normally closed and normally open

[jhonnygaddar]

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

 

[leitmotif]

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

 

[Brijm]

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

Normally Open and Normally closed can be confusing in a PLC. Allen Bradley uses the "Examine if closed" (XIC) and "Examine if open" (XIO) instructions. Many other PLC's use "Load" (LD) and "Load inverse" (LDI), represented by the normally open and normally closed contacts respectivly in the ladder.

Remember, that a PLC, loads descrete inputs into registers, which is either a logic "1" or "0". An "XIC" or "LD" contact (normally open symbol) is true, when there is a "1" in the addressed register. "XIO" and "LDI" contacts (normally closed symbol) is true, when there is a "0" in the addressed register.

This can become confusing in the case of an Stop pushbutton, when an actual Stop condition is logic "0" in the register. In a motor control circuit, a stop button is wired through the N.C. contacts (shown on the print). However, when the logic is transfered to a PLC, the stop button is still wired normally closed (loss of power = stop condition), but the logic in the PLC is a Normally open symbol. (Logic 1 lets the output be true)

I'll often try to label the registers in my PLC programs to reflect the N.C. nature of an input for clarity. Therefore the true condition (1) reflects the comment. For example a low pressure switch, which is pressure to close (Logic 0 = low pressure stop running). I would label this "Not Low Pressure" so that when I am reading the logic, a -| |- I would read if "Not low pressure" and -|/|- would be if Not "Not low Pressue" In other cases, I may simply put a comment, that it is a N.C. contact. Another way would be to label it "Above low Pressure"

The key with this is to be consistant... I have seen some programs, in which registers are labeled both ways, irreguardless of the inputs wiring, and it becomes very difficult to keep track of what a logic 1 or a logic 0 acutally means.

-MUR

 

[bernie_carlton]

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.

 

[jhonnygaddar]

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

 

[jhonnygaddar]

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?

 

[bernie_carlton]

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?

 

[Brijm]

thank you every body...

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..

Johnny,

You say that the stop switch should turn open, and that the start switch should close. Are you pressing both buttons at the same time?

how is the circuit complete with a contact open

Think about what the logical value of your inputs are when in their stable state (nothing pressed as shown on your print)

Now go to your Ladder Logic. The logic doesn't know anything about how you have your pushbuttons wired... all it knows is 0 or 1. -| |- is true if the address is equal to 1. -|\|- is true if the address is equal to 0.

-MUR

 

[jhonnygaddar]

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.
i am still confused,i know i have understood it,but i feel i am wrong!!

 

[bernie_carlton]

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.

 

[leitmotif]

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

You can read all the books you want and take lots of time and maybe understand this. I tried that and all I did was get deeper in mud of confusion and ignorance.

You can also model it as I showed you and see it for yourself.
That is what worked for me.

Do you have access to a PLC to do the modeling above?

 

[jhonnygaddar]

i cant access any plc rite now,but ur reply has made me confused again..
i'l try to paste my circuit

 

[bernie_carlton]

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.

 

[jhonnygaddar]

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

 

[jhonnygaddar]

i messed up the drawng,sorry
rung 0 contains,stop,start,motor,overload
rung 1 M and lamp1
rung 2 M and lamp2