Author Topic: Converting RV Vapor Absorption Refrigerator to Freon Compressor ("Residential")  (Read 4104 times)

Lonne Mays

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I'm sharing here my successful conversion of my Norcold 1200 to "residential" freon compressor operation.  When the original cooling unit failed, I decided to explore converting the fridge to conventional freon compressor cooling like residential type refrigerators use.  There is at least one manufacturer of a freon-based cooling unit that is sized and physically configured to replace the ammonia absorption cooling units used in Norcold and Dometic RV refrigerators.  These units draw less than 100W when running, and since the compressor draws power only when cooling, the actual power used over a 24 hr period will typically be much less than 1KWhr, so you can still dry camp using the house batteries.

The caveat is that the control electronics (power board) of the refrigerator is not designed to work with a compressor.  It was designed to work with a vapor absorption system and control the heat sources (propane flame or electrical heating elements).  The conversion of the RV refrigerator from a vapor absorption system to a freon compressor system requires the freon compressor to be hooked to the terminals that used to provide power to the heating elements of the vapor absorption system boiler. The instructions that came from the supplier of the conversion system state that the refrigerator's warning beeper must be disabled and the users must simply ignore the flashing error codes on the fridge front panel that indicates the heater elements have failed. (It will give this error continuously because the freon compressor draws only 1/10th the power that the heater elements did.) I deemed this ham-handed approach (disabling the warning electronics) unacceptable!  Therefore I decided to re-engineer the fridge control electronics so that it accepts the much smaller power (load current) of the compressor as a valid operating level.

My goal, therefore, was to raise the sensitivity of the AC Heater current sensing circuitry so that it would accept the 100W load of the small freon compressor in place of the 1500W boiler heater, and not have the control logic give an error/fault indication of Open Load/Sensor ("Sr OP" was the fault displayed by my particular board).

Having no schematic for the control electronics, and having been told by the manufacturer of the control board that the schematics and circuitry are proprietary and cannot be shared, I began the task of reverse engineering the control electronics. I soon determined, by tracing out the pathways on the printed circuit board, that the heater load power was sensed via a 500:1 ratio current transformer.

"Aha!", I thought, "All I have to do is increase the value of the 'burden resistor' on the output of the current transformer to achieve the desired increase in sensitivity. (Since the output of current transformers behave essentially like constant-current sources, their secondaries are typically loaded by a burden resistor to achieve the desired volts-per-amp output (i.e. turns-ratio x burden resistance = volts per amp). However, and much to my surprise, I discovered that the burden resistor footprint on the circuit board was not populated on the board (no part was soldered thereon)! Without a burden resistance the secondary of a current transformer will try to rise (trying to push current through an infinite impedance) until it reaches the saturation voltage of the coil! (Not the usual practice, to be sure.) I did note that current transformer's secondary was provided some loading through a diode and capacitor feeding a voltage divider (comprising, I believe, a passive integrator/filter circuit to translate the signal to an appropriate trigger level).

Not having the option of increasing the burden resistance (since a missing resistor is the same as a resistor with infinite resistance) to achieve the increased Volt-per-amp sensitivity, I was still left with the option of altering the turns ratio on the current transformer to achieve the desired sensitivity. This necessitated unsoldering the transformer from the board, taking it apart and replacing its one-turn primary with six turns of 18AWG magnet wire. This lowered the turns ratio to ~83 and increased the sensitivity, allowing me to populate the burden resistor footprint with a 5.6K burden resistor to achieve a sensitivity of ~2.5V across the burden resistor per amp of primary current.

