# Latching Relay Wanted



## greenhouse (Feb 16, 2010)

Hi:
I am looking for a 12VDC coil SPST Latching Relay with contacts rated at about 5 amp at 110VAC.

Also, I need a 12VDC power transistor to trigger it's coil.

Would like these parts to be of reasonable quaility.

Could someone please suggest part numbers for the combo and where to get them without paying an arm and a leg.

PS: There are zillions of them out there and I know so little that I can not tell if they are laching or not.


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## Carl2 (Feb 17, 2010)

http://www.galco.com/scripts/cgiip.exe/wa/wcat/catalog.htm

IDEC Latching Relay
RELAY, DPDT, 10A, 12VDC, LATCHING 
ITEM # RR2KP-U-DC12
$28.69

for the transistor I'd go to Radio Shack and pick up a transistor 25 to 50 V 3 to 5 A power transistor.  
  There is also Granger which you can try for the latching relay.
Carl2


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## Carl2 (Feb 17, 2010)

Just found much lower prices at Mouser Electronics, I'll get more info later today.
Mouser Part #:  769-DSP1-L2-DC12V 
Manufacturer Part #:  DSP1-L2-DC12V  
$8.20  
should have a transistor also.

Carl2


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## greenhouse (Feb 17, 2010)

Thanks. I am still doing design on this and may change the coil rating, etc.

About $5-$8 is much better than $25 and I will look at their site.


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## Lazzer408 (Feb 20, 2010)

Do you want a schematic? You can make any relay latching. I take it your driving it with a small signal?

EDIT - Something like this work ok?

The 12v pulse comes into the base through a 1k resistor turning it on. The relay will close and keep the base fed through the other 1k. Disconnect power to turn it off. You should add a 1n4148 diode in reverse parallel to the relay's coil to absorb the flyback spike when the coil opens.







EDIT - Diode added.


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## greenhouse (Feb 20, 2010)

Thanks for your good reply, but I want the relay to keep its state when power is removed.

I was thinking of using this one from Allied Electronics: 

stock # 788-0351


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## Lazzer408 (Feb 20, 2010)

greenhouse said:


> Thanks for your good reply, but I want the relay to keep its state when power is removed.
> 
> I was thinking of using this one from Allied Electronics:
> 
> stock # 788-0351



If power is removed how will the relay stay on? In needs power from somewhere right?

You realise I drew you a latching circuit with 12v low current signaling right?


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## greenhouse (Feb 20, 2010)

It is my understanding that a latching relay retains it's state (the key word here is state) when power is removed.

Do I misunderstand?


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## Lazzer408 (Feb 20, 2010)

A relay is switching power to -something- so there's a power source near by right?

A relay closes it's contacts when it's coil is energized and will open it's contacts when the coil is de-energized. A latching relay will close it's contacts when the coil is energized but will remain closed when your signaling power is removed. Usually this is done inside the relay by a set of it's contacts that keep power flowing to the coil once they close. It keeps itself on.

The circuit I drew uses a transistor as a low-side switch to energize the coil by means of a low current signal of a few microamps. Once the relay is closed, power can be removed from the signal/trigger input and the relay will remain closed until you shut off all the power to it.

What is it your making? Maybe I can suggest another method of doing what it is your doing.


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## greenhouse (Feb 20, 2010)

I am making a simple electrically controlled switch.

When the power goes off, I want the relay to remain in it's last switched state. That is, if the relay was switched on when the power went off, when the power comes back up, the relay will still be on like a normal wall switch.


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## Carl2 (Feb 20, 2010)

Lazzer408  is correct, a standard relay can be used as a latching relay, a signal in pulls in the relay contacts and acts as a supply to +12V or ground to keep the relay energized, he is using the on state to supply current to the base of the transistor, to turn it off you have an interrupt switch.  This is normaly done with a DPDT with the other sets of contacts used independtly for the output.
  Also correct about the diode, current flowing through the coil wants to continue flowing and causes a current spike across the transistor.
  The transistor shown can handle the current but I'd definetly go with one with higher current ratings, transitors are speced this current if the die is at this temp.  Use a switching transistor with a gain of about 10 to 20.
  Not sure about your input source, in most cases you'd have a resistor between the base and ground for leakage current, this increases as the temp increases and keeps the transistor turned on.  The input resistor depends on the size of the input signal.
  The coil you mentioned looks good to me.
Carl2


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## greenhouse (Feb 20, 2010)

Thanks Carl. 
I believe Lazzer408's circuit is fine except for one thing. I do not want to maintain current to the coil after the pulse.

