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Thread: Programmable Logic Controllers

  1. #11
    Join Date
    Apr 2008
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    Ann Arbor, Michigan
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    6,349
    171101-0636 EDT

    Continuing from last night.

    IF I have a simple relay circuit with one normally open pushbutton spring return switch contact in series with a relay coil and a source of power, then whenever the pushbutton is pressed in the relay coil will be energized and a normally open relay output contact will close. The output contact will only be closed while the input switch is closed (held in).

    Call the input button A and the relay output D, then D = A.

    Take the normally open relay contact, D, and put it in parallel with the input contact A. Before power is applied to this circuit A = 0, and D = 0, because D is de-energized.

    Apply power to the circuit, and D is still equal to 0, because both A and D are open, and there is no power path to the coil of D.

    Next momentarily press the button A and this energizes the relay closing contact D. The normally open, but now closed D now provides a parallel path of power to coil D around the A contact. Releasing A to its normally open state does not drop out the relay D because the D contact maintains power to the relay coil. The relay D will never drop out until power is some how removed.

    Add a normally closed pushbutton C in series with the D contact. So long as C is not pressed in, asserted, the above relay circuit will remain energized once initially energized.

    If A is not held in, then asserting C, opening the C contact will de-energize the relay D, and power to the whole circuit did not have to be removed to drop out the relay. Releasing normally closed pushbutton C does not reclose D because D is now open, and A is also open.

    This is a simple two pushbutton relay control circuit.

    .
    Last edited by gar; 11-01-17 at 08:22 AM.

  2. #12
    Join Date
    Dec 2006
    Posts
    108
    Quote Originally Posted by revlution4 View Post
    Thank you all for the replies, I will do some more research into the various items. I do understand some basic ladder logic, I just need to expand on it more and implement it.
    Don't know if you had a chance to visit the link I provided but if funds are not available right now I suggest you take a look at Ron's FREE stuff. He has some sample lessons, quizzes and links to his YouTube series that are very informative. Just click the "Sample Lessons & Videos" link on his main page.

    Good luck!

  3. #13
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    15,657
    many community colleges have PLC classes. usually not for credit but they are usually inexpensive and pretty good.

    relay ladder logic is the common language for most PLCs. It is not exactly the same from model to model but is pretty close. learn it once and it readily transfers to other brands.

    buy something cheap to practice on. you can get a clic from automation direct for $100 with free software.

    even ab has small plcs with free software these days.
    Bob

  4. #14
    Join Date
    Sep 2010
    Posts
    1
    In college we used a computer simulation program based on Allen Bradley's RSLogix 500 called "LogixPro 500" for learning to program PLCs. You can download the program from "http://thelearningpit.com/lp/logixpro.html" and use it for free for 15 days to try it out.

  5. #15
    Join Date
    Mar 2003
    Location
    Washington
    Posts
    9

    My approach to learning PLC programming

    Back in the 90's i bought a used Allen Bradley Micrologix 1000 from EBAY for $75.00 and then downloaded the free RSLogix lite and Linx from the AB website. I then built a small trainer with input switches and output LED's and I was off to the races. My company sent me to the AB programming classes later and with a lot of help from our company engineers I was able to become proficient in the AB software. Network proficiency definitely will help.

  6. #16
    Join Date
    Feb 2003
    Location
    Bremerton, Washington
    Posts
    7,637
    Quote Originally Posted by b1miller View Post
    Back in the 90's i bought a used Allen Bradley Micrologix 1000 from EBAY for $75.00 and then downloaded the free RSLogix lite and Linx from the AB website. I then built a small trainer with input switches and output LED's and I was off to the races. My company sent me to the AB programming classes later and with a lot of help from our company engineers I was able to become proficient in the AB software. Network proficiency definitely will help.
    That's kind of what I suggested. But get the MicroMentor book has practice exercises
    Moderator-Washington State
    Ancora Imparo

  7. #17
    Join Date
    Oct 2014
    Location
    Connecticut
    Posts
    26

    PLC

    Quote Originally Posted by revlution4 View Post
    Thank you all for the replies, I will do some more research into the various items. I do understand some basic ladder logic, I just need to expand on it more and implement it.
    You can find inexpensive plc controllers out there, a free software download of a PLC emulator would be good ( at this learning level, scan (PLC) versus deterministic (PC) should not be an issue, so a PLC emulator running on a PC, should be a good free learning tool) In essence basic ladder logic, is relay logic where the output coil would be the same as a coil on a relay, and the contacts would follow the state of the relay. 2 things you may want to consider, 1) each vendor has its own programming software used to program the ladder logic, typically unique to its product, 2) Some PLC'S have several programming languages, with ladder logic being one of the easiest. Are you looking to design systems, or an understanding of how to install, startup and maintain , as these would be 2 different directions. I was a corporate trainer for PLC and other industrial control systems for over 10 years ,training electricians and engineers ,and happy to provide insight.

  8. #18
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    6,961
    You can buy a very basic small PLC from A-B, a Micro810, for under $100, and the software can be downloaded for free. It's only a few I/O but it makes for a great self-trainer and once you learn the A-B programming techniques, you can expand your knowledge from there fairly easily. A-B supplies roughly 45% of all of the PLCs used in North America, so learning their system makes you valuable wherever you go.

    If you want to go overseas, then learn Siemens.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

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