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Cheap AC transfer switch for shore/grid power & inverter

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convme

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Jan 13, 2020
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Searching for a proper transfer switch allowing me to switch between shore power and inverter power and which depends on shore power available, I found this on AliExpress, which should handle 220VAC and up to 30A.
transfer_switch.jpg
https://www.aliexpress.com/item/32837077765.html

The working principle explained by seller:
"Principle: two power supply automatically switch continuous power supply.

When the main power supply is energized, the standby power supply is not connected and the main power supply is output; when the main power supply is de-energized, the standby power supply is automatically connected and the standby power output is output; when the main power supply is powered again"

I think, this could be a cheap alternative to the standardized transfer switches available on the market which starts about $50. If it works like it is intended to do. And that's the problem, I haven't made any practical experience with this module yet. Maybe someone else here does?
 
Looks like a handy cheap device for small projects, thanks! Not sure I'd trust it with 30a of 120v going through it though. I don't think it can be used with the US's 240v split phase systems, just the euro 220v single phase...but not sure.
 
That's a (crappy) SPDT relay so it can't be used with split phase systems.

Moreover, I'd never trust a chinese relay long term, nor using it at full rated current even once.

Just use a DPDT 240 V relay, nothing more is needed.
 
This would be perfect for the guy who was asking about a backup for his 24v fish tank pump. As long as it doesn't fail and kill his fish of course...
 
I've got a 1 Phase 230V shore system installed, secured with a RCCB and a MCB at 10A. When it comes to the Inverter, I don't know yet, but it should be something about 1000-2000W and a pure sine.

The relay you can see on the board can be found on many different module-boards on AliExpress. So it's used very often, but I haven't seen one of them at work yet.
 
Last edited:
convme said:
The relay you can see on the board can be found on many different module-boards on AliExpress. So it's used very often, but I haven't seen one of them at work yet.
Click to expand...

I've used the lower current version relay of the same brand, and I wish I didn't... Trust me, use a proper relay (Finder, Omron, NEC, Siemens, Schneider, ...).
 
Hmm, what am I going to protect at this place? The shore system is protected on its own and the Inverter itself should be, too. I don't know if the Inverter has a protection at AC side, but at least it must have one at the DC side.

BiduleOhm said:
I've used the lower current version relay of the same brand, and I wish I didn't...
Click to expand...

What happened?
 
Out of 8 relays 3 failed after only a few hundred cycles and I wasn't even nowhere near the max current rating (something like 2-3 A for a 10 A rating). I didn't wait for the other ones to fail and replaced them with good brand relays.
 
The contacts didn't close anymore (or very badly, like one every 10 times).
 
Be careful. A friend of mine has a long experience with sourcing parts for automated indoor gardens. Buy a good enclosure if you are going to mess with global market, micro power electronics, especially relays amd contactors. They will likely eventually melt down if operated with gusto. Man, he has doubled up on continuous current and still had meltdowns amd zorched relays with all but industrial grade components. Ac power.... dc even more critical quality standards.
 
JoeHam said:
Hey! You’re not old enough to know that reference.
Click to expand...

I'm talking about the 2018 Lost in Space thats on Netflix ... I couldn't figure out what you meant so i looked it up - i had no idea that there was one done in the 60's .. i watched an episode on YouTube (really slow night at work) and man - the older stuff is kinda funny in a cool way.

www.netflix.com

Watch Lost in Space | Netflix Official Site

After crash-landing on an alien planet, the Robinson family fights against all odds to survive and escape. But they're surrounded by hidden dangers.
www.netflix.com www.netflix.com
 
kernel said:
Be careful. A friend of mine has a long experience with sourcing parts for automated indoor gardens. Buy a good enclosure if you are going to mess with global market, micro power electronics, especially relays amd contactors. They will likely eventually melt down if operated with gusto. Man, he has doubled up on continuous current and still had meltdowns amd zorched relays with all but industrial grade components. Ac power.... dc even more critical quality standards.
Click to expand...

That's one of the reasons I'm still not sure about it.

My system is limited to 10A, so I'll never push the relay to its limits, not even half of it. I'm also very often offgrid, so the relay would last most of the time in its NC state passing the Inverter.

I've researched for other DPDT and SPDT relays, but all I've found have a low voltage DC signal/coil line depending on the system used. None of them can directly handle a 220VAC signal. That's also the reason, why there's a little step-down converter installed on the board of this module, which is by the way missing on the 5/12/24VDC variant. The whole board is very simple in its construction and almost everything depends on the quality of the relay installed.

When it comes to the enclosure, I'm thinking of a standard IP65 junction box. Something similar to this: https://www.ebay.com/itm/1PC-Plasti...l-Junction-box-100-100-70mm-IP65/173963024740
 
Last edited:
convme said:
That's one of the reasons I'm still not sure about it.

My system is limited to 10A, so I'll never push the relay to its limits, not even half of it. I'm also very often offgrid, so the relay would last most of the time in its NC state passing the Inverter.

I've researched for other DPDT and SPDT relays, but all I've found have a low voltage DC signal/coil line depending on the system used. None of them can directly handle a 220VAC signal. That's also the reason, why there's a little step-down converter installed on the board of this module, which is by the way missing on the 5/12/24VDC variant. The whole board is very simple in its construction and almost everything depends on the quality of the installed relay.

When it comes to the enclosure, I'm thinking of a standard IP65 junction box. Something similar to this: https://www.ebay.com/itm/1PC-Plasti...l-Junction-box-100-100-70mm-IP65/173963024740
Click to expand...
Its your rig. I would find a metal enclosure simply to contain hot burning plastic ;)

Solar rat was talking generator transfer and mentioned 50$ transfer switch units. They are widely available and some even have regular wall plug amd outlet arrangements. Plug or wire the inverter on one side and the ac grid or generator or auxilliary inverter to the other, distribution on the line out. Listed, safe trouble free.

The desire to make it yourself set whole new parameters and the thing may be fine.

Heed the experience of others regarding automation relays. @JeepHammer can likely help with sage selection of the type you are shopping/designing.

The relays used in the automated gardens were for low current 120 and 240v ac circuits and plc controlled. They burned repeatedly until he started using industrial contactors. Well within ratings too.

Some host of companies make reliable relays for your application that will have nameplate and duty that will provide rock solid reliable amd safe operation. I just couldnt start to specify which.....

One look at digikey, automation direct or a mouser electronics catalog selection of just relays alone is a head scrambler to me amd i hqve years of parts distribution experience. Specifications are specific as are intended use and usability for any application.

I guess that is where the joy is?
 
I wouldn't trust this unit for anything more than powering a night light. Sorry, what are you planning to use this for again?
  1. No safety features e.g. under/over voltage, phase reversal etc Doesn't even have minimum safe PCB track distances between AC and DC, nor any tracking protection e.g. PCB cutouts.
  2. 30A requires a minimum 12AWG cable, those tracks don't look capable of supporting the relay's rated load.
  3. Any significant heat and the terminal block and relay's soldered connections are going to melt, potentially causing dry joints and arcing.
  4. Connecting shore and inverter neutrals together doesn't sound like a good idea (different earth points?)
  5. Automatic Transfer Switches usually have an electrical or mechanical safety interlock system, other than the relay contacts themselves, this doesn't.
Definitely agree with @ghostwriter66, "DANGER, Will Robinson!" (from the original!!)

But seriously, saving a buck on a component isn't worth it when the component has the potential to kill you.
 
Looks like I have to research something else, thanks.

If I'd decide to use a relay or contactor for this I should prefer a DPDT electromechanical one, this could solve some of the mentioned problems by design.
 
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