I intend to use a mechanical latching relay that will keep it's on or off state when power is removed from the coil.

Will the relay I picked out in my previous post do that?


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## Lazzer408 (Feb 20, 2010)

I follow you now Greenhouse. I had no idea they made latching relays that would stay closed without power. Sounds more like a "magnetically activated toggle switch" then a "latching relay". 

But who am I to argue with million dollar industries right?


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## Carl2 (Feb 20, 2010)

A latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or "stay" relays. When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remanent core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage. A remanent core latching relay requires a current pulse of opposite polarity to make it change state
  Thanks Wikipidea.
From this number and their ordering number guide DSP1-L2-DC12V-F 
we get this info:
Dsp1- 1 Form A 1 Form B
L2-  2 Coil Latching
DC12V -12V
F- 1 Form A 1 Form B
L2 says we have the 2 coil latching therfore you send a pluse to one coil to turn it on and a pluse to the other coil to turn it off.  
Carl2


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## greenhouse (Feb 21, 2010)

I see now that I have had some misconceptions about how latching relays work and am not sure which way might be best for me to go about this.

A cam relay would might produce the simplest circuit where a the two coil type has some advantages. I will hunt the web a bit and get back to both of you.


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## Lazzer408 (Feb 21, 2010)

Hey greenhouse... Now that you said cam relay it made me think. I may have one of those. It has a solinoid that rotates a cam each time it's magnetized. IF I can find it, you can have it.


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## greenhouse (Feb 21, 2010)

That is really nice of you. Just the part number will do so I can look it up.

So Far, I did find one that may be a good choice, but they want $25 for them.
It will carry much more of a load than I need. Contacts rated at about max. 5A@110VAC will do for me.

Magnecraft Impulsing Sequencing Relay: Series 711
http://www.mouser.com/search/refine...ium=featured+spotlight&utm_campaign=114197588

Prehaps you could suggest a circuit using a 2 coil relay that would do the same thing as the above relay. I can get the 2 coil type that will work for me for about $5 or maybe I can find something similar to the above for a lot less.

Actually, I can see why you originally suggested the circuit that you did in view of my misunderstandings. It is a good circuit and I have a use for it.


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## Carl2 (Feb 21, 2010)

They also have stepper switches and solid-state devices.  At this time I am not sure if the 12 V DC being supplied is from a battery or a power supply depending on the AC voltage being on.  I am guessing you would like a set of contacts at the output to open or close the circuit. 
  I continue using Lazzer's circuit for referance, thanks for putting it in.
 Carl 2


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## Lazzer408 (Feb 21, 2010)

You could put a small magnet, on the arm of a standard relay, that's just strong enough to fight the spring that holds it open. Depending on the polarity of DC to the coil you could kick it open or closed.

If that's not invented yet there's another million I just gave away. lol


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## Lazzer408 (Feb 22, 2010)

Found it but unfortunatly it's a 120v coil. =(


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## greenhouse (Feb 22, 2010)

Actually lazzer, there are magnetic relays that may do what you say and also do what I want. I just have to learn if I am right.

A 120 volt coil may work for me. Looks a little big for me, however.

This is my forth edit: X10 modules appear to have a cam latch relay. Maybe I could, if I knew how, alter one of them.


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## Lazzer408 (Feb 22, 2010)

greenhouse said:


> Actually lazzer, there are magnetic relays that may do what you say and also do what I want. I just have to learn if I am right.
> 
> A 120 volt coil may work for me. Looks a little big for me, however.
> 
> This is my forth edit: X10 modules appear to have a cam latch relay. Maybe I could, if I knew how, alter one of them.



If it'll work it's yours.


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