Thread: NO vs NC

  1. #1
    Hey everyone!

    Been trying to wrap my head around electonics for my controlcabinet.

    Cant really understand NO vs NC..

    I mean.. for proximity switches NO means there is always contact exept when triggered.. right?
    NC always closed exept when triggered?

    How about relays and contactors?
    Here is where I get confused
    To me NO relays should be closed exept when you power up the coil... then it should closed.. right?
    But it seems to be the other way around.. that the condition NO or NC talks about a powered up coil?

    Or am I still confused?

    Still working on the mechanical for my routerbuild so there is no hurry for me to complete the Electrics, but I would really like to understand what Im doing when I get there

    Oh! And one more question :) Where do you all buy cheap relays? And caps?
    Heard that I should stay away from ebay for these ones!

    Thanks
    The mad-one

  2. #2
    Ger21's Avatar
    Lives in Detroit, United States. Last Activity: 5 Hours Ago Has been a member for 6-7 years. Has a total post count of 603. Received thanks 83 times, giving thanks to others 0 times. Referred 1 members to the community.
    No.
    NO means that the circuit is open, and no electricity is flowing through it.
    NC means the circuit is closed (as in fully connected), and electricity is flowing through it.

    Say you have a normally open limit switch. Elctricity only flows through the circuit when the switch is triggered, closing the circuit. If the wire were to break, you'd never know it.
    That's why limit switches are normally set up as NC. So that a broken wire will open the circuit, and trigger the limit input.
    Gerry
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  3. #3
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 7 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,121. Received thanks 233 times, giving thanks to others 5 times.
    NO = Normally Open aka open circuit
    NC = Normally Closed aka circuit complete

    NO/NC is always described in the non actuated or unpowered state.

    As Ger has explained, for anything safety critical, you would normally use a NC circuit.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  4. #4
    That makes sense! I guess the problem is slightly language connected for me. I tend to think "contact" instead of circuit..

  5. #5
    Damnit!
    I still dont understand it.. I think its backwards!

  6. #6
    Quote Originally Posted by Nr1madman View Post
    Damnit!
    I still dont understand it.. I think its backwards!
    Doesn't matter really what you think the fact is only works one way.

    Always think in terms of NO power and open or closed states. So when says NO (normally open) the switch is open when no power is applied. Soon as power is applied it flips to opposite state.!

    Ok I think might figured whats confussing you.?

    Are you trying to figure out swithces or Relays.? Both work on same principle but do confuse people.

    When talking limits or E-stop best to think in terms of completing a Circuit. For the Circuit to be closed NC switches are required then in which case if switch is opened or wire borken the circuit goes open and controller knows about. For this to work you would set the Input active state to be Active low ie: if circuit goes low/open ie no voltage is seen the control goes into E-stop/fault mode.

    The opposite can be done with NO switch and Active high state. The input state will then go active when it see's voltage. Down side being if wire breaks the control never knows about it.
    NO sw are mostly used on things like Home Sw or ATC etc when you want to inform control something as happened or completed.

    Relays however while working the same do confuse people because they flip state only when power is applied so really you work kind of in reverse.
    Where you would use a relay is say with fault signal from the drives.
    In this case you would run say E-stop wire thru NC contacts on relay who's coil is connected to drive fault signal output. Then if the drive goes into fault it would turn ON the relay which in turn OPEN the NC contact and E-stops the machine.

    End of the day it's just Binary Logic states. ON/OFF OPEN/CLOSED LOW/HIGH all the same.
    Last edited by JAZZCNC; 19-02-2017 at 07:04 PM.

  7. #7
    Quote Originally Posted by JAZZCNC View Post
    Doesn't matter really what you think the fact is only works one way.
    Of course you are right :)
    Thanks for the explanation, I needed that and will come back to it to read it again!

    The way I think of a circuit is if it's open, then the current is free to run through. If it's closed its a broken component or disconnected switch or something :)

    As I said, language difficulties!

    Cheers

  8. #8
    Quote Originally Posted by Nr1madman View Post
    As I said, language difficulties!

    Cheers
    Would work the same if you came from Mars, so even in your native language electrical circuits work the same so it's not really language at all it's you who doesn't understand electrical cicuits. So go learn electrical Circuits in your native language if it helps.

  9. #9
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 11 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,156. Received thanks 209 times, giving thanks to others 5 times.
    Quote Originally Posted by Nr1madman View Post

    The way I think of a circuit is if it's open, then the current is free to run through. If it's closed its a broken component or disconnected switch or something :)

    As I said, language difficulties!

    Cheers
    What you say makes perfect sense - if you are thinking about a tap or valve controlling water! Tap closed - no flow. Tap open - flow. However, when you are talking about electrical contacts, you have to imagine the contacts themselves. "Closed" means the contacts are touching, so current flows. "Open" means that there is a gap between them so no current flows. When you hear open or closed referring to switches, visualise the contacts themselves and it might help. Same principle applies even when it's an electronic switch like a proximity switch - closed and current flows, open and current can't flow. Then, "normally closed" means that current will flow until the switch is operated. Same for a relay.

    Good luck!

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