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SPST vs SPDT
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SPST vs SPDT

by baalexander on Tue Aug 31, 2010 12:23 am

I am looking for a basic On/Off switch for my robot. If I understand correctly, an SPST (single-pole, single-throw) switch connects the terminals when switched on and disconnects the terminals when turned off. An SPDT (single-pole, double-throw) switch connects one terminal to the pole or connects the other terminal to the pole based on the switch position.

What I'm confused about is which switch to choose for an On/Off switch and why? I had assumed an SPST, but I have seen an SPDT set up so that one state has the positive battery power connected to the rest of the circuit and the other state has the positive battery power disconnected from the rest of the circuit.

If it helps, I am using the switch on a breadboard for a 12 V battery with a current draw around 600 mA.
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Re: SPST vs SPDT

by adafruit_support_bill on Tue Aug 31, 2010 5:53 am

SPST is sufficient for an on-off switch.

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Re: SPST vs SPDT

by henk on Tue Aug 31, 2010 12:56 pm

You really need a DPST switch. A SPST can only disconnect one wire, either positive or negative.

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Re: SPST vs SPDT

by len17 on Tue Aug 31, 2010 1:33 pm

henk wrote:You really need a DPST switch. A SPST can only disconnect one wire, either positive or negative.

Is that really necessary? All the on/off switches I've seen only disconnect one of the power supply lines.

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Re: SPST vs SPDT

by henk on Tue Aug 31, 2010 1:53 pm

It certainly is safer. With a SPST switch you always have one leg of the power supply connected to the device.

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Re: SPST vs SPDT

by zener on Tue Aug 31, 2010 2:21 pm

I don't think I have ever, in my life, seen a DPST switch used to switch both legs of the power supply in a DC application. You might do this for AC, if the equipment is not insulated.

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Re: SPST vs SPDT

by oPossum on Tue Aug 31, 2010 4:06 pm

As a general rule, ground and neutral lines should not be switched.

A DPST switch would be used for two phase AC or bipolar DC.
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Re: SPST vs SPDT

by uoip on Tue Aug 31, 2010 4:52 pm

Back to the original question, SPST is all you need for simple on/off, but I've seen half of a SPDT used. The reasons may have to do with stocking common available parts -- if you need a SPDT switch somewhere else on the product (or perhaps even on a different product manufactured at the same facility), you may want all your switches to look alike and come from the same supplier, and you may not want to have to stock two separate bins full of parts. There's no electrical reason to use an SPDT and leave half unused, but there may be other practical reasons in some situations.

I've even seen 1/4 of a DPDT switch used for basic on/off. I've never seen both hot and ground switched on a low voltage DC device, though. In fact, most AC devices leave the neutral always connected, and just use a SPST switch.
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Re: SPST vs SPDT

by baalexander on Wed Sep 01, 2010 10:39 pm

Thanks for the replies. I wasn't expecting the varied responses. From what I gather, an SPST will meet my needs just fine, but an SPDT and even a DPST could work.

I'm currently looking at this SPDT switch http://www.mouser.com/ProductDetail/Mou ... bf30LUI%3d because it supports up to 1 A (my motors draw 600 mA), has 0.1" pin spacing for the breadboard, and uses a toggle switch. I was having a tough time finding an SPST meeting those requirements for the price ($2.60).

Do y'all see anything I overlooked? I'm iffy on the fitting in the breadboard, but it says the center of the pins are 0.1" apart and the pins have a max width of 0.03". I *believe* that's acceptable, but not sure the hole size of a standard breadboard.
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Re: SPST vs SPDT

by adafruit_support_bill on Thu Sep 02, 2010 6:02 am

A 0.03" pin will fit, but they may not be long enough. That's a fairly standard switch for PCB mounting, and the pins are typically not very long.

You could temporarily solder a pair of wire extensions to it for the breadboard. Given that this is for a robot, I assume that you will be building a more robust circuit once you get things debugged on the breadboard.

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Please be positive and constructive with your questions and comments.