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SPST SPDT DPST DPDT and other Q's

fireplug

fireplug

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I'm not grasping the idea of a double throw switch?

I get that a SPST is ON/OFF to a single circuit.

And I 'think' a DPST is a basic ON/OFF to two separate circuits. Though I don't understand what application this would be used for.

But I'm completely lost with the DT? Maybe this would be like an ON/OFF/ON switch that you would use with a winch?

While on the topic, if you are using relays, does it really matter what amperage the switch is rated for?
 
fireplug said:
I'm not grasping the idea of a double throw switch?

I get that a SPST is ON/OFF to a single circuit.

And I 'think' a DPST is a basic ON/OFF to two separate circuits. Though I don't understand what application this would be used for.

But I'm completely lost with the DT? Maybe this would be like an ON/OFF/ON switch that you would use with a winch?

While on the topic, if you are using relays, does it really matter what amperage the switch is rated for?
Click to expand...
SPST is on/off
SPDT is on/on

A SPDT looks like this:

---
1 2 3

In that example, the switch is in position #1, and pins 1 and 2 are connected... now lets flip the switch...

---
1 2 3

 Now pins 2 and 3 are connected. Pin #2 is the common pin--it's always connected to something

You can use a SPDT switch to choose between two inputs or outputs or disregard an outside pin and use it for on/off, just like a SPST.

The "SP" in SPST or SPDT means "single throw". A DPDT or DPST means two poles, or, two single switches in one unit.
1 2 3
---
4 5 6

You can see in the above example that pins 1 & 2 are connected to each other. At the same time, pins 4 & 5 are connected to each other. Note...they act as two independent switches, therefore for example, pins 1 and 4 are NOT connected to each other.

Slide the switch to the right, and pins 2 & 3, and 5 & 6 are connected. Pins 2 and 5 are the common pins in this example.

The on/off/on switch would have a 3rd position, or a "neutral" in the middle. Using a SPDT switch as an example, in the middle position, pin 2 wouldn't be connected to anything.

Does that help???
 
fireplug said:
I'm not grasping the idea of a double throw switch?

I get that a SPST is ON/OFF to a single circuit. Yes, a two terminal switch.

And I 'think' a DPST is a basic ON/OFF to two separate circuits. Though I don't understand what application this would be used for. You are correct. For basic wiring, it isn't used too much, but when you start getting into wierd situations, it becomes useful. This would be a four terminal switch. Think of it as two SPST switches in one switch body.

But I'm completely lost with the DT? Maybe this would be like an ON/OFF/ON switch that you would use with a winch? Pretty much. Think of it as two switches/independent circuits in the same switch body. It could be an on-off-on, it could be an off-on-on, or an on-off-momentary on etc. A SPDT would have three terminals or you can also have a DPDT which would net you six terminals. A winch could use a DT switch, but it would probably be in a momentary on-off-momentary on configuration so when you let go of the toggle the switch automatically returns to the off (center) position.

While on the topic, if you are using relays, does it really matter what amperage the switch is rated for? Not really. It does, but typically small relays draw so little current that *most* typical switches will handle the current necessary to activate a relay. If you start getting into large (80A+) relays then it will become much more important.
Click to expand...

Kind of a basic explanation, but it might get you going.
 
bug the people at the store by laying a few relays on the shelf, counter, etc. and studying the diagrams. when you see them side-by-side, you can see the logical progression as complexity increases. for my purposes, a spst or spdt usually gets me by.
While on the topic, if you are using relays, does it really matter what amperage the switch is rated for?
Click to expand...


within reason, not really. but remember that activating the internal coil on the relay does require power, just like any other circuit or device. i use 20 amp switches for mine, usually. less would be fine, i'm sure.
 
Some additional info, as most of the basics have been covered.

Double -throws (the xxDT switches and relays) are by convention ON - ON. The ON-OFF-ON ones are notated as "center-off", i.e. "DPDT center-off."

The spring-loaded ones like you'd use for a winch, as somebody mentioned, are call momentary. You can get those in SPST but are more common in the double-throw center-off, i.e. "DPDT center-off" or "spring-return to center."

Your typical Bosch-style auto relay coil draws 70mA, i.e. .07A, so you can use any dang switch you like to run them. ;) Other relays -- Ford starter solenoids -- take serious juice (coupla amps, IIRC) so you need something more than a submini.

Also, I hate to say it, but I looooove Forest Mims' books. My personal fave sounds dumb for a half-EE like me, but works well for this kind of stuff:

http://www.amazon.com/Getting-Started-Electronics-Forrest-Mims/dp/0945053282

I guess Radio Shack finally stopped carrying it after, geez, like 20 years ;)

-- A
 
stay away from that type of book! once you read up on all the fun stuff you can do with relays, capacitors, resistors, coils, etc, you'll get into all sorts of fun trouble. :shame: :D. but of course, i never did anything naughty with that info:whistle:
 
dremu said:
Some additional info, as most of the basics have been covered.

Double -throws (the xxDT switches and relays) are by convention ON - ON. The ON-OFF-ON ones are notated as "center-off", i.e. "DPDT center-off."

The spring-loaded ones like you'd use for a winch, as somebody mentioned, are call momentary. You can get those in SPST but are more common in the double-throw center-off, i.e. "DPDT center-off" or "spring-return to center."

Your typical Bosch-style auto relay coil draws 70mA, i.e. .07A, so you can use any dang switch you like to run them. ;) Other relays -- Ford starter solenoids -- take serious juice (coupla amps, IIRC) so you need something more than a submini.

Also, I hate to say it, but I looooove Forest Mims' books. My personal fave sounds dumb for a half-EE like me, but works well for this kind of stuff:

http://www.amazon.com/Getting-Started-Electronics-Forrest-Mims/dp/0945053282

I guess Radio Shack finally stopped carrying it after, geez, like 20 years ;)

-- A
Click to expand...

I used to love those Forest Mims books too when I was younger. I looked for them a few years ago, but it looked like they didn't carry them anymore.
 
Great, great, great info. Thanks for all the replies. This is all related to wiring up my Windstar fans.

Which by the way, I've got some useful info I think. I'm going to update a couple of old threads with some stuff keep your eyes open for threads regarding Carling switches and Windstar relays...
 
fireplug said:
Looking to order some relays. I have the choice of 'protection' diode or resistor. Any thoughts?
Click to expand...



The diode is to prevent back current from the coil... basically the coil, when it opens (i.e. is turned off), zaps out a tiny bit of juice back into the circuit.

If you're using illuminated switches, they MIGHT be sensitive to the backzap. I think it'd really only be an issue if you were driving the relay with solid state, i.e. a transistor.

Can't think what the resistor is for, except maybe to reduce the back flow ... but then it'd soak up more juice ALL the time, which is just a waste (though as I mentioned before the current flow through the coil on your typical Bosch style relay is so miniscule anyway it's all silly.)

-- A
 
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