Author Topic: Why don't modern ovens keep their time after mains power failure?  (Read 3131 times)

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Offline Halcyon

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For years I've noticed that most microwave and wall ovens don't keep their time after a blackout. Both of mine are under 5 years old, yet each time the power goes out, even for a moment, the clock resets.

Why aren't they designed with super caps to keep the clock ticking for at least a few minutes?
 
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Online Berni

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #1 on: August 31, 2018, 10:23:25 am »
Because supercaps cost money and the clocks use the 50/60Hz from the mains to get the timing from.
 
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Offline ogden

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #2 on: August 31, 2018, 10:23:39 am »
For years I've noticed that most microwave and wall ovens don't keep their time after a blackout. Both of mine are under 5 years old, yet each time the power goes out, even for a moment, the clock resets.

It's feature - blackout indicator. How would you otherwise know that blakcout happened? :-DD

Yes, it's weird that for 20 years or so kitchen appliance electronics have virtually no progress (well except capacitive sensors and "internet-connected madness").
 

Offline mikeselectricstuff

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #3 on: August 31, 2018, 10:24:03 am »
Cost. (not just the cap, but the low power circuitry needed)
It's not a feature that sells any more units.
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Offline joseph nicholas

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #4 on: August 31, 2018, 10:27:58 am »
It's a feature not a bug.  But it does seem kind of stupid not to put a supercap in there, how much more would it cost?
 

Offline mikeselectricstuff

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #5 on: August 31, 2018, 11:07:25 am »
It's a feature not a bug.  But it does seem kind of stupid not to put a supercap in there, how much more would it cost?
How many extra ovens would it sell ?
How often to power failures happen in the countries these appliances typically sell to ?
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Offline glarsson

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #6 on: August 31, 2018, 11:09:30 am »
It is not only the time that is lost. My microwave needs a complete configuration after power failure to set things like language of the display. Without a set language it just displays numbers. It doesn't even default to English. Expected better from such an expensive name brand unit.
 
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Offline Alex Eisenhut

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #7 on: August 31, 2018, 11:15:45 am »
Noticed that a long time ago too! There used to be frequent blackouts here until they sealed up the underground transformer properly.

It is frustrating, but rather than find a simple solution I think the way forward is an i7 with webcam in every appliance that runs unpatched Windows, so it can get the time from NTP. Then within two hours of being plugged in people are downloading your high school pics from your toaster oven.

Progress.
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Offline David Hess

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #8 on: August 31, 2018, 11:22:04 am »
They are just cheap in the quality sense.  They all use a limited number of mass produced controllers and none of them support clock and memory backup.

As far as how often blackouts are, in my experience the number per year has been increasing for decades.  Where I live now, we get at least three a year and two will be hours long.
 

Offline Zero999

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #9 on: August 31, 2018, 12:13:57 pm »
Do they make IoT microwaves? They wouldn't suffer from this issue. Of course until it's hacked so the magnetron only turns on when the door is opened and you're fried.  >:D
 

Online xrunner

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #10 on: August 31, 2018, 12:22:14 pm »
It's a power-failure notification feature. If you are gone from the house, you will know if a power failure happened. Important information easily displayed for you.
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Offline BradC

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #11 on: August 31, 2018, 12:42:34 pm »
Every appliance I've owned with some form of backup, from a pool filter timer to a standard oil heater has required repair or replacement of the backup source (battery or capacitor) long before the appliance might have reached the end of its useful life. The sort of intervention that had I not been handy with a soldering iron, would have seen it on the kerb and the wife off to buy a new one.

I'd rather have an appliance last >30 years and suffer the inconvenience of having to set the clock a couple of times a year.
 

Offline joseph nicholas

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #12 on: August 31, 2018, 02:45:52 pm »
It probabily the annoyance factor that companies love to embrace.  When something is a constant irritant it sticks in your mind but as time goes by you tend to forget it was annoying and just remember the products manufacture's brand name.  But of course all modern microwave ovens use the same chip to control itself.  This market is extremely competive with lots of sales.  They all act the same way.  Almost nobody, rich or poor fixes a broken microwave, they just throw it out and buy a new one.
 

Online Mr. Scram

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #13 on: August 31, 2018, 02:53:39 pm »
Now you have a very accurate and reliable clock, bar outages. