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Normally Open and Normall Closed in Push Button Switches

2020-05-19

Normally Open and Normally Closed in Push Button Switches

What is open and closed? Before we get into what normally open and normally closed are, let's clarify what "open" and "closed" mean. As with so many topics that we try to simplify by associating with something we are familiar with, associating electrical current flow with water flow causes a tremendous amount of misunderstandings. Whereas, we open a water faucet to start water flow, we close an electrical contact to start current flow, and instead of closing a water faucet to stop water flow, we open an electrical contact to stop current flow.

IMPORTANT CONCEPT:

  • Closed = Current flow
  • Open = No current flow


What is normally?
This is simply the state that the contact is in when something else is not affecting it. If it is a switch, then it is off.

Normally open - Is a contact that does not flow current in its normal state. Energizing it and switching it on will close the contact, causing it to allow current flow.


Normally closed - Is a contact that flows current in its normal state. Energizing it and switching it on will open the contact, causing it to not allow current flow.

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Example: Now let's go through a real-world example of normally open and normally closed contacts that we are all familiar with. A 3 way switch is an excellent example of using normally open and normally closed Stainless Steel Push Button Switch. And maybe we'll clear up a few things for the household electrician or do it yourself. First let's break it down into the basics.

  • Why do we use 3 ways switches? 3 ways switches are used to turn on a light from two different locations.
  • What if I need to turn a light on from more than two locations? Then you use a combination of 3 way and 4 way switches.
  • What is a 3 way switch? A three way switch is simply a normally open and normally closed switch in which one side of each contact is tied together. In an industrial environment we call this a single pole double throw (SPDT) switch.
  • What is a single pole double throw (SPDT) switch? It has three terminals. A common (COM) which is the left terminal in the image below, a normally closed (NC) terminal which is the top right terminal in the image below, and a normally open (NO) terminal which is bottom right terminal in the image below.
  • Why isn't a 3 way switch called a single pole double throw (SPDT) switch? I don't have a good reason for it. There are lots of speculations about it but it was probably someone trying to over simplify electrical terms. If you know what a SPDT swtich is then you will immediately understand what a three way switch is and the principles of it. If you know what a 3 way switch is then you don't necessarily know what a SPDT switch is or the principles of a 3 ways switch.

spdt.png

  • What is a 4 way switch? A 4 way switch is a double pole double throw (DPDT) switch with a few jumpers preinstalled. In the image below, the jumpers that are added to make a DPDT switch a 4 way switch are shown in red.
  • What is a double pole double throw (DPDT) switch? It is two single pole double throw (SPDT) switches mechanically linked together.

dpdt.png

  • How do you wire a "3 way" circuit or two 3 way switches? Note that this only works with two switches. If you need more than two switches then see the 4 way example further down.

3waycircuit.png

  • How do you wire a "4 way" circuit? On the end closest to your power and light, use 3 way switches. All other switches in between should be 4 way switches and there is no limit to the number of 4 way switches you can have in the circuit.

4waycircuit.png

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