Isotherm fridge power supply question

[Wave34]

Hello,
is there someone who worked on their fridge and can help me?

My Isotherm fridge works perfectly on 120VAC but on a battery, it doesn't cool.

I put a LED on the control board and I have 1 flash which is "low voltage''.

I measured at the power supply DC input and I have 12V .

On the power supply, it is written : Input 120VAC and Output 24VDC.

My question is:
When powered with 120VAC I get 24VDC at the output, but when powered from the battery, even if I have 12V at the input terminals, I have only 12V at the output.

Can someone confirm that this is NOT normal, and there should be 24VDC either from 120VAC supply or 12VDC supply.

I will bring it at work today and measure some components, but without schematics, there is not much more I can do.

 

[Wave34]

I did some testing today.
By following the PCB traces I found that it is normal to have 12VDC at the output when the supply is 12VDC.

The input voltage is directly fed to the output and isolated with diodes.

So that rules out the power supply. Next step is to check the compressor module and wiring for corrosion. Everything looks OK at the fridge, but maybe elsewhere corrosion is hiding...
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[QBhoy]

Is it maybe that it’s a 24v fridge ? Sounds like it.

 

[Wave34]

Is it maybe that it’s a 24v fridge ? Sounds like it.

Yes it is a 24VAC fridge (3 phases)

Today I took the compressor module to work and did some testing.
I had the LED connected to see if there was error codes.

My findings:
The module works!
Supplied with 12VDC, the module adjusts and gives a 3 phase signal that I saw on a oscilloscope.
I then raised the power supply to 24VDC, and I had a low voltage error (1 LED blink) and no signal at the output.

After some more reading, I found that even if the module says 'dual voltage 12V/24V' it requires 25Vdc for the higher voltage.
I adjusted the power supply to 25VDC and the LED turned off and the oscillation started at the output.
The frequency was quite high, like 250Hz.

Sooo, both modules test ok on the bench.
The only thing left to check is my connections in the boat.
I will bring a battery in the cabin and feed the fridge directly with 12V, just for my satisfaction of seeing it working on 12V and if it is concluant, I will have to start chasing for bad or corroded connections.

 

[QBhoy]

Yes it is a 24VAC fridge (3 phases)

Today I took the compressor module to work and did some testing.
I had the LED connected to see if there was error codes.

My findings:
The module works!
Supplied with 12VDC, the module adjusts and gives a 3 phase signal that I saw on a oscilloscope.
I then raised the power supply to 24VDC, and I had a low voltage error (1 LED blink) and no signal at the output.

After some more reading, I found that even if the module says 'dual voltage 12V/24V' it requires 25Vdc for the higher voltage.
I adjusted the power supply to 25VDC and the LED turned off and the oscillation started at the output.
The frequency was quite high, like 250Hz.

Sooo, both modules test ok on the bench.
The only thing left to check is my connections in the boat.
I will bring a battery in the cabin and feed the fridge directly with 12V, just for my satisfaction of seeing it working on 12V and if it is concluant, I will have to start chasing for bad or corroded connections.

Great stuff.

Must admit, I’m not as good as I should be with this, but 3 phase and DC un rectified is a new thing to me.
That aside, it sounds very much to me like you have a dual voltage fridge there...as in 220-240v Ac (with an inventor built in) and 24v DC. If it is....12v won’t do it.

 

[Wave34]

Great stuff.

Must admit, I’m not as good as I should be with this, but 3 phase and DC un rectified is a new thing to me.
That aside, it sounds very much to me like you have a dual voltage fridge there...as in 220-240v Ac (with an inventor built in) and 24v DC. If it is....12v won’t do it.

It's a bit confusing. The fridge is sold as dual voltage (120VAC and 12VDC). But the 120VAC is converted to 24VDC and goes to the compressor module. The 12VDC goes directly to the compressor module.
The compressor module has some intelligence in it. It can adapt to 12 or 24V supply.
That DC voltage input is passed to a circuit that creates 3 AC signals that are delayed from each other to create 3 phases of about 24V amplitude and the frequency is variable to change the speed of the compressor, and also do a soft start to reduce the initial current demand.
It is a good system I find.

 

[Wave34]

I found the problem.
There was a big voltage drop to the fridge when operating on DC.
The 10A fridge breaker had a big resistance. I temporarily replaced it with the front bilge pump breaker, and now the fridge works well on batteries.

The only thing I see that is left to fix, is a bad ground. There is near 1 volt drop in the GND wire, but for now I will use it like that until I have time to find the bad connection or pass another GND wire.