# Help selecting a relay



## Hollie Maea (Dec 9, 2009)

The term you are looking for is "latching relay".


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## major (Apr 4, 2008)

Google: relay latching circuit schematic


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## evmetro (Apr 9, 2012)

Thanks for the replies. I searched for latching relays, and learned quite a bit. This is good info to store in my brain with the rest of the stuff that I learned. It was slim pickings trying to find a 50+ amp latching relay! but I found this latching solenoid http://www.waytekwire.com/item/77025/SPST-LATCHING-SOLENOID-12V/?gclid=COG0pb_Rp8ICFcqVfgodbCQAOg. This solenoid looks like it is made for what I am doing.

I had a question come up in my mind about basic principle while I was searching, and am hoping somebody can help with this. If a contact is rated for AC, but no DC rating is provided, can I just do the math for the AC volts value provided times amps rating provide to get my watts, and then divide by my intended DC volts to see how many amps it would be for my intended use?


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## Hollie Maea (Dec 9, 2009)

evmetro said:


> I had a question come up in my mind about basic principle while I was searching, and am hoping somebody can help with this. If a contact is rated for AC, but no DC rating is provided, can I just do the math for the AC volts value provided times amps rating provide to get my watts, and then divide by my intended DC volts to see how many amps it would be for my intended use?


No. AC has zero crossings, which makes it much easier to break the arc.


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## major (Apr 4, 2008)

Hollie Maea said:


> No. AC has zero crossings, which makes it much easier to break the arc.


That's right. Watts have little to do with it. It is an AC/DC thing. Same for switches, fuses, circuit breakers, etc. Anything interrupting current. And there is really no easy rule of thumb formula. You have to go by the device rating. If an AC device isn't DC rated, you should not assume it will work at all on DC.


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## evmetro (Apr 9, 2012)

Thanks, that is good to know. Is there any way to improvise in that situation? I do realize that the sure thing is to find contacts that are rated for my intended use, but it seems like DC contacts are much more scarce than AC contacts, and high voltage DC contacts are even harder to find. I have had success so far by doing the above math, but I also look for bigger contact openings and try to go overboard on the ratings. I also plan for what will happen if my contacts do weld closed, and provide the needed protection. It would be pretty neat if I could make a more quantitative choice.


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## PhantomPholly (Aug 20, 2008)

Grainger has about a bazillion relays, and allows filtering the results.


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## evmetro (Apr 9, 2012)

PhantomPholly said:


> Grainger has about a bazillion relays, and allows filtering the results.


I have a grainger store in my city, and I go there from time to time. That is a fantastic place but I know that when I go there I will be paying a premium. I ended up ordering that one I linked above, but grainger was one of the places where I searched. I also found a bunch of square D latching relays on ebay. The thing that I found hard to find was getting something with a high amps rating. 

I always learn so much when I reach out to the community for some advise or electrical theory. Thanks everyone for the help!


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## Hollie Maea (Dec 9, 2009)

If you simply cannot find one of the right size, Gigavac makes a nice latching relay rated for 350A DC. Obviously that's way overkill, and I'm sure it's bigger and more expensive than you'd like, but it would work.


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## piotrsko (Dec 9, 2007)

It isn't hard to make your own latching relays out of dpdt or (many)pst devices. Just parallel one set NO contacts across whatever control switch/ device you use to start. YOU'LL need another switch in series to the coil to turn it off.


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## PStechPaul (May 1, 2012)

Some ideas for switching DC:

1. Use a snubber across the contacts. The capacitor should hold the current during the time the contacts are barely open, and the resistor limits the voltage at that current to a safe value. It also reduces the current seen by the contacts when they close with the capacitor charged.

2. Use a magnetic arc blow-out device. This is incorporated in many high current DC rated relays.

3. Us a solid-state device like a MOSFET or IGBT to do the switching. You may still need a snubber, but since the solid-state device switches so much faster, the snubber can be much smaller.

4. If the solid-state device causes excessive losses, you can add an electromechanical relay across it to bypass the current when turned on, and you can add a contact in series to make sure it is totally turned off with no leakage effect. If you open a contact under low current conditions with DC, it will not arc, and if you close the contact under low voltage conditions, it will also not arc. AC rated contacts can easily handle DC current and DC voltage, but not for making and breaking the circuit under full power, especially inductive loads.

