Relay contact ratings.

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Depends on the micro. Some can save to EEPROM. That's why i though about pots in the first place. Pots are non-volatile. We need an A2D to read from thermistor anyway, so perhaps using 3 channels A2D. One port for the defrost heater and one more for the compressor. One more set of decoder/LEDs for temp. settling. So, to do all the fridge control:

PIC32MX256DA206 (using 11 digital ports and 3 analog ports)

I've taken apart a lot of things and I've never seen a power relay with 120 volt rated contacts. Do whatever you want, I'm done with this thread.

Seriously, PICAXE 08M2 - 3 x 10 bit digital ports, configures and reads as 8 or ten bits with with a single "readadc" or "readadc10" command (IIRC) and 256 bytes eeprom reads and writes with simple single line instructions. PICAXE is the simplest and fastest solution to any one off that needs a little smarts and not too much speed bar none. If you need lots of LEDs one of the bigger PICAXE chips will do.

I have programmed (and still do) PIC's in assembler (very badly - I still do that very badly too), Microchip C, Swordfish Basic, and PICBASIC Pro. For speed, cost effectiveness and ease of producing a one-off PICAXE easily cleans the floor with all of them.

(I don't really like the guy who invented the PICAXE or his business model but it is a good product)

"Moore's Law" fails when you get to one atom sized conductors (in practical terms we can expect it to fail before then)

"Moor's law" is about one, specific and small situation. By appealing to "Moore's Law" as a general solution you are burying your head in the sand.

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Actually, PIC32MX575L256, 11 digital and 4 analog ports. Just checking if anyone is awake here.

I can do a one-off PIC32 faster than PICAXE. I don't like doing decimal bit maps in BASIC.

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Its a better idea to stop defrost cycle when the exchanger temp rises above freezing. Otherwie you're wasting energy, and at some point the machine will likely ice up.

NT

FWIW, those are oft-cited misconceptions. Media drones repeat them without understanding their basis.

The number is ~32 million, and these are people who do not have medical insurance. That is not the same as "extremely poor access" to care. Numerically, some 10-12 million of that figure are illegal aliens, and most of the remainder are young people who voluntarily choose not to buy it. Access is generally good to excellent.

Last, everyone can get care. There are countless medical programs for the poor.

I personally have an acquaintance getting the full royal treatment, for cancer, for nothing. She's better than most at working the system, but it's there for people who need it, and seek it.

-- Cheers, James Arthur

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Yes, we will need one or more thermistors or thermocouples to monitor the temperature.

The 'standard' self-defrost implementation has an additional thermostat on the cooling element, which trips open when the element reaches some predetermined temperature (11C on mine), and closes again at some lower temperature (0C on mine).

Switching the cooling back on immediately after the thermostat trips off seems attractive, to avoid defrosting the stuff in the freezer, but I have some misgivings about the impact on the compressor of starting when the element is that warm.

It's not clear to me why the machine would ice up if the defrost cycle is left running too long.

Sylvia.

Well, I obviously can't say what you've see, but here's relay with 120V rated contacts.

Sylvia.

Mike said _power_ relay. The one in the url you posted is a signal relay.

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You could design a defrost system like was on the New Inventors last year. IIRC a resistive strip was placed in the freezer, a known voltage passed through it periodically and the resistance of it measured (heat causes resistance to rise, and the more frost/ice formed on the strip the longer it took to heat up and resistance to rise) and turn on the defrost system only when sufficient ice was detected.

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I have worked on a couple of newer Australian made ones here and now they have no relays at all in them. Other brands might have, or older models that are being replaced.

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I have checked a few from my junk box, any that are intended to switch mains power and not signal circuits have a 240v rating. Many were out of junked US made equipment and made in Japan, Mexico or Taiwan.

As they are for the world market, I would think that most would be rated 240v.

One Japan made one that is UL rated has an inductive rating of 5a

5A 120V and 32v for resistive loads.

The contact clearance is about 1mm.

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A lot of the modern ones use butane as a refrigerant so may be they do not want open contacts in the very remote case of leaks

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Did not see that but it may also be the case with the newer fridges.

I suppose it depends what one is trying to achieve. Clearly, it's wasteful of energy to heat up, and then cool, an element that doesn't have much ice on it.

On the other hand, keeping it that way means that the defrost cycle doesn't have to take long, which is helpful is one's primary goal is to keep the freezer compartment below a certain temperature.

Sylvia.