LAMPS CONTROLLED FROM TWO POINTS: THE SPDT SWITCH (THEORY)
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Welcome to the home’s electrical systems course organized by NPR Online Technologies!
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In this lesson we will see how to control a group of lamps from two distinct points by using a pair of single-pole double-throw switches.
Let’s start this lesson with a question: what happens if in the room considered in the previous example, we have two access points from which we want to control our group of lamps?
In particular, suppose we enter the room through the door on the left,
turn on the lamp and want to leave the door on the right.
How do we turn off the light?
With the configuration currently available, we should go back to the only switch placed in the room.
It is evident that this is not an optimal solution because once the light is turned off, it forces us to cross the dark room, if we want to go out the door on the right.
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The solution to this problem is based on the use of another device widely used in our homes: the single-switch double-throw switch, or briefly “SPDT switch”.
Apart from the acronym, SPDT, there are different names used across the world for this kind of switch. If you live in the USA, electricians are much more likely to talk about three-way switches, while if you live in Europe, you will hear electricians talking about two-way switches. In some countries electricians talk about “diverters”, thus indicating the capability of these switches of diverting the current path from one terminal to the other. Our opinion is: if you want to be precise and don’t want to be misunderstood then use the term single-pole double-throw switch, but keep in mind that the other names are frequently used in technical jargon.
Well, let’s focus now on the use of the SPDT switch. The single-switch double-throw switch consists of three terminals and a movable contact or lever, which on one side is permanently connected to one terminal, while on the other side it is able to connect alternately to one of the other two terminals of the SPDT switch.
If we use a pair of these switches, we can switch on and off a lamp or a group of lamps from two different locations.
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In particular, with two single-switch double-throw switches, it is possible to place one of them at the left entrance of our room, and the other one at the right entrance of our room, thus allowing the control of the lamp from two different places.
Starting from a situation in which the lamp is off, if we operate the first switch, its movable contact establishes an electrical connection that allows the hot conductor to reach the lamp and consequently to turn it on.
We are showing in red the path of the electric current that flows in the circuit.
If we now flip the first switch again, the movable contact will interrupt the electric connection and consequently the hot conductor cannot reach the lamp, which will be turned off.
In fact, we can see that the electrical continuity is interrupted at the second switch.
It is clear that by operating the first SPDT switch several times, we are able to alternately turn the lamp on and off.
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A similar mechanism occurs for the second SPDT switch. Starting from a situation in which the lamp is on, if the second switch is flipped, the lever inside it modifies the internal electric connection preventing the hot conductor from reaching the lamp, which consequently is turned off. If we operate the second switch another time, the lever inside it will modify the electric contacts by establishing an electric connection between the hot conductor and the lamp; in this case, the lamp is turned on.
For better graphic representation, we are showing in red the path of the electric current.
The mechanism just analyzed represents the solution to the need to control a lamp or a group of lamps from two different points.
Let’s perform now some simulations in order to verify if an approach with two SPDT switches can solve the problem introduced at the beginning of the lesson.
Now let’s suppose, we enter the dark room from the left and flip the switch.
In this case, it can be observed that we are able to turn on the light.
If we now exit from the right
and flip the second switch, then we will interrupt the connection between the hot conductor and the lamp
and, as expected, the room becomes dark.
If now another person enters the room from the left entrance
and flips the switch, the connection between the hot conductor and the lamp is established again and consequently the lamp is turned on.
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By exiting from the right and flipping the corresponding switch, the connection between the hot conductor and the lamp is interrupted again and, as expected, the room will become dark again.
We observe that, as required by the standards, for safety reasons, all the connections of the single-pole double-throw switches have been made with the hot wire and not with the neutral wire.
We note also that, to make it easier to identify the various connections, the wires that connect the two SPDT switches are black, which is one of the colors allowed in Europe for the hot wire.
In this regard, there are electricians who also prefer to differentiate the color of the hot wire that is connected to the light bulb. In this case we can use the gray color which is another color allowed in Europe for the hot wire.
It is important to recall that there are specific rules concerning the color of wires. However, even for wire colors there are different codes depending on the geographic region. In this regard, in our course, we are using the color coding adopted in Europe where for example light blue is used for the neutral wire, while brown is used of the hot wire. Several resources on the Internet provide the color coding adopted in the different geographic regions.
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Concerning the installation phase, we can see that the two SPDT switches are put on the two walls in easily accessible positions once the corresponding door is opened.
The points where the two SPDT switches are positioned are indicated in the diagram with the letter D, where “D” is a mnemonic letter that stands for “diverter” or “double-throw”.
The neutral and hot wires are available in the room through an electrical box embedded in the wall called junction box, indicated in the following diagram with the letter “J”.
Starting from this box, we have the wires that run towards the two switches
and the wires that run towards the lamp (or the group of lamps).
For this reason, it is necessary to lay three electric conduits
inside which the wires of the designed electrical system will run.
As previously said, when possible, the electric conduits are typically embedded in the walls or in the floor or in the ceiling.
Well, we have come to the end of this lesson. We have analyzed the circuit that allows to turn on and off a groups of lamps from two distinct places by using two single-pole double-throw switches.
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