The board now functions properly with the compressor as its load, and still has all the load-sensing functionality and other features intact!  The refrigerator cools more quickly and efficiently than ever before, and the ice maker produces much more ice than we could ever use.
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Stan Simpson

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Nicely done. Congrats.
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Bill Lampkin

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Lonne, Nice work reverse engineering the control board. The unit you installed was the one mfg by JC Refrig, correct? I've read on line they have a problem with the cooling fins inside the fridge icing up over time. JC's fix was to install a pair of computer fans in front of the cooling fins, these fans are supposed to keep the ice from forming on the fins.  Does your unit have these cooling fins? How much room do they take up in the fridge? From the one photo I've seen, it looks like they are mounted in a metal box about 1.5" deep. I would think that this takes  up quite a bit of room in the fridge. Do the fans reduce the ice on the fins? Can you hear the fans? Have you had to boondock since your installed the fridge? Did you install a seperate inverter for the new cooling unit or does the stock one work ok. Have you measured the power consumption yet? As you can tell, I have been looking at this conversion for my Norcold. My main issue now is with the space the cooling fans take up inside the fridge, and do they eliminate the issue of the fins icing up. Thanks for the post!!
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Lonne Mays

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To answer your questions:  Yes, I used the 120vac unit from JC-refrigeration; The defroster fans do not take up significant space, and they really do work (even before the conversion I was considering installing something similar to deal with the frost-on-the-fins issues I have always experienced with RV refrigerators); the compressor is very quiet (much more quiet than a home refrigerator), but the defrost fans do make a bit of noise;  I installed a separate 1000W true sinewave inverter in the battery compartment and then ran romex and an outlet to the fridge compartment (I leave the inverter on all the time so it is like an uninterruptable power supply for the fridge); I have measured the power consumption with an inline wattmeter, and the compressor draws 700W for a few seconds when starting, then drops to less than 100W. 

Because the control board is designed to control a vapor absorption system, it does not have appropriate temperature hysteresis (it tries to regulate the temperature to just a one degree delta on the thermister on the fins).  This can lead to the control board trying to turn the compressor back on too soon after it has just been running (freon compressors should have at least 3 minutes rest after running to allow the pressure to equalize on the high and low pressure sides).  To address this, I added a relay to parallel a 390K resistor across the temperature sense thermistor whenever the compressor is running.  This effectively puts a 4 degree temperature hysteresis into the temperature control loop.

If I were to do this again, I would use JC-refrigeration's 12V DC compressor, and use a circuit I have devised to run the fridge in "propane" mode and have the control board think it is running a propane burner.  (See attached picture.)  This way I would not have to modify the power control board.  The circuitry attached to the spark coil fools the control board into "thinking" it has a successfully lit propane flame and is detecting the "flame rectification" phenomenom.
« Last Edit: September 21, 2019, 06:05:44 PM by Lonne Mays »

Bill Lampkin

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Lonne, Always thinking! So, the 12vdc compressor option would be your choice if you were to do again, but you would still need the parallel resistor and relay for the hysteresis issue, right? Other than not having to modify the control board for current sensing, what other advantages do you see for using the 12vdc compressor? Lower power consumption as you don't have inverter efficiency and standby losses? Is your refrig a two door model, or the larger 4 door Norcold? The fins on our 4 door Norcold frost up, but I don't know if that lowers the cooling of the fridg, as ours works well. This summer we ran from the searing heat of California's Northern Sacramento Valley (Oroville) to the cool coast of Wash and Or. It did hit 90 one day, otherwise temps were around 60-65F.

So the wife wants to keep the deep pot drawers under our 4 door fridge. Replacing the Norcold with a residential fridge would require the removal of the pot drawers. So you say go with the 12vdc conversion over the residential replacement??

Thanks again!
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Lonne Mays

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Hi Bill Lampkin, yes, I would do the 12Vdc option not only because of not having to unsolder and rework the current sense transformer and then resolder it into the board along with a surface mouint resistor, but also because the 12Vdc compressor is also a more efficient compressor (it uses a brushless DC motor and BLDC motor controller). When combined with not having to run it on an inverter, you are able to get the same cooling while saving another ~30W or so.  I do not like how JC-Refrigeration recommends hooking up the 12Vdc unit (they have (in my opinion) a bizarre configuration where they apply 12Vdc to the 120VAC input terminals on the power control board, and then hook the compressor to the heater element terminals.  Then they recommend breaking the warning buzzer off of the fridge's operator panel, and ignoring the heater fault or sensor fault messages the operator panel will give when operating the compressor. 