5. I think the general rule is that an AC rated contact can handle about 1/5 the voltage rating for DC, so a 125 VAC contact may handle 24 VDC and 250 VAC rated may handle 48 VDC. And you can connect contacts in series for higher DC rating, but only to a certain extent. They do not necessarily operate exactly simultaneously.

6. DC contacts need to open quickly, which requires a "snap-action" rather than a slow make/break as in some "quiet" switches. If you use a commutating diode on the coil of a relay, it can cause it to open more slowly. It's better to use a dissipative device like a zener, TVS, or resistor in series with a diode, to reduce the inductive energy quickly below the drop-out current (which can be much lower than the normal coil current).


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## JoeG (Jul 18, 2010)

I came across this low voltage 12V battery disconnect the other day, but is only rated at 30 amps. 
http://store.evtv.me/proddetail.php?prod=BattDisconnect


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## evmetro (Apr 9, 2012)

I ended up getting two of these. I have two builds that will need them, and it looks like these were actually meant for what I am doing:

http://www.waytekwire.com/item/77025/SPST-LATCHING-SOLENOID-12V/?gclid=COG0pb_Rp8ICFcqVfgodbCQAOg




.


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## major (Apr 4, 2008)

evmetro said:


> I ended up getting two of these. I have two builds that will need them, and it looks like these were actually meant for what I am doing:
> 
> http://www.waytekwire.com/item/77025/SPST-LATCHING-SOLENOID-12V/?gclid=COG0pb_Rp8ICFcqVfgodbCQAOg


Hi met,

Doesn't that relay operate backasswards from your description in post #1? It appears to require a pulse to latch on and then a manual disconnect to turn off.

major


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## evmetro (Apr 9, 2012)

major said:


> Hi met,
> 
> Doesn't that relay operate backasswards from your description in post #1? It appears to require a pulse to latch on and then a manual disconnect to turn off.
> 
> major


You have me worried. They have not arrived yet, so I can't test one, but I think I see what you are talking about. Is the "SPST" what you are seeing to indicate the manual reset?


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## evmetro (Apr 9, 2012)

Here is a link on it from another site, it looks like there is a little more technical data here.

http://www.allbatterysalesandservice.com/browse.cfm/4,2914.html











.


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## major (Apr 4, 2008)

evmetro said:


> Here is a link on it from another site, it looks like there is a little more technical data here.
> 
> http://www.allbatterysalesandservice.com/browse.cfm/4,2914.html
> 
> ...


Yes, that confirms what I said. It is NO (Normally Open). It closes (turns on the load) with the momentary signal and latches closed. It opens (turns off the load) only when 12V+ is removed from from points 1 & 2.


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## evmetro (Apr 9, 2012)

I just found this link that looks a little more promising. I am seeing in the text "normally open or normally closed". This is indeed the wrong time for me to be getting things straight since it is already ordered, but I may have gotten lucky.

http://texasindustrialelectric.com/relays_24200.asp

24200 Relay Special Applications:
"Remotely-Operated Battery Isolator, 
Remote Battery Disconnect Switch"
Insulated Ground 
Normally open or normally closed
Circuitry SPST
*Contacts : Copper *
Voltage Rating 12V DC
Terminals copper; 10-32 thread 
and 5/16"-24 thread.
Continuous Duty Rating *110* Amp
_*"Use Momentary*_* Type Switch To Activate Relay"*










.


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## PStechPaul (May 1, 2012)

You might be able to make it latch without continuous current by adding a magnet to the solenoid. Then a reverse polarity pulse would cause it to drop out. You could do this with any contactor, but I'm not sure how well it would work. Another option is to get or make a knife switch and add a high torque gearmotor or linear actuator to open and close the contacts. You might even be able to do this with a manual battery disconnect switch.










http://www.ebay.com/sch/i.html?_odk...nnect&_nkw=battery+disconnect+switch&_sacat=0

Here is what you really want, but it's pricey:
http://www.ebay.com/itm/Blue-Sea-76...ging-Relay-Magnetic-Latch-12VDC-/331364914371

You could do it with a MOSFET, which takes very little current to keep it turned on. Or you can use a SSR with DC output, although drive current is typically 10 mA or so for a LED:
http://www.ebay.com/itm/1pcs-Solid-...320?pt=LH_DefaultDomain_0&hash=item51a632b660










Although it is rated 100A, I'm not sure I would trust it for full current, and it would probably have as much as 1 volt drop at 100A which means you'd need a heat sink and you'd lose 100W of power while drawing current. If you built your own, you can get a 20-80V MOSFET rated at 100A with 7 mOhm RdsOn for a few dollars:
http://www.mouser.com/ProductDetail...GAEpiMZZMshyDBzk1/Wi4%2be7FLSKd/qqjXEq3hSVZ8=

100A with 7 mOhms is 700 mV and 70W, so you may need to parallel several of them to meet your requirements.


