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Join Date: Mar 2009 Location: houston Posts: 25 |
hii,i am new to plcs and need to know about NC and NO. when power is supplied through both NO contact,it closes and powre flows through the rest ofthe circuit. when a NC contact is used,it must open and power should not flow,but in case of a stop switch,this doesnot happen i mean when using a start and stop in a ladder program,the normally open strt switch closes and motor starts and if this happpens,then the normally closed stop switch must open and power should not flow...but this doesnt happen i hope a few of them understaand what i meant |
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March 2nd, 2009, 07:10 PM | #2 |
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Join Date: Nov 2004 Location: Seattle Wa. USA Posts: 3,680 | Yes I understand fully. This is the reason that when I tried to learn at home I decided I had an incompetent for an instructor and replaced him (me) with a qualified instructor. Was much better off. Here is what I did to understand (in class) what is going on here. Wire a NC to input O configure as NC The output is 0 ------ NC to input 1 ------------ NO ------------- 1 -------NO -------- 2 ------------ NO ------------- 2 -------NO -------- 3 ------------ NC ------------- 3 Wire one light to each of the outputs 0, 1, 2, 3. Now predict what will happen when you have it deenergized will any lite energize will any lites lite? What will the corresponding light do when you press the button (ie if switch is NC on input 0) what will lite on output 0 do? Etc etc Do this on paper and then go ahead and energize and test your predictions? Dan Bentler Last edited by leitmotif; March 2nd, 2009 at 07:14 PM. |
March 2nd, 2009, 07:42 PM | #3 | |
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-MUR | |
March 2nd, 2009, 08:38 PM | #4 |
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Join Date: Apr 2002 Location: Yakima, Washington Posts: 6,386 | I too - like Brijm - name our inputs according to the condition which causes the input of the PLC to turn on (eg. 'Start' or 'Not Stop'). This though as he pointed out, can lead to some confusing double negative concepts. Let's examine your start circuit. The 'Start' pushbutton is wired Normally Open, it requires a press to close it allowing current to flow to the input pin turning it on. The 'Stop' pushbutton is wired Normally Closed. It is normally, without being pressed, passing current to the input turning it on. It requires a press to turn the input off. Why wire a Stop button Normally Closed? This is for safety. If a wire should come lose or break from the Stop button, a Normally Open method would provide no way to stop the process. A Normally Closed, by its very nature, would automatically stop the process in this case. Note: Stopping is usually a safer outcome than continuing to run, or worse, starting unexpectedly. By naming the inputs by the condition which turns them on it removes from consideration the nature of the sensing switch, though it may introduce, as noted above, some interesting double negatives (Not Not Stop). But safety should always win out over easy readibility. Use extended comments as needed to make the condition clear to future programmers on the system (which includes yourself a few weeks down the road.) If your particular system is not behaving as you expect, give us more information on the type of wiring, the type of PLC, and a portion of the ladder which deals with these inputs. __________________ |
March 3rd, 2009, 12:01 AM | #5 |
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Join Date: Mar 2009 Location: houston Posts: 25 |
thank you every body... i'd like to ask mr.bernie carlton..(others are welcome to answer) sir,i understand the concept of using NC for stop switch. lets take a case,in a ladder program the stop switch is place ahead of the start switch,and lets say there is a motor connected after the strt switch, so in a ladder program...the logic starts from left to right.in this case the stop switch which is normally closed should turn open and the NO start switch should close...the result..no complete path and motor shouldnot start.but the motor starts..how is the circuit complete with a contact open
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March 3rd, 2009, 12:14 AM | #6 |
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Join Date: Mar 2009 Location: houston Posts: 25 | I-------------I I------------( )----------I I NO lamp1 I I I I--------------I/I------------()------- I NC lamp2 if this is the circuit,the normally open contact closes and lamp1 glows the logic comes to rung 2 and the normally closed contact opens and lamp2 doesnot glow... is this true? |
March 3rd, 2009, 01:00 AM | #7 |
