Ladder Logic Beginner (need help with a question)

[Hassman 0]

Hey all I'm starting right from scratch, and would really appreciate some help. I have a question straight from my textbook that we are taking up tomorrow, ive been rattling my brain but i don't know if i am designing the ladder correctly The question says: use NEMA symbols and design a ladder diagram that has an amber pilot light (PL3) on when NO pushbutton PB5 is momentarily pressed and remains on until NC pushbutton PB6 is pressed. I have attached a word document with my feeble attempt to draw a ladder diagram, if this is correct please let me know, and if not can you explain why and help me out??? ladder diagram PDF.pdf Edited 1 Feb 2013 by Hassman

[Mark- 17]

Hello, I must be missing something because I do not see the attachment.

[Joe E. 259]

Then I'm missing it too...

[Crossbow 297]

2 things. 1. Those are electrical symbols, not ladder logic symbols. 2. It wants the lamp to stay on after you release the momentary, so you need to write a latch around PB5 with PL3.

[Hassman 0]

Hey crossbow thanks for your reply! They aren't ladder logic symbols?? Sorry...I have been taught to draw ladder diagrams with those symbols, i guess ladder diagrams are different than ladder logic? Ok, so with that in mind, how does a latch work?? Im not sure, how do we draw one out and how do you write a latch around a pushbutton and pilotlight?

[OkiePC 14]

You need the relay to stay on when the button is released. This is a very common thing you will see in both hard wired controls and PLC code. Edited 1 Feb 2013 by OkiePC

[Hassman 0]

Hey okiepc thank you very much for the diagram, and as I'm looking at it I'm wondering, why do i need the relay contact beneath PB5? Im sure i'm missing something but... If PB5 is NO and then i press it, doesn't it just allow current to pass through the circuit? why do i need the symbol for a contact beneath it? What does that do exactly?

[plcdp 11]

Yes when you push the push button the current flows through but think about what happens when you let go of the push button. By using the contact to seal around the push button the path stays closed until the n.c. stop button opens the circuit up when pushed. Hope this helps, Dave Edited 1 Feb 2013 by plcdp

[OkiePC 14]

Yes. plcdp nailed it. Have you handled real push buttons? I highly recommend you get familiar with them. A momentary push button has a spring that pushes the contact back open as soon as you lift your finger, so you wouldn't meet the "and remains on" part of the assignment, because the relay A would drop out. By the way, you need to add a branch for your amber pilot light which should look like the one in the middle right of this chart. http://www.industria.../image/15-5.jpg I read your A in a circle as the armature of a relay or relay coil. Edited 2 Feb 2013 by OkiePC

[Hassman 0]

Ok so let me this straight... I approach this set of push buttons and press PB5 (momentary push button) closing the circuit and passing current through. However, since it normally releases the second i let go, i add a normally open contact, which upon being energized closes as a result of the momentary push button being pressed and now in a sense "locks in" the current in the circuit until i choose to press PB6 (normally closed push button) Is that correct? Yeah, I play around with push buttons in my lab every monday , but i don't really get a chance to just play with them since we always have an assignment or something. It would be nice to get some free time just to play around with it and see how it all works. I should drop in next week and see if I can do just that! Thanks again for your help guys, really appreciate it!

[Crossbow 297]

To clarify my statement before, in the picture above, the contact marked A is the standard ladder logic symbol which I was referring to. PB5 should also use that symbol, and PB6 that symbol with a slash through it. Those are ladder logic symbols. The symbols you drew for your switches are the symbols you would use on an electrical schematic, not a ladder logic program. PLC programming software does not use those symbols.