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I've come across multiple mentions of switches described as 1NO1NC.

They often described as having 2 options: ON/(OFF) and (ON)/OFF and from what I gather they have 3 terminals for wires: a NO terminal, a NC terminal and a C (?) terminal.

Can you explain what these mean?

When would you use an NO or an NC switch?

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NO = Normally open (open = open circuit = not creating a path for the current)
NC = Normally closed (closed = short circuit = creating a path for the current)
C = Common

Schematic

(The drawings show the state in the absence of force.)

When you press a normally-open pushbutton, you provide a path for the current.
When you press a normally-closed pushbutton, you impede the current from flowing.

In a relay, when the coil is not energized, C and NC are connected. When the coil is energized, the magnetic field attracts the movable metal, and C and NO are connected instead.

Uses:

  • NC pushbuttons are used in emergency stop buttons. You press them when an accident has occurred, or may occur, and you need to immediately stop some machine, whose action could damage someone or something. Normally-closed buttons are preferred for two reasons:
    a) they don't rely on creating a good contact, to signal something. They just have to open a circuit, which is much easier. An NC is more robust and therefore safer.
    b) they react quicker. For an NO button, the signal event happens at the end of the movement (when the movable part makes contact). For an NC button, the signal event happens at the beginning of the movement (when the movable part stops making contact).
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  • \$\begingroup\$ Thanks for the clear explanations, that's awesome. Also nice uses examples \$\endgroup\$ – Ben Apr 30 '12 at 23:03
  • \$\begingroup\$ oops, forgot I hadn't accepted an answer here. All great answers. I'll go with the people's choice :) \$\endgroup\$ – Ben Feb 28 '13 at 11:47
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NO is normally open i.e. the contacts are normally open and close when the switch is actuated.

NC is normally closed i.e. the contacts are normally closed and open when the switch is actuated.

1NO1NC is generally used to describe contactors (industrial power relays) and manual switches like emergency stop buttons. It means that there is one pair of normally closed and one pair of normally open contacts with their own terminals i.e. there will be four terminals.

The switch you describe with 3 terminals would normally be called SPDT or single pole, double throw.

Generally normally open contacts are used in most electronics especially for momentary contact push buttons- it is more intuitive to work with switches like this.

You will find normally closed contacts in a lot of industrial applications as logic is entirely implemented by switches and contactors. The NC contacts are used on an emergency stop button to interrupt the current holding the contactor coil energised, for example.

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  • \$\begingroup\$ Thanks to you too. Great reference on the SPDT terminology \$\endgroup\$ – Ben Apr 30 '12 at 23:07
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The abbreviation "NO" means "Normally Open". The abbreviation "NC" means "Normally Closed".

These switches are usually of a momentary type rather than latching. "Momentary" means that they're only on while you're pushing them like a key on your keyboard, rather than latching, which means that they stay where you put them like a light switch. This might be a spring-loaded toggle switch, or they might be a pushbutton. It may also be a pair of switches or contactors sharing a single activation lever.

"C" almost certainly stands for "Common", which would be the terminal that is shared between both switches. When the switch body itself has a common terminal, that's usually known as an"SPDT" or "Single-pole Double-Throw" switch. The single pole is the common terminal, and the switch can be "thrown" in either direction.

Here's a simple schematic diagram of a 1NC1NO switch. Imagine that the <--- is a spring pulling the center lever to the left:

    <---|
        |
NC-|    |     |-NO
   |----|--|  
   |    O     |
C1-|          |-C2 

In this default setup, the left-side terminal "NC" is closed; it is connected to C1. There is a break in the "NO" switch; it is open or not connected to C2. When actuated, the situation changes:

    <- - - |
           |
NC-|       |  |-NO
      |----|--|  
   |       O  |
C1-|          |-C2 

Now the "NC" terminal is an open, while the "NO" terminal is closed. When released, it will spring back to the left and be in the original configuration.

You could use an individual NC or NO switch in many situations. For example, a reset button is commonly used for a reset circuit:

enter image description here

Normally, the resistor R1 keeps the output wire high. When the button is pressed, the line is pulled to ground and the output is low. You could reverse the positions and use an NC button, but that would draw power continuously through the resistor.

One application for a 1NC1NO combination switch is in emergency shut-off devices, like the 'red mushrooms' common on industrial equipment. By using both switch types, the system can go into a shut-down state in most failure modes:

  • The button is pressed: NC becomes open and NO becomes closed.
  • The cable is severed cleanly: NO remains open (because the wires are disconnected) but NC becomes open.
  • The cable is severed and shorted out: NC remains closed but NO becomes closed.

This type of switch is commonly available with the mushroom and each switch available as separate parts, later assembled like this:

abb emo

The three units at the back of the switch are individual units with their own actuators.

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  • \$\begingroup\$ @KevinVermeer I don't think that latching/non-latching is the distinction between 1NC1NO and SPDT. Microswitches with 3 contacts are always described as SPDT, whereas industrial switches and contactors are always 1NO1NC or 1NO2NC etc. I can't find any examples of a correctly described SPDT as 1NO1NC, only badly described switches on ebay. \$\endgroup\$ – Cybergibbons Apr 30 '12 at 14:11
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    \$\begingroup\$ @KevinVermeer: I would expect "1NO1NC" to describe a switch with two pairs of terminals, with the terminals of the first pair being shorted when the switch is "pushed", the terminals of the second pair being shorted when the switch is not, and neither of the terminals in the first pair being connected to either terminal of the second pair in any situation. It seems odd to apply such terminology to a 3-terminal device, which would more typically be described as "SPDT". \$\endgroup\$ – supercat Apr 30 '12 at 15:20
  • \$\begingroup\$ Thanks for the clarifications on SPDT vs 1NO1NC. I've learned something today and edited accordingly. \$\endgroup\$ – Kevin Vermeer Apr 30 '12 at 15:57
  • \$\begingroup\$ @KevinVermeer Thanks for your input too. Nice illustrations too :) \$\endgroup\$ – Ben Apr 30 '12 at 23:10

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