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Author Topic: SPDT Relay with disconnected NO state?  (Read 1092 times)
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Somerville, MA
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Hey all, back with a basic relay question. I can see that an SPDT relay allows the ability to switch between two poles. I guess the assumption is that one of those poles is nominally closed, and the other is nominally open.

Fine. However, I was thinking there should be a relay configuration which has two poles, plus a nominal state in which neither pole is connected. Is there a common name for this, or must one create one out of combinations of relays like an h-bridge?

See the attached picture for clarity...



* RelayQuestion.png (5 KB, 598x348 - viewed 5 times.)
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Left Coast, CA (USA)
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Fine. However, I was thinking there should be a relay configuration which has two poles, plus a nominal state in which neither pole is connected. Is there a common name for this, or must one create one out of combinations of relays like an h-bridge?

Not possible as a electro-mechanical relay can have only two states, energized or non energized. However some mechanical toggle or slide switches can have such three discrete states or positions, and those that do are described as "center off".

Lefty


« Last Edit: August 31, 2011, 02:45:33 pm by retrolefty » Logged

Phoenix, Arizona USA
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A better question, which might lead us to suggest a solution for you, braddo_99, would be why you think you need such a relay? In other words, what is the application or question you are trying to solve?
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Manchester (England England)
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Well you could do this with what is known as a carpenter relay
http://www.flickr.com/photos/russell_w_b/5075821618/
This has two coils one each side of the contacts, energising one coil pulls it one way and energising the other pulls it the other way. With nothing energised it remains in the center position.
These were used in old fashioned teleprinter.  
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Well you could do this with what is known as a carpenter relay

LOL, well could ' a you ' buy a new one today?

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He wasn't asking where to get one, he was asking what it was called.  smiley
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He wasn't asking where to get one, he was asking what it was called.  smiley

You have properly identified the 'unicorn of relays', LOL

Lefty
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Somerville, MA
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Thanks folks, I appreciate the feedback, and if such a thing did exist today I would certainly be interested in checking on price and availability.
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Somerville, MA
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By the way, the reason I'm looking for such a thing is that I'm "impersonating" a center off switch. It's a spring loaded job that energizes different contacts on either side, just like the unicorn I sketched. :-) I can certainly figure out something to get the same effect but would rather just use one unit for this since, I have three such switches to emulate.
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if such a thing did exist today I would certainly be interested in checking on price and availability.
You can still get them on ebay but they are not new.
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If you do end up with 2 relays you have to figure out what happens if BOTH go active at once.  You have the options of default to one side, last or first takes precedence, all contacts connected, both open.  Some of those being harder than others.
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Seems to me you can construct an H-Bridge using two DPDT relays, but then the motor is either moving forward or reverse, there's no off. To get that you'd need another SPST relay. To control 4 motors (I think I said 3 before), you would then need 12 relays. Too many.

Or, you could make the H-Bridge with 4 SPST relays, which is when you would run into the complexities mentioned just above, to make sure you didn't directly short the power. Here you would have even more, at 16 relays... yikes, that's a lot of space.

One can also make/buy MOSFET based relays, but I have the sense that these would be more expensive, get hotter, and maybe couldn't handle the same current the relays could.
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Well if you wish to use relays and only require full speed forward, full speed reverse, and stop, you can do that with just two relays, a DPDT for polarity selection, and a SPST to turn power on and off.

Lefty

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If you are talking about motors the position where both ends of the motor are connected together is the desirable fly wheel breaking situation.
See:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html
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