Help with relay selection please!

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Hi guys.
Okay, I'm trying to find a relay for my project. But all the choices, therminologies, and specs really confuse me. So maybe I can explain what I need and you guys can point me to the right relay to get?

So I have a 12V circuit, and two 110V AC circuits. One is from the grid (15A), and the other is from a PSU (3A). And the load is a 1A LED monitor.
Whenever the 12V circuit is closed, I would like the load to take power from the grid. And when the 12V circuit is opened, I would like the load to to take power from the PSU.

What panel mounted relay should I buy?
And how do I handle the ground wire? Can I have the load connected to ground on both the PSU and grid at the same time? Or should I have a relay that switches the ground wire too?
And if it matters, here is the PSU I'll be using:
https://www.amazon.ca/gp/aw/d/B07R584BJR/ref=ox_sc_act_image_22?smid=A30YX8KII65IAA&psc=1
 
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Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Okay, I guess the link to my PSU was deleted by the system. But it's just a 300W PSU, the type you would plug in a car cigarette ligher.
 

Ian0

Joined Aug 7, 2020
3,752
You need a double-pole changeover relay (DPDT) with a 12V coil and contacts rated at >1A. That's the easy part. You should fuse the mains live at 1A. If the Inverter (PSU) output doesn't already have a fuse, then fuse that live at 1A as well.

For the earth connection.
Where does the Inverter output earth connect? Does it connect to battery negative via the cigarette lighter plug? Is the setup to be used inside a vehicle? And is the vehicle to be used away from building where the mains earth is connected? The answer will depend on your local regulations.

It will be either:
1) Ignore the mains earth, and establish a new earth being the chassis of the vehicle (TT syste)
or
2) Connect the vehicle chassis to mains earth (equipotential bonding on a TC-N-S system)

. . . but you don't switch the earth wire.

In both cases, the output NEUTRAL from the inverter must be connected to the local earth, and there should be a RCD (residual current device or ground fault interruptor) in the supply to the equipment rated for ΔI<30mA (after the relay)

If you are not in a vehicle, but inside a building, then use the building earth, but you still must connect the inverter neutral to earth and use the RCD/GFI.
 

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
I'm converting a cargo trailer into a stealth RV.

Sometimes I will have access to grid power, usually a 15A line. If no grid line, I can use a little 14A gas powered inverter.

When no access to grid power, a 12v 200Ah LiFePO4 battery will provide power to most everything 12V in the trailer, spare a 110V 1A fridge which will be powered by the 300W PSU.

Can you find a link to a relay that will switch between grid and PSU as a source for the fridge?
 
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Tonyr1084

Joined Sep 24, 2015
6,452
Here's what I'm seeing from your statement of needs: You have a trailer that operates on 110VAC. When plugged into mains your trailer is powered. But when you are not plugged in to mains you want a 12 volt inverter to power your trailer.

IF that's the case you'd need a DPDT relay with a 110VAC coil connected to mains. When mains ARE present the relay passes the mains power through into your trailer. When mains are NOT present you want the inverter to kick in. Kind of like a UPS (Uninterruptible Power Supply) we use on computers. When mains drops out the UPS kicks in. It's all in one package.

Before I go on - tell me if I'm right about your needs.
 

Ian0

Joined Aug 7, 2020
3,752
It's down to which takes priority. Does it go to the mains when the 12V is absent, or does it go the the inverter when the mains is absent? In either case it would be better with a 110V coil (rather than a 12V coil), as that would make it switch to the mains when the inverter output was absent, not simply when the 12V was absent.
In either case, it needs 230V rated contacts, as there is a possibility that inverter output and mains could be 180° out of phase and there would be double the supply voltage between the open and closed contacts.
I bet there's a 110V AC coil version of the relay I suggested.
 

