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Switch help

  1. Oct 31, 2013 #1
    In Home Wiring the Switch that is used to tun on and off just one light is called a S.P.S.T Switch.

    Now when you have one light that has two Switchs that can turn it on and off the Switchs are 2 way Switchs. But are they called S.P.D.T Switchs?
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  3. Oct 31, 2013 #2


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    SPST stands for Single Pole Single Throw. I think the 3 way switch is a SPDT, Single Pole Double Throw.
  4. Oct 31, 2013 #3


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    Integral is correct a s.p.s.t., or more commonly a single pole switch, is just an on/off switch. A 3 WAY switch is a s.p.d.t. and the ones used in homes/offices have 1 terminal either a different color or the word common in the plastic near the terminal. They are called 3 way because you have 3 different combinations of positions- on/on , on/off , off/off- and each position change on EITHER switch will change the load state. There is also a 4 way switch which when toggled will change from [top right connected to bottom right ] and [top left connected to bottom left] to {top right connected to bottom left} and {top left connected to bottom right}. These are 4 terminal devices and can be installed in any number between (2) 3 way switches to allow control from any location. (Do not confuse a 4 way switch with a 2 pole switch (2 single pole switches in 1 handle[ D.P.D.T]) , they look identical)

    I've been an electrician for approx 25 yrs and know some more experienced guys who can't wire a 3 way or 4 way setup. Troubleshooting and replacing them is the most confusing home project you will encounter (including rough framing, drywall finishing and plumbing.)
  5. Nov 17, 2013 #4
    OK thanks for all the help and yes I do know how one switch connects to one Light.

    And I do know how two switchs connect to one Light.

    But there is one thing I never could understand.

    When you see a Light with a Fan and one switch I do understand how this connects.

    But I do not understand how they connect two switchs to a Light with a Fan.
    You know when one switch turns the Fan on/off and the other switch turns the Light on/off.

    I did a google search for photos of wiring and they are not that good.

    Can you show me a good photo or diagram of how they connect?
  6. Nov 17, 2013 #5


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    Not sure I understand the question.
    Is that not simply one switch for the light, one switch for the fan, each with a separate circuit?
  7. Nov 17, 2013 #6
    Well whenever I look at how they wire two switches to a Fan with a Light they are very very diferant.

    Can you maybe post a link to a photo?
  8. Nov 18, 2013 #7
    Thank you for all the help I am getting it now.

    But I would like to go over the diferant switches to see if I have it right.

    A way switch is called a S.P.S.T

    The two switches in a Hallway that connect to one light are called D.P.S.T

    Now you may see three switches connected to one light. And the first and last switch are D.P.S.T but the middle switch is one with Four Screws so am I right this is called D.P.D.T ?
  9. Nov 19, 2013 #8


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    Biferi, the two switches are S.p,D.t. and the third can only be described as a [straight through / criss cross] switch (It is NOT a D.P.D.T- that switch has 6 terminals).
    In one position the terminals go straight through (top right to bottom right) and (top left to bottom left), and in the other position the criss cross (top right to bottom left) and (top left to bottom right). Sorry, I can't draw on a computer to save my life so I can't help with a diagram.
  10. Nov 19, 2013 #9


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    I wired up my house with three switches controlling one light, about 25 years ago. I think it involved a special "4-wire conductor": 3 hots/neutrals, plus that inevitable "Ground".

    The magic isn't in the number of screws on your switch, it's the "black box" circuitry inside the switch.

    Learn boolean logic.
  11. Nov 19, 2013 #10
    Just to see if I have what you tell me right.

    The two switches that work one light are called DpSt

    Now I am watching a video

    And it has three switches and one light.

    The first and last switch are a DpSt switches.

    And the middle switch is just a Pass through you say.

    Please watch the video and tell me if I am right I thought it may have a name?
    Last edited by a moderator: Sep 25, 2014
  12. Nov 20, 2013 #11


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    They are not DpSt(double pole single throw) they are SpDt (single pole double throw). Please reread my above post. The switch in the middle is called a 4 way switch.
  13. Nov 20, 2013 #12


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    A quick google search clears it all up:

    A quick look at switches:

    indicates to me that I'm drunk, as there should be no such thing as a single throw switch.

    I would call that a wire.......
  14. Nov 20, 2013 #13
    Thanks for all the help but what I ment was there is
    S.P.S.T switches.
    S.P.D.T switches.

    And then the 4 way switches.

    But don't the 4 way switches have a name?
  15. Nov 20, 2013 #14


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    Do you not trust wiki?

    I don't after what they said.

    This makes more sense:

    It has been many decades.....

    I really did used to know all this stuff. :redface:
  16. Nov 21, 2013 #15


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    A 4 way switch is in fact a DPDT switch with the 2 sets of throws (can they be called that?) criss-crossed and tied together.
  17. Nov 24, 2013 #16
    Thanks for all the help again.

    But I have one more Link to a video I was watching and I just wanted to know if I am looking at it right.

    On a 4 Way Switch are the Top 2 Screws Brass and the Bottom 2 Screws Black?
    Last edited by a moderator: Sep 25, 2014
  18. Nov 26, 2013 #17


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    Colors and position may vary from switch to switch. I usually just check continuity. What the vid says makes good sense though. One set of colors going one direction and the other set going the other direction.
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