Electronic – Can you clarify what an 1NO1NC switch is

switches

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?

Best Answer

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).