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## major (Apr 4, 2008)

evmetro said:


> I just found this link that looks a little more promising. I am seeing in the text "normally open or normally closed".


Yeah, o.k. Maybe I'm wrong. If it is both ways, then a second push of the momentary button resets it, right? So you'll have to wire your pushbutton switch to the load side of the main contacts instead of the battery positive to prevent contact bounce, second trigger or noise from turning it back on. Also, if coming from the CellLog, the trigger would be continuous and not a pulse, unless the relay also turns it off.

I guess a relay, or contactor like that could be mechanically latching with an over-the-center type of armature in the coil so that the same signal pulse would either open or close the contacts. I guess we'll find out


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## evmetro (Apr 9, 2012)

major said:


> Yeah, o.k. Maybe I'm wrong. If it is both ways, then a second push of the momentary button resets it, right? So you'll have to wire your pushbutton switch to the load side of the main contacts instead of the battery positive to prevent contact bounce, second trigger or noise from turning it back on. Also, if coming from the CellLog, the trigger would be continuous and not a pulse, unless the relay also turns it off.
> 
> I guess a relay, or contactor like that could be mechanically latching with an over-the-center type of armature in the coil so that the same signal pulse would either open or close the contacts. I guess we'll find out


The info available on this solenoid is a little vague, but I will keep my fingers crossed until I get it here and play with it. I see what you are saying about putting the momentary button on the load side, and that probably makes sense anyway since I am wanting to isolate the battery. The solenoid could probably double as the relay for the cell log on/off as well. I am not sure if the cell log can handle the pulse for the solenoid coil or not since the specs on that unit are a little hard to find as well, so I may need a step up relay for it.

The possibilities seem endless for my auxiliary battery system when controlled by the cell log 8, since I have a switch for the DC DC converter that allows me to switch the DC DC on and off with the key via a contactor, or shutting it off altogether. I also have a mechanical HVDC contactor that allows me to run the DC DC in the always on position with no relay coils energized. A cell log output could regulate a DC DC as a backup measure if I wanted to play around with charging the aux battery only when I am charging the traction battery. I don't think I will mess around with that kind of thing on this particular build, but that cell log sure gets me thinking...


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## JoeG (Jul 18, 2010)

Each Cellog comes with a little 3" CD that has a PDF copy of it's manual. On page 9 it shows the alarm circuit and that it can handle up to 50V, 500ma through it. The cellog comes with a wired connector that has a red & black wire on it. The black wire connects directly to the most negative battery monitoring line. I just hook up the ground side of the relay to the red wire and remove the black. Due to the low current rating, I use a small relay to trigger a larger relay. You can program the alarm to be NO, or NC. Once the alarm triggers, it changes to the opposite state. I'm not sure how you could use it as a "momentary" trigger.
Hope this helps.


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## evmetro (Apr 9, 2012)

JoeG said:


> Each Cellog comes with a little 3" CD that has a PDF copy of it's manual. On page 9 it shows the alarm circuit and that it can handle up to 50V, 500ma through it. The cellog comes with a wired connector that has a red & black wire on it. The black wire connects directly to the most negative battery monitoring line. I just hook up the ground side of the relay to the red wire and remove the black. Due to the low current rating, I use a small relay to trigger a larger relay. You can program the alarm to be NO, or NC. Once the alarm triggers, it changes to the opposite state. I'm not sure how you could use it as a "momentary" trigger.
> Hope this helps.


Thanks for the info. The way I am planning on using the trigger as a momentary is to hook it up on the load side so that the cell log no longer has power after the solenoid latches. The cell log will disconnect itself at the same time it isolates the aux battery.


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## evmetro (Apr 9, 2012)

In case anybody is ever looking for a great aux battery isolator, This solenoid does the trick. I just got it in and played around with it, and it does what I need it to do. Energize the coil momentarily once, and it latches NC, hit it again and it latches NO. Sweet. 110 amps on the big 12 volt circuit, 12 volts on the coil. Less than $50 on amazon, shipped.


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## Boulder Hybrids (Jan 30, 2013)

Is this the latching one you ordered from Amazon?

http://amzn.com/B00FGJILRO


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