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Re: Relay contact ratings.
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OK, changed my mind, let do digital.

Let start with perhaps 4 buttons. Two to change cycles per day and two
to change durations.  Three bits driving 138 (one of eight) to
indicate cycles (eight cycles should be plenty).  Another three bits
to indicate durations such as 5,7,10,15,20,25,30 minutes.  I doubt we
really want to defrost beyond 30 minutes anyway.  So far, we can do it
with 10 port pins and 16 LEDs.

Just a quick first draft:

char cycle[8] 3D% ;
char duration[8] 3D% ;
int c_idx;
int d_idx;

while(1)
{
 if(port(c_up))
  c_idx++;
 if(port(c_dn))
  c_idx--;
 if(port(d_up))
  d_idx++;
 if(port(d_dn))
  d_idx--;

  defrost_on();
  delay(d_idx);
  defrost_off();

  delay_hrs(24/cycle[c_idx]);
}

...




Re: Relay contact ratings.
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Are c_idx and d_idx in non-volatile RAM? I'd prefer my fridge to
continue working, without intervention, after a power outage.

There appears to be a defrost-cycle period between each opportunity to
change the settings by one. Could take a while.

Sylvia.



Re: 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)
100K Pots (x3)
74XXX138 (x3)
LED (x24)
Thermistor
240V/5A AC relay (x2)
etc.



Re: Relay contact ratings.
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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)

--
We have failed to address the fundamental truth that endless growth is
impossible in a finite world.

Re: Relay contact ratings.
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Actually, PIC32MX575L256, 11 digital and 4 analog ports.  Just
checking if anyone is awake here.

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

Re: Relay contact ratings.
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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

Re: Relay contact ratings.
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Yes, we will need one or more thermistors or thermocouples to monitor
the temperature.

Re: Relay contact ratings.
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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).

11C seems rather high, but I suppose it's intended to ensure that the
entire element has defrosted, not just the part near the thermostat. It
also means the thermostat need not be that accurate.

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.



Re: Relay contact ratings.
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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.




Re: Relay contact ratings.
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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.

Re: Relay contact ratings.
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Sound like we should wire the whole fridge with sensors.  But that's
fine, thermistors are cheap.  PIC32 can handle up to 16 analog
channels.

BTW, i also have a busted microwave and fridge to play with.  If only
i have a robotic arm to move something from the fridge to the
microwave and nuke it in the morning.


Re: Relay contact ratings.
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wrote:
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wrote:
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long
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Bad idea. All that would happen would be that you'd discover that the
distribution of emperatures in the fridge are not what you'd like them
to be, but that since all you have is on/off switches for the
compressor, fan, and defrost heater, there's little you can do about it.

But if you're also adding some mechanically operated vents, and some
additional fans, then now you're talking.

Sylvia.

Re: Relay contact ratings.
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At least all around the heat exchanger and pipes.

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We can control how much to defrost, by measuring temperature changes
based on compressor and fan timings and loads.  There are plenty of
parameters we can measures and to adjust in heatings.

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I would add additional hot water pipes.  It's more cost efficient than
electric heat.

Re: Relay contact ratings.
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not at all, ice doesnt conduct heat well, so removing a thin layer of
ice makes the cooling more energy efficient.

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Re: Relay contact ratings.


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the closer the inside temperature of the fridge is to the outside
temperatue the less work the compressor needs to do to circulate he
coolant.

a warm evaporator means the compressor's input is at a higner pressure
than normal and thus it needs to do less work to compress the gas
enough to condense in the condenser,

--
⚂⚃ 100% natural

---

Re: Relay contact ratings.
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 From context, I assume you meant "more" in that last sentence.

The question is - does it matter? That is, will running the compressor
when the element is warm do anything more than consume some extra
electricity. I suppose it's no different from having the fridge turned
off for a while, and then turning it on, but it may have some long term
implications for the life of the compressor if it happens every six
hours. I'm speculating - I don't know.

Sylvia.

Re: Relay contact ratings.
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This is an interesting question.  Whether the compressor starts
immediately as the heater
turns off, or waits a few minutes for the evaporator to "cool" , would
make no difference IMHO.


The reason being that even if the heat is allowed time to dissipate
from the evaporator before re-starting
the compressor this heat is only going to dissipate into the freezer/
refrigeration compartment and if nothing else, warm the compartment
and food slightly more.  The compressor still has to work to remove
this heat, as it is still in the cabinet, just in a different place.


It will just take longer for this dissipated heat to "come back:" to
the evaporator and be removed via the normal refrigeration process. If
turned on straight away, this heat is removed first, then the unwanted
heat from the rest of the refrigeration cabinet, including the food
within.  Theoretically the instant changeover is better for the food
from preventing this temperature rise  , by admittedly a very very
tiny amount that is certain to be irrelevant in this application.


If you were defrosting by a heater something like a heatsink or
radiator that was being refrigerated to cool it, but was operating  in
open air and not in a closed cabinet, it would be slightly more
beneficial from a power saving viewpoint to let the defrost heater's
heat dissipate first before re-starting the compressor.



PS: this discussion motivated me to defrost the small fridge in the
office.  Thanks for that :)

What a mess.

Re: Relay contact ratings.

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Given that our natural gas is 80% cheaper than electricity, and going
lower.  I might replumb the fridge with (gas) hot water pipes, with
electromechanical water valves.  Can i use plastic sprinkler valves
for warm water?  I can install the valves at the outlet, so they won't
be too hot anyway.

Re: Relay contact ratings.

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   Items made with the gray plastic for sprinklers isn't certified for
indoor use.  It is low pressure & low temperature rated and easy to
break.
--
You can't have a sense of humor, if you have no sense.

Re: Relay contact ratings.
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After passing through the fridge, the warm water should be cold
enough, if not too cold.

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I know, i just rebuilt six of them and five were internally broken.
But they are dirt cheap.
The high quality replacement part should outlast the cheap factory
part.


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