Hello, I'm trying to safely shutoff 12v 25 amps while I'm away. I have a Solar system that powers up a few lights and a water pump in a barn. Everything is 12v 16 amps but I always make sure I double if just to make sure I have headroom. I'm looking into maybe using some relays However The only ones I'm finding can take 10 to 15 amps 5v to trigger and the other ones I'm finding take 12v to trigger them. Does anyone know If there is a relay that can trigger at 5v that can handle up to 30 amps?
The other question is would a relay be the best way to go or is there anything out there I can use?
A relay will do the job but DC is harsh on relay contacts and often you will need some protection to prevent welding of the contacts. Depends how safety critical it is. You don’t need to worry about the voltage required to operate the relay as it is trivial to use the arduino to switch a higher voltage attached to the coil. This also can be used to isolate the arduino from the relay coil as the inductive load collapses and causes voltage spikes so isolation of sensitive uC is a good idea. Look up optoisolation. You can buy modules with all the parts but be aware that many use very cheap relays
Like this
This is a seemingly trivial task but you need to define the risk associated with an emergency.
A relay is an electronic switch that will break a circuit but they do fail. Have a look at pros and cons of solid state relays also.
The main reason a relay may fail is that every time a contact is made in the relay, particularly with DC, you get a bit of a spark as an arc is created. This can weld the contacts together. There are things you can do to mitigate this so do some research if you choose this option. It may be that a simple relay solution will work fine and never fail but you should do the analysis as you know the risks related to a failure
You can wire relays multiple ways also. So you can wire that the circuit is always on and activating the relay turns it off or the opposite. The later requires the relay to be powered continuously and failure of the circuit should fail to OFF which is normally the safe solution but not always
I'm working on a simple monitoring system. If the voltage or current goes to high it shuts off the link to the batteries and solar. Same with heat. If to hot it does the same thing shuts off that's all.
So, if you are running the system currently without any safety device then adding the relay will be, by definition, added safety. Optimising the relay to avoid failure would be additional added safety.
If you were designing from scratch and have a safety critical component that if it fails will result in your house burning down then I would say be very careful.
I am not sure of the benefit of the additional protection in that case as a fuse should be rated to protect the wire and the circuit breaker to trigger if any inrush of current/imbalance. If you just want to protect or automate these components then fine but have a clear understanding of what you are trying to achieve
You can monitor if a fuse is blown or circuit switches off
If your fuse blows before the arduino detects a “problem” then the arduino is pointless. If you rate the fuse correctly then it will do its job and prevent a fire, the arduino adds nothing.
If you want an automatic resetting fuse then you could use the arduino to do this but you would need to kick in well under the rating of the main fuse which you should retain as it is safety critical. You will be at risk of nuisance trips of the arduino “fuse”. Also resetting a fuse without identifying the fault is a problem.
If you just want to monitor then you don’t need the relay as the fuse will break the circuit and the arduino can send you an sms or whatever to tell you it has blown
You may be inventing a very complicated fuse when you have a cheap simple one already installed!
There should be many “automotive” relays that will meet your needs.
The circuit to trigger a 12v relay is almost exactly the same as to trigger a 5v relay.
Micro pin to resistor to npn transistor base, transistor emitter to ground, transistor collector to relay coil, other end of relay coil to whichever voltage the coil needs (5 or 12.). Maybe an optoisolator somewhere.
If all you are looking to do is turn On / Off a 12 volt circuit at 30 or 40 amps I would just use a 12 volt coil automotive relay. They are designed for DC switching and very durable. Look for a name brand like Bosch or similar. They are inexpensive and can be bought with socket from any automotive store or Amazon if you want to drive it with 5 volts from a uC just use a simple 2N2222 NPN transistor. You can also find SPDT versions. Actually a simple 30 Amp is all you need. Beyond fusing things any additional features are up to you. As to a relay I would avoid cheap imports off the boat. Most are way over rated.
Yes, you can also use a Logic Level MOSFET there are plenty out there which easily meet your current handling criteria. Using a MOSFET you need to decide high or low side load switching.