of the ocean. Same applies on the output side. If the device has it's + supply and the PLC is switching the -, if the wire between the PLC and the device breaks and shorts to ground, the device will turn on. But if the PLC is providing the +, there won't be any power unless the PLC output is on.

[panic mode 246]

because it's grounded system and everything is NPN.

[BobLfoot 302]

OK I know I half slept thru EL 101 what with working 60 hours a week and all at the time, but can someone lay it all out clearly for me. I undersand NPN and PNP as relates to transistors, but the water gets muddy when the terminology is applied to sensors and PLC cards. For NPN which is grounded the 0V or 24V terminal ? I am thninking the 24V but might be wrong. For NPN what is a true case or logical 1 0V or 24V? I am thinking 0V but who knows for sure. For PNP which is grounded the 0V or 24V terminal ? I am thninking the 0V but might be wrong. For PNP what is a true case or logical 1 0V or 24V? I am thinking 24V but who knows for sure. And for the tie breaker which is sink and which is source as AB calls em?

[Ken Moore 23]

I'll probably get bashed for this, but I use the uneducated method. NPN, the last letter is N, so this device sends N for a true condition (0V). PNP, the last letter is P, so this device sends P for a true condition (24V) With some vendors, a Sinking I/O module requires Sinking (NPN) field devices. BUT... With AB a Sinking I/O module requires a Sourcing (PNP) field device. I don't know if this is still the case with the Logix platfrom, but can confirm it is true for the 5's and 500's.

[BobLfoot 302]

Skillfully dodging which gets grounded are we ken?

[TimWilborne 108]

That's how I was taught. NPN for NO Power, PNP for Power The thing I don't like about NPN in traditional wiring is if a cable is cut or a switch faults it to ground it will turn the input/output on. PNP would blow a fuse or circuit breaker which I see as better. Of course I had a rep of a company who product was strictly NPN argue that it was designed that way to keep from burning things up in the same scenario. I didn't buy his explaination.

[panic mode 246]

I think mitsubishi uses first (sinking inputs means "to be wired to sensors with sinking outputs") while pretty much everybody else uses oposite (sinking input means circuit inside card is sinking so sensors would have to source). But what ever the case, when I think about it, I don't like PNP/NPN any better than sink/source terminology. The only clear way to show how to use device is diagram, regardless if sensor or PLC I/O. The thing is that many "PNP" sensors don't have open collector PNP transistor as final switching element anymore, it may as well be MOSFET or NPN (usually with another few componentes to protect that output in case of a overload or short circuit). in case of PLC I/O, it is important to see on block diagram if the I/O are isolated or not, what is common, how are I/O grouped (multiple commons etc.) as for P and N meaning Power and No Power ....imho that is bad teacher. that may work for particular case where negative is reference (0V) but this is only one case and life is more complicated.

[BobLfoot 302]

I was curious because for 13 years I worked for an Asian-American concern and all our equipment was built overseas and came in with Omron, Toshiba, Panasonic and Mitsubishi controllers. On these they always either grounded 24V or nothing. And in one case they sent an entire machine with Red #12 ground wires. This to a facility in Upstate NY. Then I went to work for a Domestic concern and all we used was Rockwell and here 0V was always grounded. So you can understand my confusion a little.

[Wulfgar 5]

I'll add my opinion, this has always confused me as well especially with floating ground. From my experience there is no benifit either way if you use PNP ground the negative 0V giving +24VDC NPN ground the positive giving -24VDC This way you are always switching the hot wire and never the neutral. So a short to ground will not turn on the sensor or output. I've seen machines every which way; Honda is NPN with positive grounded. I've built machines with NPN inputs and PNP outputs and floating ground. This to me is confusing and I don't think having a floating ground is good but I not an expert on this. As far as sinking or sourcing, I find these terms confusing as it is relative to your reference point like left and right, so I never use them. Anybody know the pros and cons of using grounding the power supply verses floating? Edited 9 Nov 2006 by Wulfgar

[panic mode 246]

in my case everything is grounded and I never had any issue.