Relay contact ratings.

part

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

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.

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.

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

Sylvia.

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.

There is no neutral connection to the timer?

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"?

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

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

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

Sylvia.

Huh???

te:

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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 {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]); }

...

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.

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

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

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.

You've taken one apart?

Sylvia.

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.