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Join Date: Apr 2002 Location: Yakima, Washington Posts: 6,386 | If the contacts represent the same input then the top rung would energize the output when the input has power applied, the second rung would not energize its output. When the input is not energized then the state of the outputs would be opposite. You labeled the outputs with different names. This implies that these are separate outputs. This is good as ladder logic with identical outputs tends to obey the state of the last rung. Here is a typical start/stop/latch rung using a NO wired start and a NC wired stop on a single rung NO Start NC Stop Output ---+---| |-------+--------| |----------------( ) | | | Output | +---| |-------+ If the Start button is pressed and the Stop button is not pressed then power passes to the output. During the very first evaluation of the rung (scan) which turns on 'Output' the contact for 'Output' is not closed yet but that doesn't matter. On the scans after 'Output' turned on a path is present through the contact of 'Output', retaining the coil even though the Start pushbutton may be released. Once the 'Stop pushbutton is pressed the path to the coil is broken and 'Output' turns off. This diagram ensures that the 'Output' will be off if the Stop pushbutton is pressed even if the Start pushbutton were to be pressed at the same time. Here's a test - you could have a type of circuit where, if the Start AND Stop conditions were true at the same time that you wanted the 'Output' to turn on. Where would you place the Stop contact such that it would turn off the 'Output' only if the Start condition was not also true? __________________ |
March 3rd, 2009, 12:03 PM | #8 | ||
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March 3rd, 2009, 02:10 PM | #9 |
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Join Date: Mar 2009 Location: houston Posts: 25 | what i have understood is,the circuit cheks for the NO contact to be on,if its not then outoput is 0.it hceks for the NC contact and if off output is 1.so when circiut is powered the NO contact closes and output changes to 1.the NC ontact which was in off state turns 0. |
March 3rd, 2009, 02:22 PM | #10 |
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Join Date: Apr 2002 Location: Yakima, Washington Posts: 6,386 | Can you redraw the ladder diagram which has you concerned? Please use Notepad to make the drawing using characters like dash and vertical elements like my post. Make sure to clearly identify all contacts and outputs. Leave Notepad open. Then begin a post. Click the ladder contact ( -||- ) symbol. You will get a pair of bracketed sections with the word LADDER in them. Click between them and press ENTER a few times to get some space between the two bracketed sections. Now, go to your Notepad. Select and Copy your drawn ladder. Come back to your post. Click an area between the bracketed LADDER sections. paste your ladder there. Please make any comments outside of the posted ladder. This may make your question clearer. __________________ |
March 3rd, 2009, 02:25 PM | #11 | |
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Join Date: Nov 2004 Location: Seattle Wa. USA Posts: 3,680 |
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Do you have access to a PLC to do the modeling above? | |
March 3rd, 2009, 02:40 PM | #12 |
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Join Date: Mar 2009 Location: houston Posts: 25 | i cant access any plc rite now,but ur reply has made me confused again.. |
March 3rd, 2009, 02:59 PM | #13 |
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Join Date: Apr 2002 Location: Yakima, Washington Posts: 6,386 | Have you tried the PLC tutorial available on this site? Click the 'Learn PLCs' at the top of this page. That course may help in clearing up your confusion. __________________ |
March 3rd, 2009, 03:39 PM | #14 |
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Join Date: Mar 2009 Location: houston Posts: 25 |
I-------o 0------0 0------( M )-----]/[------ I stop I start I OL I I I I I----][------I I I M lamp 1 I-----]/[-----------------------( )---------------- I I I M lamp 2 I-----][-------------------------( )-------------- when processor is placed in run mode,the start swithc is open,hence it closes..this is true at the same time the stop switch which is closed should open,but it remains closed.how can this be.if open switch closes,then the closed switch should open,but this doesnot happen...on the other hand,the NC on rung 1 turns the light off and the NO on rung 2turns the light on. so when its true in case of lights and the start switch,y not in case of stop switch this might sound stupid,but it really doesnt get into my brainnnnnnnn |
March 3rd, 2009, 03:41 PM | #15 |
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Join Date: Mar 2009 Location: houston Posts: 25 | i messed up the drawng,sorry rung 0 contains,stop,start,motor,overload rung 1 M and lamp1 rung 2 M and lamp2 |