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Relay contact ratings.

From your description, it appears that your fridge looks like this: (View using a fixed-pitch font)

--
                          
MAINS>--+---------------+----------------------------+ 
        |               |                            |
        |               O C                          |
  [DEFROST MOTOR]- - - -|S1                          |  
        |           O-> | |  
        |           |                     |      |NO
        |           |                     | [COMPRESSOR]
        |           |                     |      |
MAINS>--+-----------+---------------------+------+

Am I right?

If I am and you're having problems with the defrost timer and its
switches, then it seems you could replace the motor with a timer and
the timer motor's set of switches with a SPDT relay, like this:

MAINS>-+-----------------+----------------------------+ 
       |                 |                            |
       |                 O C                          |
    [TIMER]-[COIL]- - - -|K1                          |  
       |             O-> | |  
       |             |                     |      |  
       |             |                     | [COMPRESSOR]
       |             |                     |      |
MAINS>-+-------------+---------------------+------+

You've said the defrost only happens every six hours, but for how long
is the heater energized?

knowing that will allow us to determine the duty cycle of the timer
and cause it to mimic the motor timer.
Reply to
John Fields
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=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0|

=A0| =A0 =A0 =A0 =A0 =A0O C

=A0 [COIL}- - - -|

| =A0 =A0 =A0O-> |

=A0 =A0 | =A0 =A0 =A0|NO

=A0 =A0 | [COMPRESSOR]

=A0 =A0 | =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0|

=A0 =A0| =A0 =A0 =A0 =A0 =A0O C

=A0 =A0 [COIL]- - - -|

=A0 | =A0 NO O-> |

=A0 =A0 =A0 | =A0 =A0 =A0|

=A0 =A0 =A0 | [COMPRESSOR]

=A0 =A0 =A0 | =A0 =A0 =A0|

It should be long enough to melt the ice and let the water drain off. It can easily be programmable in a micro, at least during development. Perhaps a 5 to 10 minutes range.

Reply to
linnix

part

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>>>>>

It's hard to tell whether it's 'defrosting' an element that is in any case more or less ice free. The element isn't visible without detaching the backplate of the freezer, and running the fridge without the backplate in place would so change the airflow as to make any observations irrelevant to the normal operation of the fridge.

The reason I got started on this whole exercise was the fact that at times the bread in the fridge was clearly not frozen, even though the freezer temperature certainly drops to less than -15C at times. I hadn't previously looked into the defrost mechanism, and indeed, didn't realise that there is actually a quite powerful heater element involved. Nor did I realise that the defrosting occurs every six hours.

A particular issue with the defrost mechanism is that it has a defrost termination thermostat mounted on the cooling element (set to open at

+11 degrees celsius, closes again at 0C), but regardless of what it does, the fridge remains in its non-cooling mode for about half an hour because of the defrost timer. I put a temperature sensor onto the thermostat, and it shows that the actual temperature there rises to nearly 20C, presumably because even after the heater is turned off, it's still very hot.

In addition, there's no link between the defrost timing and the thermostat. Defrosting may start just as fridge has reached its highest normal temperature and the termostat is about to turn the compressor on. So the day to day outcome will vary considerably, even ignoring ambient temperature changes, making assessing whether the thing is working properly rather problematic.

Now the bread is probably the thing in the freezer that has the least thermal inertia. Maybe the fridge has been working correctly (in the sense of "as designed") all along, and I've just been overreacting to the occasional thawing of the bread.

I've now learnt stuff about self defrosting fridges that I might have been happier not knowing.

Sylvia.

Reply to
Sylvia Else

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I could. Using it wouldn't be so straight forward. The timer has a single pin for input, which is both the source of power for the clock, and for running either the compressor and heater.

Sylvia.

Reply to
Sylvia Else

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Strange that I didn't find them. It's not as if I didn't look.

Sylvia.

Reply to
Sylvia Else

I should buy one of them new-fangled phase-correcting "power savers" and see how much my bill drops by when I reduce the load from zero to zero.

Sylvia.

Reply to
Sylvia Else

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There is no neutral connection to the timer?

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

You mean we have found a way to avoid having to do anything unpleasant to ourselves by foisting the problem on a latter generation. And you think this is a "solution"?

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

--
Certainly you must have misunderstood.

In the context of Moore's Law:

http://en.wikipedia.org/wiki/File:Transistor_Count_and_Moore%27s_Law_-_2011.svg

we _have_ addressed the fundamental truth that endless growth is
impossible in a finite world by diminishing the size of transistors in
order to pack more and more of them into smaller areas as time goes
by.

This allows us to build more powerful machines which, rather than
foisting out problems on future generations, will help us solve some
of ours, and theirs as well. 

Your suggestion that we should make life more unpleasant for ourselves
by not taking Moore's Law into account and building computers out of,
say, 2N2222's is puzzling
Reply to
John Fields 

--
Thanks, but I'd prefer some real numbers from Sylvia so that I can
work up a hardware solution for her.
Reply to
John Fields

You don't like my numbers? Then go with the number from commerical defrost timer.

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ost-5X454?Pid=3Dsearch

4 minutes to 110 minutes in 2 minutes step. 4 to 12 cycles per day.

I would go with 2 pots + 2 A2D. adjust and time it with a micro.

Reply to
linnix

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OK, I suppose that would work, though the thing would still be switching voltage which is double its design rating.

Sylvia.

Reply to
Sylvia Else 

--
I don't care for conjecture, which is all you're offering.
Reply to
John Fields

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Huh???

--
JF
Reply to
John Fields

te:

ong

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 {1,2,3,4,5,6,7}; char duration[8] =3D {5,7,10,15,20,25,30}; 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]); }

...

Reply to
linnix

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Since I'm in Australia, both the defrost heater and the compressor are rated at 240V. Anything that switches them on and off has to switch 240V.

Sylvia.

Reply to
Sylvia Else

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It would have to switch half the current it would on 120 VAC, and the relays are the same for either line volktage.

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

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Really? You think the relay can only handle 120 VAC, just because that's what poweres the timer circuit?

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

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Unless you've actually pulled one apart, and determined the exact specification of the relay, or semiconductor device, involved, it's only safe to assume that it's specified to perform the particular task intended for it. It may well be a part that's specified to handle higher voltages, but it would be rash to assume that.

Sylvia.

Reply to
Sylvia Else

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You've taken one apart?

Sylvia.

Reply to
Sylvia Else

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