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Switches

So, why would I write an article about switches - a very simple little device that everyone uses everyday starting with baby toy noise maker buttons? Surely, everyone understands what switches are and what they do! - or do they??

Ok, so everyone does know if you turn a switch on, some device starts working. Most who have ventured onto this site also understand that the switch is completing a path for electricity to get to that device. It turns out however, that like most things in life, engineers have come up with many ways to expand that little device, making them even more useful - but maybe a bit more complicated.

Ultimately, all switches are composed of two or more contacts. These are simply pieces of metal designed to make a good electrical connection with each other, and of an appropriate metal alloy to withstand the heat and arcing of making and breaking an electrical connection.

SPST, SPDT, DPST, DPDT (and stuff like that)

Commonly, a switch will be designated as SPST, SPDT, DPST, and DPDT. Lost now? If so, great .. you are the exact reader I am writing this for!

These groups of letter designations describe how many contacts there are, and their operating relationship to each other. These will commonly be written all in a lump like in the previous paragraph, but we are going to break them in half for simplicity. The first two letters (ie SP-single pole, DP-double pole,3P,etc) tell us how many circuits, or pathways, the switch can control. The second group of two letters (ie ST-single throw, DT-double throw, etc) designate how many combinations the user can select from the operating lever.

SPST

SPSTs are by far the most common switch you will encounter. You'll find them in use for everything from common wall light switches to the keyboard keys on your computer.

According to the description above, the SPST designation tells us there is a single circuit and a single user option. This might seem a bit confusing at first; because this type of switch has two positions - on or off; however its a single 'throw' because it has only a single on position.

DPST

It probably follows pretty easily that DPST switches are not much different then, essentially just two switches built together. This does not mean two separate switches however. It means literally two separate circuits physically built and operated together by the same mechanical mechanism.

Switches of this type are used when it is necessary or convenient to turn two circuits on or off simultaneously. An example of this use might be a power indicator light and a heating element. The indicator light might operate on a low voltage supply for safety or efficiency, but the heating element could be using direct line power. Obviously, you don't want the two circuits directly connected together, but you do want them to come on and turn off at the same time.

SPDT

That brings us to the SPDT then. Single Pole, Double Throw. You should recognize from the paragraphs above that this switch has a single circuit in it; but what's with the throw bit? Turns out, that's sort of two switches built into one unit as well! Instead of turning on and off at the same time, they turn on and off opposite of each other - depending on which direction the user moves the lever. That might mean you can turn the same light on to a high power source for a bright light, or a low power source for a dim light - but never both at the same time. It could also mean connecting a single power source to either of two different devices but not both at the same time.

Some common applications you will find for this type of switch include the heat/cool switch on a thermostat or the left/right positions on a video game joystick.

DPDT

The final designation, DPDT, is just a combination of the above two switches. It has two circuits, and two on positions the user can select. Note, it may or may not include a center off position - remember the throws are the available on positions. The two circuits(or more) - while not electrically connected to each other - still operate at the same time.

While that may sound a bit redundant, its actually a very useful combination that is commonly used to provide forward and reverse control. It does this by flip-flopping the the two wires going to a motor - thereby causing it to run in the opposite direction.

You might also run into extensions of this nomenclature, such as 3PST or SP5T, etc. Those mean just as you would expect, more than two of which ever is replaced by a number. I won't get into much detail with those, but you've seen them as selector knobs when some device has multiple settings for some function.

In the next section, I'll discuss momentary switches, and the electrical ratings.

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