Learning about plc's

Transducers connected to PLC'S generally provide either a 0 -10v dc signal or a 4 - 20 mA signal.
Can someone explain why these signal levels are used.

I don't know exactly who and when came up with it but
this is the standard so we all have to live with it.
If I get to choose, I take the 4-20mA because it can be
used on long cables as well and it can be used to
detect if wiring is ok.

So, the HNC course now includes analog signals! You should add 0-5 VDC to your list of common signals. The 0-5 VDC signals were used because 5 VDC is a common rating for transistors and electronic devices. I suspect that signal level traces back to the voltages Morse picked out for the telegraph, but I can't prove it.

Pneumatic control signals pre-date electronic ones, and they were typically 3-15 psig. The 20% offset (3 psig) is a "live zero" and was used to distinguish between a 0 process value (3 psi) and a failed transmitter (0 psi). When current signals were adopted to transmit analog signals that were more immune to noise than voltage signals the 20% live zero was maintained. By using 20 mA for 100% a reasonable signal strength and power consumption resulted, and a conveient 250 Ohm resistor could convert the 20 mA to 5VDC for compatibility with existing devices. The result was a 4-20 mA current signal.

0-10vdc signal levels

yes steven , we can clearly see here that ur trying to rip of the system, so please keep your hnc questions off here as we people only love plcs, not college students!!!! people lets revolt against this junior frausdster!!!!!!!!!!!!!!!!!!!!!!!!

I agree. lets make sure that these <CENSORED> research their own assignments and dont rely on impossing on the good will of others.
*DO YOUR OWN WORK*

WOW, this from fellow countryman and an American. I disagree, a student is a person attempting to learn, the aforementioned HNC course has not shown that it is a good curriculum (outdated) and those claiming to be Instructors have not shown much.

I know a person online from "over the pond" that I consider a friend and hope to one day meet. This friend went thru the HNC course and is very knowledgeable about industrial electrical and plc's in general.

Asking a question about a subject like 1-10vdc/4-20ma is a legitimate question for this site. The way the question was asked was acceptable, a simple explanation of how/why they were used. I have asked questions far more simple than that...and I cant be nor never will be an HNC student.

I am well known for my controversial nature. I think that in the case of HNC and all students there is information that is excluded from the material given them. I also believe that the missing or excluded material is simplistic and should be freely offered. I will include most of this simple information at another location in the near future.

IMHO it is not time nor will it ever be time to "rally" against anyone, especially students. If you are willing to freely participate in a forum then you should offer information pertaining to any subject you can.

NOTE: I do not advocate you give them the code for any program they have been assigned to do. I see no harm in explaining how a TON works, how 0-10vdc/4-20ma works OR came to be, what is unitary, modular and rack mount...the list goes on.

Dont condemn a person for wanting to learn. If you must condemn "something", condemn those instructors for not teaching how to ask a question.

I also agree that some students are being lazy at times and this site has seen many of those. I also think that if the animosity/criticism was toned down a little they may stay and eventually become an asset.

Those that dont would never have anyway and would never be either.

BTW: This the best you, surgenor5555 and davidjw, could offer as your first post?

A few responses here amazed me. Here is why...I have been poking around here for awhile now and have seen quite a few students questions. Many have been lazy questions, but this one to me is not. And it definetly deals with PLCs. He isn't asking for some one to do his work for him, just a why question. How many people here have posted messages pertaining to a project they are working on? What is more morally acceptable? To ask questions to gain knowledge or to ask questions for your project(that you are getting paid for)? Seems that quite a few people around here use these forums to get answers for projects(me included). Try to play nice.

Dale

I personally gained from the constructive answers to Stevens post and learned a little more why 4--20ma signals have been my preferred option.(Thanks for your background on 4-20ma singnals Tom)
Although I can see how a Student could take advantage of the forum members I think this needs to be balanced with an obligation to pass on Knowledge to the younger members. I am in complete agreeance with srdoran