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Electrical Contacts - Normally open and normally closed contacts

mlbuy.jpgElectrical contacts make up electrical switches, relays, circuit breakers, and most any other electrical component switches something on/off or can be switched on/off.

What is open and closed?  Before we get into what normally open and normally closed are, let's clarify what "open" and "closed" are. 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 relay then it is not energized.  If it is a switch, then it is off.  If it is a high limit such as a temperature alarm then the current temperature is below the limit. 

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.

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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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Don't over think these two concepts.  That is all there is to it.  Also a very important note for those continuing on to our PLC lessons.  These two symbols don't mean normally open and normally closed in ladder logic.  Right now we are learning about wiring.  We will address what these symbols mean later.

Example:  Now let's go through a real world example of normally open and normally closed contacts that we are all familiar with.  Rarely are these "familiar" examples good for learning, but a three 3 way switch is an excellent example of using normally open and normally closed switches.  And maybe we'll clear up a few things for the houshold electrician or do it yourselfer.  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 (NO) 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.

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  • 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

Trainer Exercises

  1. Wire Push Button 1 to Light 1 so that when you press Push Button 1, Light 1 turns on.
  2. Leaving the wiring of step 1, add wiring to make Light 1 turn on when Push Button 1 is not pressed.
  3. Leaving the wiring of step 1 and 2, add wiring to make Light 3 turn on when Selector Switch 1 is turned to the right.
  4. Sketch out a circuit that will use Selector Switches 3 and 4 to create a "3 way" circuit where cycling either selector switch through any of its positions will cause the light to alternate between on and off.  After sketching out this circuit, wire your trainer and test it.

These lessons are designed for use with our PLC Trainers.