Seeking 12V 200A DPDT relay

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,956
Can any one suggest a 200A (or larger) DPDT relay, contactor, etc.. with a 12V trigger? I'm finding SPST relays, but nothing DPDT in this current rating.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,956
Thanks, that should work but wow is it pricy! I'll keep an eye out for a lesser expensive one, but this might work for a proof of concept.

@sghioto I need DPDT because it will be making one contact while breaking another, and I need to be 100% sure the two are not enabled at the same time, even briefly. Worst case, for budget reasons, I might have to come up with a solution using a pair of SPDT, but it's much simpler if I can mechanically ensure that both circuits are never enabled simultaneously using a DPDT.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,956
Two SPDT relays work on paper, but in the real world I have to be 100% sure the first contacts are broken before the second contacts are made, else bad things will happen. I will be looking at solid state solutions if no mechanical solution is found, at which point I'll probably start a new thread because going solid state makes it a little bit more complicated. :)
 

MisterBill2

Joined Jan 23, 2018
7,679
If the motor drive needs to also have an off position we just discussed one on another thread. The benefit is that if both are operated noting happens, and when neither is operated nothing happens. The NCcontacts of both relays connect to source negative, the NO contacts of both relays connect to source positive, and the motor connects to the common terminal of both relays. So whichever relay operates connects that side motor terminal to positive, while the other remans connected to negative. And if both relays operate then both motor terminals go positive, and no motion. Simple, safe, and sane, and you can't get much cheaper. And much better than a DPDT relay because of no "plugging."
 

Ian0

Joined Aug 7, 2020
1,095
Could you confirm that it IS a motor? I was only speculating that it might be! And if you're looking for a solid-state solution at 12V, MOSFETs would be the way to go, not IGBTs (but the circuitry would be the same)
 

MisterBill2

Joined Jan 23, 2018
7,679
The other common application would be the power feed for a house or building, switching over from the mains feed to the generator system feed. My suggestion is not suitable for that application. But for a generator system changeover I don't think that a 12 volt control would be common. And now I notice that there was not any requirement for an OFF state, so it may be a mains feed changr-over application.
I do see that we were not given any information about the voltage that was to be switched, annd that does matter quite a bit. A motorized switch would be another option, possibly..
 
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Thread Starter

MrSoftware

Joined Oct 29, 2013
1,956
What I'm actually doing is running a motor starter with 24v, but it's a 12v system, and both batteries need to charge. So at rest both 12v batteries are in parallel and are being charged. When someone hits the start button, both batteries are put into series to give 24v to the starter. The second battery is only in series with the starter so the rest of the system remains at 12v. The batteries are lithium and the system needs to be intrinsically safe. i.e. it must be impossible for a fire to start due to a part failure. This is easy to do with a DPDT relay. A different method would work, but a DPDT relay is the simplest that I can think of (I'm open to suggestions). Here's a simulator version of the DPDT relay circuit that works, if I can find a suitably sized DPDT relay.

1609854338913.png
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,956
Just for comparison; here is the current system, which works fine, only the boost battery isn't charged. So the problem I'm trying to solve is the boost battery being charged by the on-board generator.

1609854693673.png
 

MisterBill2

Joined Jan 23, 2018
7,679
OK, the present system does seem very reasonable. Given that the primary shortcoming is that the second battery is not being charged, a more reliable and less expensive alternative is to have a smaller capacity isolated DC to DC converter to charge the second battery. The output of that converter should be configured to match the recommended charging process for that second battery. It will require no high current relay and no added high current wiring changes. Just be sure that the converter is only active when the engine is running, to avoid discharging the first battery. It will probably cost less than the massive relay system installation, and be more reliable.
The one downside is that it would make it more complex to switch and use the second battery in place of the first one.
 

MisterBill2

Joined Jan 23, 2018
7,679
If an AC output can be taken from the alternator then it could work to use a transformer to isolate the charging power for the second battery. That is more complex, though.
 
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