regulating a fridge freezer combi with an arduino

[Swake]

Got a no-frost fridge/freezer with a thermostat issue....
Logged the temperature over a 48h period as I suspected trouble. The fridge swings between 1.5°C and 7.3°C; and on the same setting the freezer is going between -17.3°C and -25.2°C. This is to much of a swing and it needs repair. Another Fridge/freezer combi of similar size has a near flat temperature line in both the fridge and the freezer, so this confirms the other one is not having a normal behavior.

While I can replace the mechanical thermostat, I'd like to have some more fun and put some Arduino like controller on it with a nice fancy LCD display :-)

Any one with experience or guidelines?
How do I regulate the no-frost thing? In the back of the fridge there is some sort of timer and from what I could read there should also be a defrosting heater somewhere, didn't find it yet but will dig a little deeper.

[MarkF]

Funny.  I just monitored the temperature in my refrigerator two weeks ago.
Only for about one hour.  I had about the same 6^oC deg temperature fluctuation.

I do NOT see that as a problem since the high temp stays well below freezing in the freezer section.
In order to maintain a small fluctuation, the unit would need to turn on/off more often.
Not necessarily a good thing in my opinion.  More wear and tear on the compressor.

I can not help on the Arduino interface.  Interesting project.
But, you need much more details on your controller board and how it functions.
What the sensors are and their electrical interface requirement.

[NiHaoMike]

Maybe the real problem is the lack of thermal mass? If the refrigerator is mostly empty, add some water bottles since water has a very high thermal capacity.

[hermit]

From some one that had quite a few years in the appliance repair industry.  Never check the air temp.  Check product temps.  The thermal mass idea above is valid.  Better yet.  Submersible probe in a glass of water in the fridge.  In the freezer stick it in your ice cream.

[Swake]

That thermal mass thing should only delay the temperature change. More or less mass should not change the switching points of the thermostat. I well nevertheless put some drinks in there and redo the test because it must have an impact and a fridge is not meant to be empty :-)

I would agree with the airtemp comment if it was a one shot measurement performed with an instrument external to the fridge. In this case however the temperature probe/logger can be considered as being a product itself as it was put for 48+ hours in the fridge/freezer.

From what I learned I can say that logging the temperature for only an hour or so is certainly not sufficient to have a good picture. Our fridges barely make half a cycle over a 1 hour period. Another fridge of similar size with an 'electronic' regulation as opposed to a mechanical thermostat has a temperature swing of only half a degree. That mechanical vs electronic thermostat thing at least partly explains the difference in behavior. Right now I can't tell if a temperature swing of 6°C in the fridge is considered normal or not. Me being a user of that thing it doesn't feel as normal.

[Circlotron]

Back in November 2002 I had the then 22 year old fridge thermostat fail so I made an electronic one. I put a thermistor in a piece of steel 3 inches diameter and 2 inches high. As has been mentioned, that more accurately represented the temperature of objects in the fridge. Ran it to a comparator with *zero* hysteresis and then to a micro that looked for a full five minutes of uninterrupted over temp or under temp and switched the motor relay accordingly. Also put a pair of 90mm fans at the bottom of condenser, facing upward, at the back of the fridge. Also a single fan inside the fridge that blew directly on the evaporator. And put an hour meter on the motor. The compressor duty cycle dropped from 60% with the original thermostat down to 31% with the new thermostat and fans. The compressor would switch on roughly once every hour. Ran it till October this year then put it out in the rubbish when other parts of the 39 year old fridge were leaking water. Interestingly, the inside fan, a Sunon ball bearing unit ran 24/7 in 4 deg C ambient for just under 150,000 hours without any trouble at all and still sounded like it had years of life left in it.

[Swake]

A stable environment is definitively the best to make things keep up for a long time. 5 minutes delay is an excellent tip to build the hysteresis.

Have put 24L of water in the fridge and 2 bottles of 5L antifreeze in the freezer. So, with this thermal load let's wait till tomorrow and see what it tells us.

In the mean time I'll research what that timer does for the no-frost thing. I guess it is activating the heater for a couple of minutes now an then.

[Swake]

Wanted to provide some feedback on the experiment:
With the added water as a buffer the fridge now oscillates between 6.1 and 9.3°C. The fridge had the exact same settings as before. Room temperature is stable too (0.5°C swing). I cannot find an apparent reason as why it is not switching on and off at the same temperature as before adding that buffer.

[m k]

My fridge has two measuring resistances.
It didn't behave with one only.

Edit,
obviously the other on broke and I tried to keep it happy with what it had, I couldn't.
After replacing the faulty one the machine has been the calm self again.

[todd_fuller]

They made bespoke controllers for this application. Typically, they are targeting commercial refrigerators/freezers, but they control the compressor, fan and defrost cycle. The controller should also protect against compressor short-cycling. With a controller like this, you can dial in the set points + dead bands. Danfoss ERC213 is an example.