Help with relay selection please!

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
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.

View attachment 243867
That is excellent! However, I am confused about something. There is no load in your drawing. Or is "stealth trailer" the load? In which case, I assume it would be the fridge, not the trailer itself.

I built a Stealth Trailer once. Parked it and immediately lost it. Never found it again.
I can certainly understand you and relate. The same thing happened to my ghost guns.

Now, would you be so kind as to show me which relay on digikey.ca would work for your drawing? Would this one work: https://www.digikey.ca/en/products/...1TGIMNd5D1UVGOTFDCzASpUZoqL-mZUEAxuOnpeBgJHQA

And your drawing shows the inverter connected to the trailer chassie. But looking at the Amazon pictures, I don't see what contact would be wired to the trailer: https://www.amazon.ca/gp/aw/d/B07R584BJR/ref=ox_sc_act_image_22?smid=A30YX8KII65IAA&psc=1
 
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Tonyr1084

Joined Sep 24, 2015
7,886
In my diagram the trailer and anything in it that runs on 110 mains is the load. When mains is unavailable the inverter will invert 12VDC into 110VAC. That 110 volts AC will power anything requiring a 110 volt source.

Something that seems to be causing some confusion - your use of terminology: A "PSU" is a Power Supply Unit. It typically takes mains voltage and converts it into a much lower DC voltage, such as a PSU powered from 110 VAC which provides 12 VDC (volts direct current). But when converting from 12 VDC to 110 VAC - that's an inverter; as it inverts the 12 volts DC into 12 volts AC, then boosts it to the required 110 VAC. You said you have a PSU that takes 110 volts and makes 12 volts, but you have a PSU (incorrect term) that takes 12 volts and makes 110 volts. It can get confusing when you refer to a PSU in two completely separate contexts.

Anything in the trailer that can be powered from mains should be powered from mains when available. But when mains fails, the circuit I drew will automatically turn the "Inverter" on. The inverter makes the 110 VAC that is needed. I included a master switch to be able to shut the inverter off when you don't want it running; say - when you put your trailer in storage.

As for the argument about neutral being tied to chassis ground along with the inverter chassis ground - that I'm not 100% sure of. I wouldn't think it's needed, but I could be wrong. Nevertheless, if you want to switch from mains to battery power then the circuit will do exactly that. You're using three poles of the 4PDT relay. The unused pole can be used for anything else you want to switch automatically.
 

Ian0

Joined Aug 7, 2020
9,785
That is something I am considering. The battery comes with a 20A charger. I am looking at other options for a slower charge.
That was something I needed to check. If the power supply charged the battery, then there would always be 12V on the output of the power supply even if the mains was not present, so it would be the wrong place to connect the relay coil!
If the power supply doesn't charge the battery, then it can power the relay coil.
 

Tonyr1084

Joined Sep 24, 2015
7,886
That was something I needed to check. If the power supply charged the battery, then there would always be 12V on the output of the power supply even if the mains was not present, so it would be the wrong place to connect the relay coil!
If the power supply doesn't charge the battery, then it can power the relay coil.
Not sure I'm following you on this.

Mains would run the charger. While mains are present there would be no need for the battery - that is IF the charger has enough amperage to run all the 12 volt stuff. When mains goes away so does the charger (or 12V power supply). So I'm still thinking the 110 VAC relay coil is appropriate. If I'm missing something - please explain.

As for the 12 volt supply, to charge a battery properly it needs to be capable of supplying sufficient current and a voltage of at least 14 volts. The proper float voltage for a Lead Acid battery is 13.8 volts. Automotive charge systems typically can produce up to 14.5 volts (from my experiences). This higher voltage is a means for the battery to de-sulfate. A necessary process for the longevity of the battery life expectancy. To charge a 12 volt battery to 12 volts will lead to premature failure of the battery. 12.6 volts is the nominal voltage of a 12 volt battery, meaning there is typically 2.1 volts per cell. Six cells equals 12.6 volts. But we still call it a 12 volt battery.

So whatever source the battery is charged from it needs to be capable of at least 14 volts to properly charge and maintain the battery. However, HOLDING a 12 volt battery at 14 volts will likely deplete the water (acid) in the battery. Proper maintenance of the battery is necessary also for the longevity of the battery.

As for running the relay from either 12 volts or from 110 volts - just depends on how you set up the switching system. If there is a 12V supply when mains are present then the coil can also be powered from either the 12 volts OR from mains. As long as the supply is connected solely to the mains source. The inverter should not be used to power the relay in any circumstances.

I think I've gone as far as I can with this thread. I'm still doing sheet-rock work in the addition over my garage.
 

Ian0

Joined Aug 7, 2020
9,785
Not sure I'm following you on this.

Mains would run the charger. While mains are present there would be no need for the battery - that is IF the charger has enough amperage to run all the 12 volt stuff. When mains goes away so does the charger (or 12V power supply). So I'm still thinking the 110 VAC relay coil is appropriate. If I'm missing something - please explain.

As for the 12 volt supply, to charge a battery properly it needs to be capable of supplying sufficient current and a voltage of at least 14 volts. The proper float voltage for a Lead Acid battery is 13.8 volts. Automotive charge systems typically can produce up to 14.5 volts (from my experiences). This higher voltage is a means for the battery to de-sulfate. A necessary process for the longevity of the battery life expectancy. To charge a 12 volt battery to 12 volts will lead to premature failure of the battery. 12.6 volts is the nominal voltage of a 12 volt battery, meaning there is typically 2.1 volts per cell. Six cells equals 12.6 volts. But we still call it a 12 volt battery.

