Well now that you've made me come down here........
S = Single
D = Double
P = Pole
T = Throw
Simple:
An SPST swith has only two terminals - if the switch is "on", the two terminals are effectively shorted together. When the switch is off, the two terminals are effectively miles apart from each other.
A DPST is the same thing, with 4 terminals. Two of the terminals will act as above, the other two will also act as above, but most importantly, the two pairs are fully isolated from each other, in terms of electrical connections. They are, of course, connected via the common actuator (that would also be known as the handle, for you non-marketing types out there).
More involved:
A SPDT switch has only one pole, like the first switch noted above. However, it has three terminals, and one of them will be "common" to the other two. If you flip the handle one way, the common and one terminal are electrically connected, and the remaining terminal is disconnected. Flip the handle to the opposite side, and the two non-common terminals swap their connection status. Easy to understand, yes?
A DPDT switch just expands the above, as the DPST did for the SPST switch.
Now, we can get fancy, but only at the risk of pissing off the Marketing 'Droids. They think they know more than Engineers and Technicians about how to label a switch. It's extremely too bad that ignorance isn't painful!!!
DPDT can be, and is, used to describe a number of possible combinations. You can have a center position, which originally meant that the 'on/off' functionality of a xPST switch was combined with the xPDT, giving you the ability to turn off both terminals. This was called, quite appropriately, on/off/on. But you couldn't turn both terminals on
at the same time, so of course, that shortcoming had to be overcome, and we now have on/on/on functionality in a xPDT switch as well. (See footnote for a small diatribe on labeling switches of this variety.)
Tricky (only a bit more so):
If you keep adding poles, you simply change the first letter to a digit, i.e. a three pole switch becomes a 3Pxt, and so on.
If you use switchs other than toggles, you can have any number of throws. In this case, you use a digit to indicate that number of throws, i.e. xP3T or xP4T, etc. Some rotary switches go up to 12 throws!
Tricky (a lot):
In the center position/throw (of a toggle switch), you don't have to have all the poles doing the same thing at the same time. Some switches will "divide and conquer", as it were. For instance, there is one model that has 4 poles, but in the center throw, two of them are on for one 'side', and the remaining two are on for the other side. You have to think about what you want before hooking up one of these. In addition, there is the so-called "Brian May" switch, which also has 4 poles, but in the center throw, only one pole turns on for one side, and only other pole turns on for the opposing side - the remaining two poles are entirely off! You
will be wearing your thinking beanie before you install one of these puppies!
Here are templates showing various layouts of what's been covered. The latter two are shown in the upper-most row, middle and right side.
guitarnuts2.proboards45.com/index.cgi?board=schem&action=display&thread=3149Reduction towards simplicity:
When you have a handful of DPDT toggle switches, all the world starts to look like a rat's nest of wires! ;D But in fact, you can use a SPDT or a DPDT as a simple on/off switch - who says that you
must use the other terminal(s)?
Presto, in the same physical space, you have just imitated an SPST switch, no problem.
HTH
sumgai
Diatribe:
The $64 dollar question, of course, is why did the Marketeers ignore the fact that a throw was a stop along the handle's path of travel, and the center position was indeed one of three throws?
Good question, eh? If I were in charge, those Marketing Bozos would be in line for a solid smackdown, just after I got through making all the spammers in the world pay one penny (American) for every spam they've ever sent. Good thing I'm not on the radar of getting put in charge, ain't it? ;D
