# Mechanical timer, what is the difference?



## goosegunner (Oct 27, 2011)

I am looking to replace a metal toggle switch for the fan on my Econoburn with a mechanical timer switch.


I found a few, but what is the difference between SPDT and SPST?


I found these,

http://m.globalindustrial.com/m/p/e...nd-Timers-amaz_shop&infoParam;.campaignId=T9I


http://m.globalindustrial.com/m/p/e...l-series-timer-w-hold-continuous-duty#nowhere

Which one should I get?

Any better choices?

gg


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## tom in maine (Oct 27, 2011)

If you just want to turn it on and off, interrupting the hot lead, similar to a light, then a SPST is what you want.
SPST stands for Single Pole Single Throw.

SPDT stands for Single Pole Double Throw, which might be used for switching to another fan for example.

I suspect the metal toggle switch is SPST (just two connections to interrupt that circuit.)


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## goosegunner (Oct 27, 2011)

Thanks, I have not looked at the back of the metal toggle. The switch now just stops the fan to load wood, so i would need the spst. The current switch controls nothing else.
Gg


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## ewdudley (Oct 28, 2011)

Tom in Maine said:
			
		

> If you just want to turn it on and off, interrupting the hot lead, similar to a light, then a SPST is what you want.
> SPST stands for Single Pole Single Throw.
> 
> SPDT stands for Single Pole Double Throw, which might be used for switching to another fan for example.
> ...



Oops, we're a little mixed up here. 

SP is single pole, where a pole is a circuit. DP is double pole, two circuits.

ST is single throw, meaning there are two terminals for one set of contacts that makes or breaks.  If you take it out of the box it comes in and set it on the bench and the contacts 'make', then it is normally closed, or NC.  Opposite is normally open, NO.  If it is is SPST and they don't specify NO or NC, then it is fair to assume NO.

DT is double throw, meaning there is common terminal and two other terminals, a NO terminal and a NC terminal.

So if you want the timer to open a circuit while the timer is running, you need a NC circuit that the timer opens, so you need the SPDT, or a SPSTNC if there is such a thing.

--ewd


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## goosegunner (Oct 28, 2011)

Wouldn't the spst switch work?

I want it to open the circuit when time runs out. It also has a hold so it can be run continuous without the timer.

gg


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## ewdudley (Oct 28, 2011)

goosegunner said:
			
		

> Wouldn't the spst switch work?
> 
> I want it to open the circuit when time runs out. It also has a hold so it can be run continuous without the timer.
> 
> gg



Sorry, when you said "The switch now just stops the fan to load wood, so i would need the spst. The current switch controls nothing else.", I thought the timer switch was needed to stop the fan for loading wood as in open the circuit with the timer.


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## goosegunner (Oct 28, 2011)

ewdudley said:
			
		

> goosegunner said:
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Boiler currently has metal toggle switch that can be placed in the down or off position to stop fan for loading. In the up or on position the fan is controlled by the main boiler controll and the set point.


Ok I am confused here what I am looking for is to

1. Have a timer to turn to about 4-6 hours to turn fan on

2. When time runs out fan stops or turn timer to zero to stop fan to load wood



gg


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## ewdudley (Oct 28, 2011)

goosegunner said:
			
		

> ewdudley said:
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Nope, you're not confused, I was.  The way I read it I thought you wanted to turn the timer on to turn the fan off for a certain amount of time while you loaded wood, and then have the timer automatically turn the fan back on again, which didn't make too much sense but it's what I though you wanted.  So you're right, you want a simple SPST countdown-to-off timer.  Sorry for the confusion.

-ewd


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## goosegunner (Oct 28, 2011)

No problem, thanks for your time.

I now need to decide if I want the timer or to try to use the Econoburn flue temp shutdown capability.

gg


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## Garth B (Oct 28, 2011)

does the econoburn have a flue temp shutdown capability? I put a snap disc on my flue to shut mine down, wasn't aware of a factory fan shut down other than overtemp from the aquastat


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## Garth B (Oct 28, 2011)

nevermind, saw the other thread.... its nice they are coming with the added functionality now


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