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Re: Heaters there for a reason
caninophile   7/6/2012 10:16:11 AM
Ditto, Al.. Did not think there could be two individuals of such similarity. I am 78 y/o and still learning everyday. I also an EE that ended up doing most of the negotiations for our company and went back to law school at nights for the primary purpose of having a better understanding of contract law. Ended up 4.5 years later with a JD degree.

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Re: Perfect Interface
Island_Al   7/5/2012 3:19:17 PM

Thanks for the reasoning behind the knob. I'm still disappointed that they use such a mechanical unit however since LEDs and display unit prices have plummeted. When I turn the knob, I still hear the cam and switches clicking away. Yet in spite of the new technology, the only failure I have had in the Maytag was the upper computer board, not the knob. Another microproc runs the variable freq for the 3 phase motor on the drum. After having the upper control micro fail I expected the lower one to fail next. Nope, the next failure was the drum support bearing. Not only did it fail, but I had to buy a new drum as the bearing was cast into the assembly! I guess "new" does not always mean "improved". Along the same lines I bought an old time double edge razor to shave with. Much nicer than the 3, 4, or 5 blade with or without Aloe, etc. I recommend these razors highly since switching.  Sort of a "saving face" thing.  Thanks for your input.


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Re: These aren't the only heaters.
CliffG   7/5/2012 2:31:29 PM
After I bought my Viking refrig several years ago (old analog controls)  I had problems with the water feed to the icemaker: it used to freeze up near the dispense end. I had to drop the icemaker out of the way and squirt hot water down the fill tube. I found out later that the Sub-Zero brand has a heater on the supply tube. For an unknown reason the tube has not stopped up in about 2 years: very strange. Its probably all the clean living.

There are some units for sale in the US, and of course in Israel, that have circuits designed for compliance with rules for the Sabbath. Jewish law (as I understand it) says that one must not kindle a flame during the Sabbath, and this has been interpreted as not doing something that would actuate an electric circuit. Thus an observant Jew must not turn a light on or off. Putting the fridge in Sabbath mode disables the light switch, but also changes the compressor/thermostat function. It does not go on based on temperature, because then opening the door would cause it to warm up and trigger the ciruit. Instead, a random timer turns the ocmpressor on and off. More sophisticated units observe the recent cycles and "learn" how much the compressor cycles and emulate that timing.

My garage freezer is half-size, about 4 feet high, and needs periodic manual defrost. My clever approach to save time is to empty it, then use my heat gun to speed thawing. During the last defrost challenge, I learned that the heat gun is hot enough to melt the interior plastic liner. This gives me the advantage of inspecting the insulation through the hole! Great new feature!

Ken E.
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These aren't the only heaters.
Ken E.   7/5/2012 12:30:30 PM
Refrigerators may also use efficiency robbing heaters to keep the water supply to the icemaker from freezing, and to keep condensation from the door seals.  Pull a schematic on a modern unit (Particularly a high end unit.) and you'll be surprised.

(Oops, I see someone scooped me on this observation.)

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Perfect Interface
tekochip   7/5/2012 11:12:13 AM

The knob is the perfect user interface! It tells the user what point of the cycle is being executed and you then turn the knob to inform the machine that you want a different part of the cycle. It's tough to design a control to drive the knob as a display device and then use its position as an input device, but this is really the perfect user interface. I worked on one of these hybrid controls and it was much more expensive than a cheap membrane switch, but was far more intuitive. Interestingly enough, the White Goods manufacturers used to give us timing charts from mechanical timers and that was what we would write code around. The OEM was so used to working in the world of mechanical timers with cams and switches that they couldn't let go of designing with that type of documentation. We eventually wrote a tool that allowed the OEM to create a timing chart on their PC and the tool would then create a cycle table in C for our micros to run from.

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Other heaters
DB_Wilson   7/5/2012 10:56:53 AM
jb222 has the defrost operation nailed.

What you may not know is there may be two other heaters in the refrigerator/freezer.  Icemakers have heaters to get the ice to release from the ice mold.  If that heater is not operating, the ice maker will have ice in it but will not cycle.  The ice eject/water refill cycle will start and stall mid-cycle as it would normally do until the ice would slide free from the mold.  With no heater, the ice will not slide and the icemaker sits stalled.  The icemaker's motor is designed to stall in this manner.

