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Old 11-23-2022, 10:20 AM   #1
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Refrigerator misfunction

My refrigerator doesn't want to operate at the dock with the AC power connected, but appears to run fine on DC power. I was wondering if any of you have an idea of where the problem is. The refrigerator is a WAECO top loading unit. It has a Danfoss BD35F compressor with a Danfoss 101N0220 controller (not sure if that's what it is?) and a WAECO Mobitronic Type MPS-50 rectifier. The attached sketch shows my setup.

With AC power connected, the compressor will run for approximately 4 seconds and then shut down. After about 1 minute, it turns on again and runs for another 4 seconds and then shuts down. The cycle repeats itself. The refrigerator doesn't chill. I measured voltage at the rectifier output (the C+C- terminals) and it's providing 26+ Volts when this happens.

With the AC power disconnected but the DC power still switched on, the compressor will run continuously and the refrigerator appears to get cold. I measured voltage at the rectifier output and it's providing 12+ Volts.

Could the problem be with the rectifier? It appears to be doing what it's supposed to do. Could it be the compressor? It seems to run fine on 12V DC power. I wonder if that "controller" is the problem and if that can be replaced. If anyone has any ideas or experience with this type of issue, I'd appreciate any input!
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File Type: pdf Refrigerator - Rectifier wiring.pdf (73.8 KB, 19 views)
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Old 11-26-2022, 05:28 PM   #2
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By your drawing, the refer always runs on DC. It appears that the AC side of the rectifier isn't putting out the correct voltage. 26v is causing a high voltage safety to cut power. Also voltage too low will stop the power. Most appliances will work with +/- 15%.
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Old 11-26-2022, 07:40 PM   #3
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Stop running it on 120VAC untill you have contacted Waeco and talked to them.
Something is haywire in the inverter , WAECO Mobitronic Type MPS-50 rectifier.
Measurement of 26VDC is haywire. IT may be something as simply as a switch you have to change but find out.

If you keep trying you may damage the fridge itself.
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Old 11-26-2022, 11:51 PM   #4
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I'd first check AC power to the MPS-50. Make sure it is good.

If the AC power into the MPS-50 is good, just replace it. They are ~$80 "bricks" that are outside of the unit. There is no install. It just gets connected to the AC and to the DC and then plugs into the back of the unit.

If the refridge works with either AC or DC it is working. It knows nothing about the source of the power coming into it. It expects good power. It, itself, doesn't do any conversion or much conditioning.

You may be seeing good voltage and still having a problem because the waveform isn't right. You'd need an oscilloscope to see that.

What I mean by that is that the brick should transform the 120v AC into a lower voltage AC, then rectify the alternating current into an all positive bouncing waveform and then smooth that into DC. It is possible, for example, that the rectifier is half bad and only the positive or inverted negative wave is being produced. It I'd also possible that after being rectified the smoothing part of the circuit is bad, e.g. open capacitors. Your meter would likely still show the right voltage in either case -- but the compressor wouldn't run. You'd need to be able to see the waveform to know.

I haven't seen the schematic for the MPS-50. There are other possible designs. But, if the refridge runs on DC and isn't running on AC, thst bricks is overwhelmingly most likely to blame. Making the AC and DC look functiomally the same, like the correct output for the refrigerator, is exactly the purpose of that brick.

Could it be something else? Sure. The controller in the compressor xoukd be hyper sensitive to something thst shouldn't matter and happens to vary depending upon the input source. Etc. But these scenarios are unlikely.

The brick is ~$80. Order it from Amazon or West Marine and, if I'm wrong, the return will be easy.
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Old 11-28-2022, 08:00 PM   #5
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Well, I hope you're right about the rectifier as that would probably be the easiest solution. I'm waiting for a response from Waeco/Dometic. If I don't hear from them soon or if they don't answer the phone, I'll just go ahead and try the new rectifier like you suggested and see if that solves the problem. Easy enough...

Thanks for the ideas!
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