DPDT?!?!

[Anonymous]

I have heard of this magical [bow](or not so magical) device called a DPDT. could anyone answer these easy( or hard) questions on this ?
What does DPDT stand for?
What does it do?
How do I use it?
How does it work?

Thank you for your help.

[Jacktal]

The DPDT stands for "Double Pole/Double Throw" which indicates that the switch will control two different circuit simultaneously on one single move of the actuating arm(DP) and also has two positions for both circuits(DT).

Let's hold the switch vertically and determine the rear contacts....The center terminals are the "commons" meaning that one different source of current is generally applied to them,as they are independent electrically but not mechanically as the switch controls both sides at the same time.In fact ,this is two switches in one,side by side that are operated by a single move.

Now the other terminals...still holding the switch vertically...the power applied to the center terminal can then be redistributed by either the top ones or the bottom ones to different accessories alternately,depending on which way the toggle has been thrown.Don't forget that right and left are electrically independent although toggled together.

Basically the DPDT switch allows to control two pairs of accessories from two different power sources(even different voltages).You could also use them to run one single track from two different power supplies simply by using the "common" terminals as a single outlet to the track and the upper pair of terminals to one power supply and the lower pair to the other supply.These switches can also be used for polarity reversing and many other applications that would be too long to describe here.I just hope I haven't confused you with its basic operation.

[Anonymous]

DPDT stands for Double Pole, Double Throw switch. Double Pole means 2 separate circuits or poles (both + / -). Double Throw means it has 2 positions and is ON in either position (ON-ON). A typical ON / OFF switch like a wall light switch is a SPST or Single Pole, Single Throw. That only has one circuit.

Some other common switch types are SPDT, SPDT/CO, and DPDT/CO. For the last two CO stands for Center Off; therefore a DPDT/CO has 2 poles and 3 switch positions—ON-OFF-ON. SPDT has one pole and 3 switch positions—ON-OFF-ON. There are more, but these are the basics.

I would suggest you to check under the heading ‘hobby basics’ on the main web page at this site and/or or get the book “Easy Model Railroad Wiring” by Kalmbach, and “Basic Model Railroading: Getting Started in the Hobby” also by Kalmbach.

[CBQ_Guy]

OK, I'll take a stab at this. DPDT stands for Double Pole Douple Throw. It describes a type of electrical switch. Basically think of a light switch on the wall - when you flip it one way, the light goes on. And when you flip it the opposite direction, the light goes off (By the way, that type of switch is known as a SPST, Single Pole Single Throw).

Now think of the light switch example again. Except this time imagine that there is no off position, per se, when you flip it the other direction; and lets now assume there are two separate lites. If you flip the switch in the other direction, one light still turns off like before, but now the other lite goes on. So basically it's like you have two switches built into one, except one or the other is always on, there is no off position (at least where you can turn both lites off at the same time.) Sometimes there would be situations where this combination would be desired. So what you have with a DPDT switch then, is either one lite is on OR the other lite is on.

That's basically the answer to your question.

I'll mention one additional thing here, though, since it applies to model railroading, at least it did in pre-DCC days when 2-throttle "cab control" was widespread (and still does if you are using DC and NOT DCC today): If you substitute the word THROTTLE for the word "lite" above, you can now see how one throttle OR another throttle (but not both at the same time) can be chosen to connect to a section, or block, of your model railroad.

Additionally, one other thing should be mentioned. You will also see a designation like DPDT, Center OFF. This type of switch operates exactly like a regular DPDT switch as described above, BUT there is an extra position in between the two extremes when you flip the lever. This extra position is the off position, for BOTH lights. So basically you have positions for, one lite on OR the other lite on, OR both lites off.

Having said all that, others would point out that the above could be accomplished using a SPDT, single pole double throw (if I'm thinking of this correctly!) switch, and they would be correct IF you are using what's called common rail wiring for your layout. This is where one rail only gets switched on or off, or from one cab to another. You're switching only one of the two leads to the track. The other rail is connected together with all the other rails on the same side of the track, through ALL the blocks on the layout, except the areas which are considered reversing sections (now you're getting into a need to wire a DPDT, non-center off switch as a DC reversing switch, which is a whole different topic). However, the DPDT switch is necessary if you want to switch BOTH wires going to BOTH rails at the same time.

Now there are other designations for other types of switches, such as DPST, SPDT, 4PDT, etc., designed for specific needs. I'll not try and explain them here except to say if you wanted to use more than two throttles in DC cab control, one usually goes to rotary switches for that application.

Finally, you will sometimes see the above designations with the added word "momentary". This simply means there is a spring inside. When you flip the switch to on, you have to hold it in that position as long as you want power to flow to whatever it is the switch is turning on. When you release the switch lever, the internal spring will move the electrical contacts inside back to the other of off position, depending on the type of switch you're using.

That's enough for now. Just concentrate on understanding the first two paragraphs at the beginning as they answer your actual question. The rest is a bonus!

Hope this helps.

[bcammack]

Let me get this out of the way first: Do the girls call you "Wedgie"?? LOL

Seriously, though, consider the simplest On-Off switch. A simple On-Off switch is a Single Throw switch. It opens and closes a single circuit. If it only controls one "leg" or wire of a circuit, it is a Single Pole switch. If it controls two legs of a circuit or two wires, it is a Double Pole switch. The number of wires is the first two letters, the second two describes the function.

Looking at your DPDT switch. The first two letters indicate that it switches two circuit legs, or wires. The second two characters tell that it is a Double Throw, which means that it isn't an On-Off switch, but an "On/Off-Off/On" switch. When it's thrown one way, one circuit is closed and the other is open and vice versa.

To illustrate (crude ascii art diagram):
A----------------------------------+
C------------------------------o// o--------------------------------E
D------------------------------o\\ o--------------------------------F
B----------------------------------+

Poles A, C, and E are isolated electrically from Poles B, D, and F.

Poles A and B are routed by the switch setting.

In the above illustration, Pole A is thrown to Pole C and Pole B is thrown to Pole D. This completes the circuits and energy is routed A->C and B->D.

Throwing the switch the other direction connects Pole A to Pole E and Pole B to Pole F, routing energy A->E and B->F.

To complicate things, you will also see terms like "center off" which means that the toggle can be centered on none of the circuits are connected.

Hope this helps.

[nfmisso]

And here is a good place to buy them, if you need quite a few, and maybe some other electronics stuff:
http://www.allelectronics.com/cgi-bin/category.cgi?category=700275&item=MTS-120PC&type=store

[Anonymous]

QUOTE: Originally posted by bcammack

Let me get this out of the way first: Do the girls call you "Wedgie"?? LOL

HAHAHA! I fell out of my chair when I read that! The answer is no. (BTW Wedge is a character from Star Wars)

Thanks for all your help. Just another question, CBQ_Guy mentioned something about cab control and two lites, could i hook this up to two cabs rather than two lites? (sorry if someone has already said this)

THANKS AGAIN
Cmdr. Antilles

[Jacktal]

Basically,you could have one cab linked to "common" terminals and feed either one of two independent tracks through the two other terminals,each side of the switch handling + and - polarities.This is the most frequent use of a DPDT switch.

Or you could just as well have one single track linked to the "commons" (+ and -) fed by either of two cabs that would be linked to the outer terminals.The same way you could let's say have a track alternately powered with DC or DCC,the DPDT switch taking care that either but never both sources could be in use at the same time.

If we knew a bit more about what you would like to do,it would be easier for us,forum members,to help you as there are almost endless applications that a properly wired DPDT switch can handle.You may also need relays and other devices to fill your needs without having multiple switches or else.