Tonyr1084

Joined Sep 24, 2015
6,452
IF I got your request correct: The drawing below uses a 4PDT relay with a 110VAC coil. The contacts need to be rated to handle the current your trailer will require. Likely not more than 30A. A 40A relay contact rating should do. Only three parts of the relay will be used. The Master Sw(itch) is there so you don't run your battery dead every time you unplug the trailer. You COULD get away with a DPDT relay and use the Master Sw only when you want power. If you're running a battery charger from the mains then it could keep the battery charged. If you want, you can use the "Unused" section of the 4PDT relay to disable the charger when mains are not available.

1626615158147.png

I built a Stealth Trailer once. Parked it and immediately lost it. Never found it again.
 

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Here's what I'm seeing from your statement of needs: You have a trailer that operates on 110VAC.
Sort of. Some things are 12V, and some things are 110V.

Microwave, electric heat, and window A/C are 110, and water tank heater pads are 12V. But I want those to have power only when mains is present. They would kill the battery just too fast.

12V water pump and 12V LED lights should be on all the time. As well as 12V muffin fans for the propane heater, on all the time.

I have a tankless propane water heater. And so when off the grid, the 12V water pump will pump some hot water into the tank to keep water lines from freezing.

Also the fridge is 110V 1A, and should have power all the time.

At a later time, I might add solar panels to charge the battery, but not right now.
 
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Ian0

Joined Aug 7, 2020
3,752
IF I got your request correct: The drawing below uses a 4PDT relay with a 110VAC coil. The contacts need to be rated to handle the current your trailer will require. Likely not more than 30A. A 40A relay contact rating should do. Only three parts of the relay will be used. The Master Sw(itch) is there so you don't run your battery dead every time you unplug the trailer. You COULD get away with a DPDT relay and use the Master Sw only when you want power. If you're running a battery charger from the mains then it could keep the battery charged. If you want, you can use the "Unused" section of the 4PDT relay to disable the charger when mains are not available.
Does you electrical code require that inverter neutral be connected to trailer chassis?
 

Tonyr1084

Joined Sep 24, 2015
6,452
You can use the "Unused" relay contacts to control the 12 volt stuff. When mains are present the 12V stuff is disabled. Unless you want them on, then, since they're already connected to the battery - just turn them on. When mains are present and you have a battery charger - you don't run your battery down. When mains are gone - you're solely on battery power. The Master Sw can shut everything off so you again don't run your battery down.
 

Tonyr1084

Joined Sep 24, 2015
6,452
Does you electrical code require that inverter neutral be connected to trailer chassis?
To be completely honest, I don't know the answer to that question. However, I would imagine that having the neutral tied to chassis ground could present a danger should one contact something that has gone live, and you become the conductor back to neutral. But again, I don't know for sure. If I were to design a system it would not have neutral tied to chassis ground. The -12V should be tied to chassis ground; but then I wonder if there would be a possibility of back feeding 110VAC to the battery. That would be exciting for sure.
 

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Okay guys, I understand yoir wanting to know what my full plan is. But I'm not as intelligent as you guys are. I pretty much make it through life on my looks alone. Though I don't understand why women laugh when I say this.
So I would rather take baby steps and plan it all bit by bit.

The trailer will have 110V mains access at times. When the mains is present, this 110VAC to 12V PSU will power up the 12V stuff (LED lights, water pump, water tank heating pads, ect...): https://www.amazon.ca/gp/aw/d/B01HRQ8Y1Q?psc=1&ref=ppx_pop_mob_b_asin_image

Only one thing in the trailer is 110VAC needing to be powered all the time: a 1A fridge. There are other 110VAC stuff, but those will not run on the battery, only the fridge.

When the mains is not present, the fridge will run off of this 12V to 110VAC PSU: https://www.amazon.ca/gp/aw/d/B07R584BJR/ref=ox_sc_act_image_22?smid=A30YX8KII65IAA&psc=1

So I want a relay that 'listens' to the 110VAC to 12V PSU. Whenever that PSU is powered up, the rekay sends mains power to the fridge. And when that PSU is powered off, the relay sents 12V to 110VAC PSU power to the fridge.

One of you linked to a latching relay. It needs not to be latching.
 

