Inverter beeps "power failure" after being briefly plugged into 12V PSU mains

[ballerburg9005]

Hello,

I connected an ordinary 12V switching power supply at home to my cheap and fully functional almost new 2600W 12V car/solar inverter for a minute or two, because I didn't have a battery to power it and I just wanted to test if the cooling fan was spinning (I replaced just the fan it was too noisy, I only took off the top lid to do it and never touched the circuit).

It beeped and complained about the power supply and it showed the "power failure" LED, with the LCD display and the USB port functioning as usual and the voltage being right on. I thought nothing by it, since I didn't connect a load or anything and the 12V from the switching PSU probably weren't clean enough for it to accept it.

Now the huge problem is that when I connected it back to the solar battery in my garden shed, it would still beep and show failure. The device had been perfectly working for weeks, mostly with minimal load.

I disassembled it and I checked all the MOSFETs with a multimeter, I watched inverter repair videos, the large fuses are fine too, the solder work looks all solid and right, I tried the old fan as well in case the device measured if it was spinning ... none of the ordinary things seem to make sense. Obviously this is not a normal kind of failure for an inverter, since it seems to have been somehow caused by powering it without a battery.

I suppose there is some kind of very small security fuse blown somewhere that's for the ICs (can't find anything), or something of the sorts, but I just don't know what to look for and where.

Do you have an idea where the problem is? Could you maybe point me in the right direction?

It has mostly SMD components. My understanding of electronics is rather basic.

[m k]

A model number and few pictures would help.

[ballerburg9005]

It has no model number. The board numbers turn out empty ( ZJ-B2H5611B1 , XZB1000226YJP )

Here are some pictures: https://imgur.com/a/TGaSGu3

http://aliexpress.com/item/1005002494340774.html

Please consider the following:

* The board was in absolutely pristine condition visually before I prodded it with a soldering iron.
* in my diagnostic attempts I actually destroyed the last MOSFET accidentally, which is why it has been replaced.
* The board is actually 24V. I previously wrote 12V, because I didn't want to give off the false impression that this was some expensive solar inverter. They make the same inverter in both 12V and 24V and it is just a cheap "car type" inverter. Everything I did and used was then of course 24V also.

[strawberry]

inverter circuit

[m k]

What was your test connection exactly, was USB included, was ground loop possible?

[ballerburg9005]

@m k:

The USB ports work without issue, and also the LCD and anything but the 220V works. It shows the red fault LED and beeps because of it.

The 24V supply I used for briefly testing the fan is a brand new typical household switching power supply from an LED lamp strip. I don't have an oscilloscope, so I can't be 100% sure about it, but it shows 24V clean on the LCD. Apparently or supposedly this power source somehow damaged the inverter when I tested the fan with it. By "fan" I mean the tiny fan that cools the unit. At least it was working perfectly before I used this mains power supply, like described. I still can't believe it though ... it is not like one of those old humming power bricks.

Btw. today I pulled out all the MOSFETs from the input side and checked them and put them back in. One of them was slightly off in values, but nothing substantial and replacing it yielded no effect.

So it is still completely mysterious what the problem is with this inverter.

[m k]

What kind of a household switching power supply exactly, was it constant voltage or constant current?

[ballerburg9005]

Just an ordinary household one. Like this:

[m k]

Have you put the old fan back?

From strawberry's circuit example you can see that some parts are connected to battery pretty directly.
On the other hand your wall wart can give out almost what ever.

Start tracing routes backwards from input side power parts and check what you meet.
Transformer primaries must have AC of some sort, if you go live with it.