pnp and npn? dry and wet?

Hello, Can someone please explain in simple terms what is the tech diff btw npn and pnp DI channels? Is it the same as having dry or wet? (What is dry and wet anyway?) And also high side and low side...Do all these mean the same thing?

Thanks

No, all these do not mean the same thing.
There may be more than one explanation for wet/dry contacts but I will share what I am familiar with.
A dry set of contacts is what you normally have on your standard relay or auxiliary.
A wet set of contacts would be found in a mercury switch.

Sinking and sourcing are terms used to describe how an associated load (the device being turned on or off by the sensor) is powered in relation to the sensor. Sinking, the more widely used of the two, involves the switching of load current (power) supplied by a power source external to the sensor. In a sinking configuration, current passes through the load first, through the output switch of the sensor second and lastly to ground. A sensor with a sinking output switches the ground, or negative, leg of the circuit. In contrast, sourcing refers to a configuration in which the sensor is the source of the current required to power the load. In a sourcing situation, current supplied by the sensor passes through the load second and then to ground. A sensor with a sourcing output switches the positive leg of the circuit.

NPN transistor; You feed a little bit of current in the base leg and it turns on to let a lot of current flow through it.
PNP transistor; You to let a bit of current flow out of the base leg to make a lot of current flow through it.

There's something else you can do with NPN and PNP transistors. If a circuit only contains one type, you can replace it with the other type, providing you reverse the polarity of the supply.

Hope this helps
Roger

Thanks for Terry and others here is a good explanation
what is the differece between npn and pnp I/O Channels

http://www.plcs.net/dcforum/DCForumID1/2645.html
http://www.plctalk.net/qanda/showthread.php?s=&threadid=1706&highlight=npn+and+pnp

Good luck

Well, Roger ole pardner, I gots to disagree with the "wet" and "dry" explanations. This may or may not be the historical basis fer them terms, but nowadays wet and dry tends t' refer to the presence or absence of voltage on the contact. Leastways, thets whut I see in my industry and in most parts o' the US of A.

"Dry" contacts is a term very commonly used, and refers to a set of contacts where there is no voltage present at the device where the contacts are located. The voltage being switched is supplied by the sensing device, and is often 24 VDC.

A typical example of dry contacts is an alarm annunciator. A panel or device will have a number of contacts that change state on a fault condition, and each contact is un-powered and changes state for a different fault. The annunciator will have a 24 VDC power supply, and the "+" output is used as a common to all the fault contacts. Each contact is switching the voltage supplied by the annunciator. The annuciator then indicates which fault has occurred.

"Wet" contact is not a term used very often, for some reason. It is simply the opposite of a dry contact, in the sense that the voltage being switched is supplied by the device where the contact is located. In the above example wet contacts for the faults would be powered by the panel and used to trip relays or lights at the annunciator. The disadvantage to wet contacts for remote devices is that coordination of voltage ratings etc. is required, and commons and neutrals have to be watched. Most contacts switching devices inside their own panel are usually wet contacts.

maybe a picture will help

Yo, neo,

Here's a little schematic taken from an Allen-Bradley sensor catalog - it represents the NPN and the PNP transistors as if they were simply switches - which is basically how they function. The "load" in the schematic is actually a PLC input - which acts as the LOAD in the circuit when seen from the sensor's point of view. Most people find these connections easier to remember once they notice that the "N"pn type sensor switches the "N"egative side of the supply - and the "P"np type sensor switches the "P"ostive side of the supply.

[attachment]

Important: If the boss hands you an NPN (also known as a "sinking" type sensor) to install, you must connect it to a "sourcing" type input module in order for it to work correctly. Conversely, a PNP (also known as a “sourcing” type sensor) must be connected to a “sinking” type input module.

The other responders should have you straightened out on the “wet/dry” contact issue. I’ll just add: most guys use the term “dry” contacts to refer to “stand-alone” contacts - for example: like those on a standard relay. In other words, if the contacts are “dry” then YOU (the electrician) “own” both sides of the switch. You can hook up any voltage your little heart desires to one side - and that same voltage will come out the other side whenever the switch is closed. In that sense, neither the NPN nor the PNP sensors shown above have “dry” contacts. Specifically, since they’re not REALLY switches after all - then the type of voltage that you’re going to get out is dictated by the type of sensor you’ve bought. Incidentally, I’ve personally never heard anyone refer to a set of contacts as “wet” as opposed to “dry” - but then I don’t get around as much as these other guys do.

Finally, there used to be - an maybe there still are - contacts available which were “wetted” with mercury and then sealed in a little glass tube. They were used for light (as opposed to heavy) loads in situations where a normal contact might corrode or oxidize if it hadn’t been used in a lengthy amount of time. The mercury wouldn’t corrode and so it made excellent electrical contact. Obviously Roger has run into these before (see post #2 above).

Tom/Ron
Thanks, your posts about Wet/Dry contacts are absolutely correct. The AUX contacts I mentioned were more along the line of what you are referring to, on the other hand I seemed to give the impression that the presents or absence of mercury made the difference between wet and dry. I guess in some conversations that may be true just like the definition of Wet/Dry on a sensing device I installed, their definition was (Wet contact = When the device was immersed in oil) and (Dry contact = When the device was not in the oil.) The makers of the sensing device are guilty of poor choice of words to describe the use of their device, as was I by using a mercury switch as an example.

Thanks again
Roger

I can see a case for both scenarios on the 'wet' contacts (which by the way, I have never heard of)

But, I certainly have heard of 'dry' contacts and this always means (in UK) voltage free contacts. These are usually for the end user to interface with.

My 2 cents

Roger, stick with your original definition.
Dry - stock standard contacts
Wet - mercury-wetted to reduce arcing

While I don't dispute what may be common (mis)use of the terms, there are better descriptions, e.g. isolated, voltage-free.

Gerry

Thanks.

I think the proper term may be wetting

Tell the truth I have never heard the use of wet contacts but dry is common and I think applies to mechanical type relays. I dont think I have ever heard an electronic type device, photoeye, prox, etc be called a dry contact. I would say that Roger's explanation makes sense in it's own fashion. There are switches that actually use liquids as part of there conduction ckt like the mercury switch or some level switches.

I have heard the term "wetting current". With many contact type relays if the current is too small then the contacts will build resistance, yes the idea is to reduce arcing but a small amount of periodic arcing can actually be good for the switch contacts, because it keeps the contact faces free from small amounts of dirt and corrosion. If a mechanical switch contact is operated with too little current, the contacts will tend to accumulate excessive resistance and may fail prematurely! This minimum amount of electric current necessary to keep a mechanical switch contact in good health is called the wetting current.

Thats why when dealing with very small current applications its recommended to use Gold contacts or bifurcated gold.

Dry vs Wet

The correct terms for contacts are dry and wetted. These terms are alive and well in the US electric power industry. Most other industries call them "isolated" and "non-isolated".

Dry contacts are voltage free, you get both sides of the contact for use in your control circuit. A PLC example is an isolated or relay output. A standard pushbutton is also "dry".

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Wetted contacts are connected to a common voltage source, you only get one side of the contact for use in your control circuit. A PLC example is a standard output module with 1 common for several outputs. All NPN/PNP sensors are "wetted".

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thanks...

Thank you guys...

What will the automation world do without you guys....sigh...

do u know anything about future trends in plc's?

Re: Dry vs Wet

Dou know anything about future trends in PLC's

02bnden04,

maybe. What do you want to know?

Jean Pierre