Posts Tagged ‘switch’

No, we’re not talking Caber Toss here (I don’t have the legs for a skirt… err kilt… call it what you will, it’s still not a good idea to wear it while you’re working around the house).

We’re talking about switches! Specifically those found in your home, used to turn on and off lights and other devices.

First let’s start by defining a switch.  A switch is a device for making and breaking the connection in an electric circuit. A switch can have one or many poles and one or many throws. All switches look about the same on the outside; a lever or button that is flipped or pushed, but internally the number of poles and throws determine how the switch is used.

In a switch, a Pole is the number of circuits that can be controlled by a switch. It might be easier to think of this as the number of inputs. Throws are the number of positions the switch can take. Throws can be thought of as the number of outputs (sort of).

Single Pole Single Throw (SPST)

A single pole single throw switch has one input, and one output. It is used to turn a circuit on, or off. “But wait…” you might be thinking “It turns the circuit on and off, isn’t that a double throw?“. You’re sort of right, I guess I didn’t explain throws so well. It’s not about the physical positions a switch can be in: up or down, it’s about the number of contacts in the switch. Maybe it’s easier to explain with a picture.

As you can see, there is one pole contact and one throw contact. With a SPST switch, turning on and off the light is easy.

Now that we’ve got the concept down, let’s take a look at some other types of switches. Keep in mind that you can have switches with as many poles and throws as you need, but we’re only going to focus on those typically found in residential wiring.

Single Pole Double Throw (SPDT)

A single pole double throw may also be known as a 3-way (2-way in Europe) switch. They are used to control a single device (light, string of lights, etc) from multiple locations.  With this kind of switch, you can turn the light on at the bottom of the stairs.  Then you can walk up the stairs and turn the light off at the top of the stairs. To do this, you’ll need two SPDT switches. One switch at the top and one at the bottom of the stairs. Internally they look like this:

As you can see, the single circuit can be switched between one of two contacts. In the US, these two contacts will be connected using wires called Travelers. These wires connect the contacts of one SPDT switch to the contacts of another SPDT switch (typically).

So how does this turn the lights on and off? Well let’s finish the drawing.

The power from the circuit is connected to one of the switch’s pole contacts, and the light is connected to the other switch’s pole contact. When both switches are in a similar position, electricity flows in the first switch, through one of the traveler wires to the other switch, and finally to the light.

“What if I want to control a light from more than two locations?” you might ask. Well, that’s where the next switch comes in.

Double Pole Double Throw (DPDT)

Double pole double throw, also known as 4-way or intermediate switches, allow you to connect as many switches as you’d like into the circuit. Internally they look like this:

To control the light, we’ll add them into the circuit on the travelers between our SPDT switches.

Now the current will flow into the first SPDT switch, out along one of the travelers to the DPDT switch, along one of the other travelers to the second SPDT switch, then finally to the light.

Using DPDT switches, we can control a device from as many locations as we want (within reason). All we have to do is add another DPDT switch, like so:

Understanding switches will help you whether you need to control a single light from one place or a group of lights from multiple locations.   Anything you can imagine can be done, if you choose the right switch for the job.  And please, no kilts while you’re working. It’s just dangerous.