[SOLVED] Replacing an obsolete transformer in Engel 12VDC/120VAC refrigerator

Thread Starter

Safteysquint

Joined Dec 30, 2018
9
I’m having trouble finding a replacement transformer for this refrigerator. I am also having some trouble interpreting the schematic. Is it converting 12vdc to AC, then sending to a transformer to run the compressor?

It runs off of 12Vdc or 120vac.The unit functions properly on DC power. It will not function on AC at this time. I assume the transformer is bad.

Being a strange winding, could I use 2 independent transformers?
 

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Dodgydave

Joined Jun 22, 2012
10,088
Yes the transformer is designed to run on AC mains and has an Oscillator for the DC supply to provide the AC to the motor.

So when running on DC there is a separate Oscillator module, check the dc supply at the 220uF capacitor and at terminal 0 on the connector, the oscillator may have blown.
 

dendad

Joined Feb 20, 2016
3,939
It also may be the relay not switching for AC, or what often fails, the transformer thermal relay.
Does the mains AC volts appear on the white and orange wires?
With power off, what is the resistance between the white and orange wires?
 

Thread Starter

Safteysquint

Joined Dec 30, 2018
9
It also may be the relay not switching for AC, or what often fails, the transformer thermal relay.
Does the mains AC volts appear on the white and orange wires?
With power off, what is the resistance between the white and orange wires?
120v on mains, 22ohms on 120vac primaries
 

Thread Starter

Safteysquint

Joined Dec 30, 2018
9
It also may be the relay not switching for AC, or what often fails, the transformer thermal relay.
Does the mains AC volts appear on the white and orange wires?
With power off, what is the resistance between the white and orange wires?
I also subbed in a known good relay and it is still not functioning.
 

dendad

Joined Feb 20, 2016
3,939
What AC voltage is on the blue wires when running on AC?
It sounds odd that it can run on DC but not AC if the AC primary winding is ok.
And then measure the blue wires when on DC. The reading may be different as it will be square wave, not sine wave I think, and your meter may see that as a different value.
 

Thread Starter

Safteysquint

Joined Dec 30, 2018
9
What AC voltage is on the blue wires when running on AC?
It sounds odd that it can run on DC but not AC if the AC primary winding is ok.
And then measure the blue wires when on DC. The reading may be different as it will be square wave, not sine wave I think, and your meter may see that as a different value.
It was reading something like .4ohms on the 2 blue 20v-0v wires. No voltage though. I don’t have a scope where I’m at now to look at the wave form, if any.
 

dendad

Joined Feb 20, 2016
3,939
What is the reading when on DC power?
You say the fridge works on DC, so there should be a reading there.
Even on DC, the motor is running on AC.
 

Thread Starter

Safteysquint

Joined Dec 30, 2018
9
What is the reading when on DC power?
You say the fridge works on DC, so there should be a reading there.
Even on DC, the motor is running on AC.
Ok, so now, the readings from the transformer,

120vac primaries 24ohm
20vac windings 1.4ohm
11-0-11 windings 1.0ohm

I think the transformer is good.

good continuity on dpdt relay in both positions.

No voltage on 0v to 220uf cap.

No voltage on 12Vdc in to 20vac output windings to compressor. (Don’t know what happened, fridge was running on DC last night)

No voltage on 20vac output windings in AC mode.

So, as from the first suggestion, is one or both of the oscillators fried?
 

dendad

Joined Feb 20, 2016
3,939
You need to get it running on AC first as both AC and DC use the transformer.
It sounds like something has an open circuit.
There should be AC volts on the secondaries when the unit is running on the mains AC.
 

debe

Joined Sep 21, 2010
1,252
The fridge is an early Engel. the compressor runs on 22V AC or crude square wave on 12V from the oscillator, or on mains is 22V sinewave. Pict of the 22V on the compressor when on 12V DC & current draw & voltage.COMP WAVE FORM.JPGCOMP VOLT & CURR.JPG
 

Thread Starter

Safteysquint

Joined Dec 30, 2018
9
Ok, that it awesome Debe. Can I just run the 20v secondaries direct to the compressor then, to troubleshoot? Or is it a dual primary transformer, with 120vac and 20vac on the primaries?
 

debe

Joined Sep 21, 2010
1,252
Bypass the relay & thermostat & just feed 110V AC to the transformer White & Orange & Un plug the coupler to the oscillator. The compressor should run.ENGEL CIRCUIT.jpeg
 

Thread Starter

Safteysquint

Joined Dec 30, 2018
9
Alright, I’ve got it fixed. So when I was troubleshooting the transformer, I didn’t even think to jumper the red wire to red wire, and orange to orange in the molex. After that I got good voltage on the secondaries.

The reason it was working on DC is the thermostat has two internals. One for AC circuit and one for the DC circuit, the thermocouple is shared. The AC contacts in the thermostat had failed. I am now running two separate thermostats as I couldn’t find a suitable replacement in short order.

Thank you to everyone for all your help.
 
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