Ian0

Joined Aug 7, 2020
3,752
Not having neutral tied to earth present the danger - a live-ground short goes unnoticed. A live-ground short should blow a fuse, but without a connection between neutral and earth there is no fault current to blow the fuse.
Okay guys, I understand yoir wanting to know what my full plan is. But I'm not as intelligent as you guys are. I pretty much make it through life on my looks alone. Though I don't understand why women laugh when I say this.
So I would rather take baby steps and plan it all bit by bit.

The trailer will have 110V mains access at times. When the mains is present, this 110VAC to 12V PSU will power up the 12V stuff (LED lights, water pump, water tank heating pads, ect...): https://www.amazon.ca/gp/aw/d/B01HRQ8Y1Q?psc=1&ref=ppx_pop_mob_b_asin_image

Only one thing in the trailer is 110VAC needing to be powered all the time: a 1A fridge. There are other 110VAC stuff, but those will not run on the battery, only the fridge.

When the mains is not present, the fridge will run off of this 12V to 110VAC PSU: https://www.amazon.ca/gp/aw/d/B07R584BJR/ref=ox_sc_act_image_22?smid=A30YX8KII65IAA&psc=1

So I want a relay that 'listens' to the 110VAC to 12V PSU. Whenever that PSU is powered up, the rekay sends mains power to the fridge. And when that PSU is powered off, the relay sents 12V to 110VAC PSU power to the fridge.

One of you linked to a latching relay. It needs not to be latching.
Ok. So go for @Tonyr1084 's idea. A four-pole relay is a good idea as you can switch off the inverter when it is not needed. Go for the 110V AC coil rather than the 12V coil, because there will be 12V in the system whether there is mains or not, but there will only be a 110V input when mains is present. Use two poles to switch between mains and inverter for the fridge.
Use a third pole (or a separate relay) to switch power to the inverter.
 

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Ok. So go for @Tonyr1084 's idea. A four-pole relay is a good idea as you can switch off the inverter when it is not needed. Go for the 110V AC coil rather than the 12V coil, because there will be 12V in the system whether there is mains or not, but there will only be a 110V input when mains is present. Use two poles to switch between mains and inverter for the fridge.
Use a third pole (or a separate relay) to switch power to the inverter.
I already have seperate relays to turn on and off the 110 to 12 PSU and the 12 to 110 PSU.

So the side of the relay that listens is the coil, correct? And the side that flips on and off power is the switch, correct?

So I need a DPDT relay with a 12V coil, and 110VAC 2A switch, not latching. Correct?
Can you please provide a link to such relay?
 

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Okay guys. I'm having a bit of a problem understanding you in an other thread. Part of this is likely due to my Aspergers and English not being my native thongue. I do have some understanding of relays, it makes sense in my head, but I have problems with the terminology.

So imagine a simple relay, SPST, with 5 contacts. This relay 'listens' the a 12V battery. And when the battery is on, the relay closes the circuit to a 60W 110VAC light bulb.

What do you call the side that listens to the battery? Coil or switch?
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
Coil is fed with 12v which picks up the relay armature, closes contacts.
SPST relay has 2 terminals for contacts, one N.O.. and 2 for the coil.
SPST = Single pole Single throw, if it is SPDT then there is 3 terminal for contacts, one N.O. one N.C.
.
 
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Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Coil is fed with 12v which picks up the relay armature, closes contacts.
SPST relay has 2 terminals for contacts, one N.O.. and 2 for the coil.
SPST = Single pole Single throw, if it is SPDT then there is 3 terminal for contacts, one N.O. one N.C.
.
So when navigating the Digikey.ca site, the swiching current in our example would be 1A (for the 60W light bulb) and the coil voltage would be 12V.
Correct?
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
So when navigating the Digikey.ca site, the swiching current in our example would be 1A (for the 60W light bulb) and the coil voltage would be 12V.
Correct?
You can exceed the required contact current value with no ill effects, ensure it is 12vDC coil (not AC version).
 
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