Fridge Defroster Operation

[lbc]

Hi All,
I've got a Kelvinator "cyclic defrost" fridge that I've had since new, and now serves as a beer fridge.
It's always been touchy with its temperature setting, with the need for thermostat adjustment to compensate for external temperature - turn it up in summer to keep the insides cold enough, and turn it down in winter to stop the drinks from freezing.

Late last year, the thermostat died, so I decided to replace it (the thermostat) with something more high-tech instead of buying a new fridge. I work with industrial machine control, so I'm comfortable working with mains power and microcontrollers, but I had no knowledge on the inner workings of refrigeration.

As a temporary measure, I bypassed the thermostat and did the switching from the mains plug. This would let me know the compressor etc was all OK, it just meant that the fridge light wouldn't come on if the new thermostat had decided the fridge was cold enough and has turned the compressor off...

An electronic sensor was put at the top-back of the inside on the fridge, and connected to a box on top comtaining a PIC which switched an SCR to turn the compressor on and off as needed. Sensor was fed to an analog input on the PIC. 30 minutes of coding later and yay, it worked! Inside the fridge stays a constant 3 degrees, no matter if the outside temperature was 15 or 40 or anything in between.

I want to improve it further (alarms, fans, LCD dispay, etc) but I've got one problem to sort out first.

The plate at the back of the inside of the fridge ices up after a few weeks. There is a wire running thru it that appears to be a heating element - this was connected to the (now disconnected and bypassed) original electromechanical thermostat.

I'm assuming this is a low powered heater which is switched on occasionally to melt the ice. It works, as far as I know, because it didn't ice up before the old thermostat died.

So, question: Am I correct in assuming this wire is a heater element, and if so, how often is this turned on to melt the ice? One an hour, once a day, or what, and how long is it turned on for?

Thanks in advance to anyone who can explain this part of the mechanics of a fridge to me in my quest to make the perfect beer fridge.

Laurence Collins.

 

[DJR]

The wire is a defrost coil, if you touch the outside of a FF fridge it is quite warm at times, it needs this to deice the inside of the fridge which is where the frost starts. If you're not using the coil you'll just have to periodically defrost the fridge (turn it off for a while with the door open)

 

[lbc]

It's not a frost free model, but a "cyclic defrost". The freezer section has always had to be defrosted manually every 2 to 3 months.

The part that is now icing up that didn't ice up before is inside the fridge (not the freezer) compartment.

Laurence

 

[Eugene]

I think You may have answered your own question, you say it frosts up after a few weeks, then use your new wizz bang controller to run the defost element each two weeks for 1 hour, se how it goes, or maybe once each week for 30 mins, it cant hurt.

I am going to put one of the Jaycar thrmostat controllers togehter this week and use it on my fridge (now I have a Beer Fridge)

 

[afromaiko]

Does the wire have insulation on it or is it bare?

I've got a white wire in mine that snakes around and is glued to a piece of thick copper foil. Looks like a standard electrical wire, but the core is very thin so that it warms up the wire. I believe it ran permanently to keep the thermostat/light panel defrosted. A mate has a different fridge but with the same thing that runs along the back of his internal drip tray to stop it icing up.

If you are talking the bare, heavy duty rod like heater around your cooling element then it will most likely be on a timer that comes on every 8-12 hours until another defrost thermostat near it trips then switches off the heat.

 

[ausdb]

I've got a Kelvinator "cyclic defrost" fridge

The plate at the back of the inside of the fridge ices up after a few weeks. There is a wire running thru it that appears to be a heating element - this was connected to the (now disconnected and bypassed) original electromechanical thermostat.

I'm assuming this is a low powered heater which is switched on occasionally to melt the ice. It works, as far as I know, because it didn't ice up before the old thermostat died.

So, question: Am I correct in assuming this wire is a heater element, and if so, how often is this turned on to melt the ice? One an hour, once a day, or what, and how long is it turned on for?

Excuse the butchering of your post above!

On that type of cyclic defrost the heater is wired across the compressor contacts of the thermostat so that whenever the compressor cycles off the heater energizes using the compressor run winding to complete the circuit. The heater is of such low wattage that it has no effect on the windings and when the thermostat cycles on the current chooses to flow through the thermostat contacts instead of the heater, so bypassing it. Quite simple but effective.

To ensure a complete defrost every time, cyclic defrosts use a fixed cut in temp thermostat (about 3or5C can't remember anymore) but the cut out temp is judged by the temperature of the coil/plate section mounted in the refrigerator cabinet. The fridge coil/plater is the end run of the evaporator section of the fridge, (the majority is in the freezer compartment) and as domestic fridges use a fixed expansion device (capillary) that works best with a constant load and ambient is why you need to adjust the temp a bit to suit winter/summer conditions.

So the answer to your question is basically switch the heater on when the compressor is off but cycle it off when the refrigerator coil/plate is a few degrees above freezing. Sounds like you need another thermistor or whatever sensor you are using attached to the coil to monitor its temp and switch the heater via another scr and some logic from your PIC

Sorry to ramble but hope it helps.
PS I hate working on domestic fridges especially the new ones, too complicated for what they are!