If I were to do this again, I would use the 12Vdc unit and deliver power to it via a 12V relay driven from the gas valve solenoid output of the control board.  Then I would set the fridge's control panel to "gas" (not "Auto" and not "AC").   The control board (AKA "power board") will activate the gas solenoid terminals when the temperature has climbed to the set value, and also send pulses to the high voltage ignitor coil to try and light the propane burner.  The control board will then check to see if the propane flame has successfully remained lit (if it does not detect the flame it will de-activate the gas solenoid  terminals and try the sequence again).  The flame is detected by sensing if current can be made to flow one way through the ionized gas of the flame, but not the other (this phenomenon is known as "flame rectification" and is used in gas burner appliances like home furnaces as well as in RV gas refrigerators).  To simulate the presence of a flame, I believe I could use the diode and resistor network I have shown.  I would also use a neon lamp like an NE-2 as a spark gap to protect the diodes from overvoltage.  I would use a couple of 1N4007 diodes in series with a 6 MegOhm resistor as my starting value, and alter the resistance via experimentation if the power board was not happy with the "simulated flame". 

My refrigerator is the 4-door Norcold 1200 with ice-maker.  I pulled the fridge out of the cabinetry and layed it face down on the floor of the motorhome to do the conversion.  The videos provided by JC-Refrigeration are a good overview of the labor involved (and yes, it is a two person job to lift and remove the old vapor absorption cooling unit).  Before re-installing the converted fridge back into the cabinetry, I lined the cabinets with EPS foam to make the fridge even more efficient (because afterall, you can't have too much insulation).  I also re-used one of the Norcold factory installed cooling fans as an additional fan to cool the freon coompressor (because you can't have too much cooling or heat-sinking either).  I also bought a roll of aluminum tape and re-coated the whole back of the fridge and much of the sides.  (See the attached pics.)  I also added foam tape wrapped around the cold return line to prevent condensation from forming and dripping down and wetting the back of the cabinet (we live in Coos Bay on the southern Oregon coast, and it is very humid here).



Bill Lampkin

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Nice job Lonne-We were just in Winchester bay last week and visited Coos Bay (Vinnies, for BBQ lunch).

I haven't checked w/JC on the wire size for their 12v unit, I'm guessing existing wiring to the back of the Norcold refrig is 12ga. Do you think 12ga would be sufficient to handle the start-up of the compressor? Using your power measurements, you would have 50+amps DC starting current and about 8 amps DC running. 

I like your control scenario using the propane circuit as a surrogate to control 12V to the compressor. To solve the hysteresis issue, one idea might be to use a 5 minute time-delay relay on the compressor start circuit. That way you would not have the compressor try to start against high head pressure.

The wife likes the idea of keeping the existing fridge but with the 12vdc compressor upgrade, now I'm concerned about the wire size for the 12vdc compressor. Its always something!
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Fred Brooks

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     Bill, I am not that familiar with this after market up-grade retrofit. I am well versed in 12 volt. A 12 gauge automotive wire is rated for 25 amps dc load. They state it runs on 100 watts of power which is 100 divided by 12 equals 8.33 amps.
   I like your idea of the time delay so you can't short cycle the compressor with high head pressure. The 12 volt compressor should have a start capacitor somewhere in the loop to launch it. Not real sure how your coach is configured with the wiring going to the refer. If it is 12 gauge it is more than adequate. Let the fun begin! Fred
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Lonne Mays

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Hi Bill, Hi Fred,
I also think the 12AWG wire size already available at the back of the fridge is sufficient.  The 12Vdc compressor uses an entirely different type of motor (a BLDC motor), and so has different start up characteristics.  The 12Vdc hook up instructions that JC-Refrigeration supplies with their 12Vdc cooling unit has one removing the 120VAC from the power board and substituting 12Vdc to these connections.  Although this set up will run the compressor, I don't think that the current sense transformer will "see" appropriate current to the 12Vdc compressor because it will be DC, not AC (and transformers do not work with DC), and so the user will still get the annoying error codes and warning beeper. 

