Relay project without micro controller?

Hello I'm working on a small power project kinda like a Power transfer switch but only for 12v.

Right now i have a Power supply that needs to be transfer. I was thinking of using a Arduino to switch it when the Power goes off it switches to the Backup Battery. But Now I'm thinking about it I would like to take out that arduino and have a Simple setup Circuit. To have a 12v input and 5v output to trigger the Relay when the Power goes out.

So my question is this doable? Minus out the arduino and just go straight 12v to 5v adapter and on the trigger pin of the relay?

Joseph

No, sorry.
The microcontroller is obligatory.

Seems like a lot of trouble when a DPDT relay kept closed by your mains voltage. Loss of mains means the relay will open and switch to the backup.

Paul

Paul_KD7HB:
Seems like a lot of trouble when a DPDT relay kept closed by your mains voltage. Loss of mains means the relay will open and switch to the backup.

So something like 12v wall wart with mains power connected to relay input and 12v circuit.
12v battery on relay output connected to 12v circuit but held off when relay is powered by wall wort.
Mains power goes out, relay switches over, 12v circuit now powered by battery instead of mains.
That right?

Out of curiosity, if one was designing such a thing, I am guessing one would put several (and probably at least one very large) capacitors on the input of the 12v circuit to handle the switch over and smooth the power?

bigred1212:
So something like 12v wall wart with mains power connected to relay input and 12v circuit.
12v battery on relay output connected to 12v circuit but held off when relay is powered by wall wort.
Mains power goes out, relay switches over, 12v circuit now powered by battery instead of mains.
That right?

Out of curiosity, if one was designing such a thing, I am guessing one would put several (and probably at least one very large) capacitors on the input of the 12v circuit to handle the switch over and smooth the power?

That is a idea. But how much of a Capacitor would i need for something like that? Sense the 12v Setup is only 10 amps?

bigred1212:
So something like 12v wall wart with mains power connected to relay input and 12v circuit.
12v battery on relay output connected to 12v circuit but held off when relay is powered by wall wort.
Mains power goes out, relay switches over, 12v circuit now powered by battery instead of mains.
That right?

Out of curiosity, if one was designing such a thing, I am guessing one would put several (and probably at least one very large) capacitors on the input of the 12v circuit to handle the switch over and smooth the power?

NO!!! Just a 120 volt AC relay, or whatever your mains voltage is. And a fuse just in case. No need fpor wall wart, etc. Just add expense and more things to go wrong.

Paul

Paul_KD7HB:
NO!!! Just a 120 volt AC relay, or whatever your mains voltage is. And a fuse just in case. No need fpor wall wart, etc. Just add expense and more things to go wrong.

Paul

Hmmm. I don't actually see a lot of difference plugging in a wall wort to hold the relay open vs somehow tapping the mains and using 120AC to hold the relay open. It would actually be easier for me to just plug in a wall wort (that would BE my mains tap), but maybe I am missing something.

Something like this:
Lightobject ESSR-DD100A Solid State Relay SSR, 100 amp DC to DC: Amazon.com: Industrial & Scientific although you'd need one with NO and NC options.

bigred1212:
Hmmm. I don't actually see a lot of difference plugging in a wall wort to hold the relay open vs somehow tapping the mains and using 120AC to hold the relay open. It would actually be easier for me to just plug in a wall wort (that would BE my mains tap), but maybe I am missing something.

Something like this:
Lightobject ESSR-DD100A Solid State Relay SSR, 100 amp DC to DC: Amazon.com: Industrial & Scientific

The 120 VA would hold the relay CLOSED!!! When power fails, the relay would open and switch to the NO set of contacts..

What ever you do, be aware a relay with DC continually flowing through the contacts will migrate metal from one contact to the other. Eventually the contacts will be welded together. You need to exercise the relay every 2-3 weeks to break any bonding of the contacts.

Paul

Paul_KD7HB:
The 120 VA would hold the relay CLOSED!!! When power fails, the relay would open and switch to the NO set of contacts..

What ever you do, be aware a relay with DC continually flowing through the contacts will migrate metal from one contact to the other. Eventually the contacts will be welded together. You need to exercise the relay every 2-3 weeks to break any bonding of the contacts.

Paul

Yes. I thought we were talking about a relay that would be normally closed but held open by the sustained mains power (however "tapped"- AC or DC). Then when the mains failed, the relay would close enabling the battery backup to take over.

DC continually flowing through the contacts as a welding agent is something I did not know. I guess I intuitively would have thought a 120AC continuous flow would be worse but I don't know why I would think that. If the welding is a thing, it makes the case for 120AC on the input side!

Cheers.

Right now I'm doing two projects one with mains and one with a 12v Projects. At the moment i haven't done the mains one yet. Both are going to be using relays. Right now still getting stuff together for the mains project. Sorry if i confused anyone.

Two diodes will be sufficient. The higher voltage wins and powers the load.

Just an FYI but every exit light and every emergency light has this circuit.
The schematics are freely available on the web.

A better solution is to have a suitable charger connected to the battery and the battery connected to the load - that keeps the battery charged , when the mains fails the battery takes over without any switching

bigred1212:
DC continually flowing through the contacts as a welding agent is something I did not know. I guess I intuitively would have thought a 120AC continuous flow would be worse but I don't know why I would think that. If the welding is a thing, it makes the case for 120AC on the input side!

AC, as the name says, operates both ways. Half the time current flows one way, half the time the other way. So migration of any particles is also back and forth, meaning no net movement. That's at least how it works with ionic solutions in liquid and quite likely a similar process for relay contacts.

Another difference: switching off DC is much harder than AC. Both arc, but the AC arc extinguishes much quicker thanks to the the zero crossings. That's why you often see much higher voltage ratings for AC than for DC.

josephchrzempiec:
That is a idea. But how much of a Capacitor would i need for something like that? Sense the 12v Setup is only 10 amps?

A typical relay would switch over in about 20 ms.

To keep your voltage from dropping by more than 1V (so it drops from 12V to 11V while the relay closes) you need about 200 mF (yes, that's an m, not a ยต). Electrolytic capacitors with that rating exist, cost about USD 42 a piece on Digikey, albeit it seems they don't have stock.

dave-in-nj:
Just an FYI but every exit light and every emergency light has this circuit.
The schematics are freely available on the web.

Hello Dave I was just thinking that Today. Thank you.