The second heater is around the door.  This heater is to eliminate condensation outside the cabinet around the door gasket if the unit is used in a high humidity location.  It usually has an operator switch to disable the heater is condensation is not as issue.

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Re: Heaters there for a reason
Island_Al   7/5/2012 10:41:04 AM
Interesting comments throughout. Several years ago, as a result of having a problem in the sealed part of a home air conditioner, I signed up for a refrigeration class at a local community college. Stayed long enough to get an associates degree! Most interesting, and they covered numerous topic including refrigerators and freezers as well as the various designs, and ended in commercial HVAC systems. When taking the last "hands on" test, the instructor had me scheduled last. He then told me I did not have to perform the exercise, as even if I failed I would still maintain an "A". But he asked me what I did for a living and I told him I was an engineer and designed electronics for a big DOD contractor. When he asked why I was there, I told him I wanted to be able to fix a bad AC I owned! "You spend two years and a ton of your own money to fix an AC?" "Yep", I replied. "No such thing as wasted learning." Like Neo in the Matrix; "I know Kung Fu!"

Yes the refrig uses a timer on a cam to drive a microswitch to set the defrost cycle and their failure is the most common problem in older units. Newer ones are starting into solid state timers and triacs rather than mechanical switches. Even my Maytag washer is mostly solid state, but has a motor driven knob controlled by the micro.  Sort of retro in design.

Everything is simple when you know how it works. It has taken me nearly 70 years of learning to get to this point. Still have lots to learn. Life is a journey indeed.

But now you know some Kung Fu! At least as far as your frigerator.



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Critic   7/5/2012 10:32:42 AM
Frost build-up in freezers is inevitable. 

One way to reduce the frost build-up would be to fill the freezer with a dry gas, such as dry nitrogen, with a slight positive pressure relative to the outside air.  Air would still enter the freezer when the door is opened, but frost build-up would be greatly reduced.  The drawbacks?  Gas consumption and risk of suffocation.

Another possibility is to use heat-pump technology to defrost the freezer, either by using a separate refrigeration loop, or by reversing the one that is already present.  This would be more energy efficient than a resistance heater.  The drawbacks?  Cost (of the additional mechanical equipment) and reduced reliability.

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There was a solution...
kf2qd   7/5/2012 10:25:31 AM
Used to be you had to empty the fidge every so often to defrost it. Those fridges had a small freazer section and it would collect the frost out where you could see it.

Newer designs offer much more freezer space and efficiency and thus the coils are moved out of sight so you don't see the frost collect.

The only way around it is to not open the fridge/freezer as the source of moisture for the frost is from the air you let in every time you open the door.

Under certain conditins of high humidity the frost can collect at such a rate than the defrost cycle is unable to clear the buildup of ice. Then the only way arond it is to either empty the fridge/freezer and allow it to warm up and melt the ice, or remove a panel(if possible) and use your shop vac to blow the warm house air on the frozen section and defrost the fridge. I have had to do that operation 2 times, both at times of very high humidity. And afterwords the 2 different units worked fine.

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Re: Heaters there for a reason
jb222   7/5/2012 9:23:34 AM
All frost free refrigerators have heaters to clear the frost off the coils.  In the early days, the timer was a clock that ran all the time and the heaters came on whether they needed to or not.  There is a thermostat that will shut off the heat once the ice is cleared.  Then when the timer cycle is done, the compressor will restart.

Later, in an effort to become more energy efficient, they started making the timer a little smarter.  For example, the timer motor would only run when the compressor was running.  Now modern appliances have circuit boards and "brains" that try and determine the optimum time to defrost. 

For appliances installed in unheated garages/barns.  All the cooling takes place in the freezer section (as was mentioned).  But the temperature is controlled in the refrigerator section.  So if you set it at 40deg, and the garage drops to 30deg, then the fridge will probably not run very often.  Not a problem if you are only keeping drinks cold, but a big probably if you are trying to keep meat at 0deg in the freezer.

The extra heater that was mentioned is used to keep the oil in the compressor from becoming to thick and causing damage.


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