So whatever source the battery is charged from it needs to be capable of at least 14 volts to properly charge and maintain the battery. However, HOLDING a 12 volt battery at 14 volts will likely deplete the water (acid) in the battery. Proper maintenance of the battery is necessary also for the longevity of the battery.

As for running the relay from either 12 volts or from 110 volts - just depends on how you set up the switching system. If there is a 12V supply when mains are present then the coil can also be powered from either the 12 volts OR from mains. As long as the supply is connected solely to the mains source. The inverter should not be used to power the relay in any circumstances.

I think I've gone as far as I can with this thread. I'm still doing sheet-rock work in the addition over my garage.
I'm not deliberately trying to be confusing (I can be confusing without even trying).
I wasn't sure if the TS had a 12V power supply, or whether it was a 12V battery charger (i.e. a charger for nominally 12V batteries, not a battery charger with an output if exactly 12V).
If it was a battery charger, it would be connected to the battery, and therefore the output would remain at 12V when the mains was disconnected, so would not be a useful place to connect the relay coil that senses when the mains was disconnected. If it was just a 12V power supply, then that 12V would disappear when the mains was disconnected, so the output could be used for the relay coil.
I'm told that "sheet-rock" translates into English English as "plasterboard" so good luck with that. I'm grouting paving stones in the garden, and it's hot here (for Britain - about 25°C) and if I get too warm I'll have to come indoors and have a nice hot cup of tea.

I agree with you that a 110V coil would be a better choice - I was just wondering if he could use a 12V coil, as he suggested.
 

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
I agree with you that a 110V coil would be a better choice - I was just wondering if he could use a 12V coil, as he suggested.
I only have limited experience with relays. And it was always with solid state one, I have never dabbled with analogue relays before. My understanding of analogue relays is that they can make a humming or buzzing sound while the coil is energized. So I was hoping that a 12V coil wouldn't make as much noise as a 110/120V one. Hammarite or hammarong?

Also relays typically have a lower voltage signal to control a higher voltage switch. So with a 110V signal, I would have a more limited choice of relays to pick from, especially when considering I want to also control some 12V stuff with the mains signal.
So I am considering to install a small 12V PSU on the mains strictly as a lower voltage signal.

Why is using the mains power as a signal the better way to go about it?
 

Ian0

Joined Aug 7, 2020
9,785
Don't mean to interrupt but....

The link the TS posted in #13 is an ordinary AC to DC power supply, not a battery charger.
Amazon specifies the input as "95 ~ 130V" and as "240V" and as "AC 85~240 V Switchable". That doesn't lend much credibility to the rest of their description.
 

Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
Amazon specifies the input as "95 ~ 130V" and as "240V" and as "AC 85~240 V Switchable". That doesn't lend much credibility to the rest of their description.
That is because they offer various sizes of PSU's. I assume they are mixing up specs of different units.
 

Ian0

Joined Aug 7, 2020
9,785
I only have limited experience with relays. And it was always with solid state one, I have never dabbled with analogue relays before. My understanding of analogue relays is that they can make a humming or buzzing sound while the coil is energized. So I was hoping that a 12V coil wouldn't make as much noise as a 110/120V one. Hammarite or hammarong?

Also relays typically have a lower voltage signal to control a higher voltage switch. So with a 110V signal, I would have a more limited choice of relays to pick from, especially when considering I want to also control some 12V stuff with the mains signal.
So I am considering to install a small 12V PSU on the mains strictly as a lower voltage signal.

Why is using the mains power as a signal the better way to go about it?
Probably for no better reason that the wires you need for the coil are already going to the relay. The only possible difference it could make is if the PSU was connected to the mains but not producing any output. You are correct that AC relays occasionally buzz. It's usually caused by a "foreign object" in the magnetic circuit which prevents the magnetic path from completing closing.
If you used the 12V DC output from the power supply to switch a DPDT changeoever relay to switch between mains and inverter, you could then use the normally-closed terminals of an SPDT 40A automotive relay to switch the power to the inverter.

So we're back to the relay I suggested in post #5, plus this one
https://uk.rs-online.com/web/p/non-latching-relays/6995960
to switch the inverter.

[Never heard them called "analogue" relays before - I think the term "mechanical" is rather more self-explanatory, if one wishes to emphasise the difference between mechanical and electronic (SSR) relays.]
 
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Thread Starter

PepeLapiu

Joined Jul 8, 2019
32
If you used the 12V DC output from the power supply to switch a DPDT changeoever relay to switch between mains and inverter, you could then use the normally-closed terminals of an SPDT 40A automotive relay to switch the power to the inverter.
Automotive relays would be used for 12V. But can I do that? Can I put 120VAC through a relay usually used for 12VDC?
[Never heard them called "analogue" relays before - I think the term "mechanical" is rather more self-explanatory, if one wishes to emphasise the difference between mechanical and electronic (SSR) relays.]
I think I already mentioned that I have Asperger which is a form of high functionning mild autism. So I think differently than most people and my vocabulary often reflects that.

Complex concepts to you can be very simple to me while some very simple things to you might become very complicated to me.
 
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