As far as a delay turn-on timer, I was also thinking along these lines, but then one would still get the annoying error code & beeping until the compressor turned on.  (This will happen during the time the fridge's power board commands the compressor to turn on and the delay timer allows the compressor to turn on.  Without the hysteresis circuit I added, this was also happening when the compressor was short-cycled and the thermal overload circuit breaker (built into the compressor) would kick the compressor AC off for about two minutes, and then when the circuit breaker would cool and reset, the compressor would turn on and the control board error and beeping would clear.)  That's why I added the hysteresis circuit to add about 4 degrees F of hysteresis between compressor turn-on and turn-off and turn-off and turn-on.  As a side note, I contacted Dinosaur Electronics (the manufacturer of the control board in my fridge) and they were very polite, but also very insistent that their board is not designed to work with a freon compressor, and that they could not share schematics of their circuitry or details of their firmware programming as these were proprietary.

As a fall back plan for 12Vdc operation, one could use an after market temperature controller like this one from amazon for $15:
https://www.amazon.com/gp/product/B019I3YCFS/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
and then us a fixed resistor in place of the thermistor, and set it value so that the fridge power board thinks it is always below the set temperature.  This controller can be set for the desired turn-on delays, etc..
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Bill Lampkin

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I've been reading all the forum posts and reviews I can find on the compressor conversion, but yours is the first to say the compressor TO breaker actually operated and shut down the compressor due to the high head pressure issue. Maybe others are not paying as much attention to the compressor controls. Anyway, Hats off for an excellent job!

I think I'll steer the wife toward a residential fridge.
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Fred Brooks

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  Very Impressed with Lonne's tenacity and knowledge especially reverse engineering. Way over my head. I realize that the Ladies don't want to give up the 3 drawers under most Dometics & No-colds. After installing over 40 residentials the majority being Samsung RF-18s, I have yet to hear of 1 Lady tell me she was unhappy. All I hear is WOW look at all the room and they love the LED lighting and features. Amazing how they find other places to stow what was in those drawers. I removed the U-Line icemaker that quit years ago and installed a 14" high pot drawer that is 24" deep that replaced the 3 drawers we gave up for the RF-18. Fred
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Bill Lampkin

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I'd say Lonne is a pretty clever guy, Fred! Do you think he could come up with something to get some more mileage out of our CATS??
Back to the refrigerator control issues; I bet Lonne could just come up with a replacement board for the Norcold using the 12v or 120v compressor, and the control panel would retain its functionality. You think JC Refrigeration would be interested?

The wife was looking at residential refrigerators on the web as we drove home today.....My work is about to begin!

Thanks!
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Fred Brooks

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      Bill and Lonne,  Not too sure how to squeeze more gidde-up out of our Cat's, I just keep on rubbing on her until she purr's.
     Here would be an interesting challenge: an upgrade with the Aladdin owners to interface the past with the present?
Also I would think that JC refrigeration would be interested with Dinosaur electronics as an interface. Fred
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Lonne Mays

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Hi Fred, I have talked with both JC-Refrig and Dinosaur Electronics about the two companies communicating with each other.  I think the issue is that JC-Refrig does not sell enough volume to create an interesting business model for the two companies to engage.  I offered my schematics and help to JC-Refrig (for free), but they said that they were satisfied with their present hook-ups and work-arounds (i.e., breaking the buzzer off and relying on the thermal circuit breaker on the compressor).  BTW, another advantage of the conversion is that the unit will operate without having to worry about off-level angles going up and down grades, etc..





Fred Brooks

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   Hey Lonne, Neccesity is always the Mother of invention. Sounds like a business oppurtunity for you! Fred
 
Fred & Cindy Brooks
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Proverbs 3: 5 & 6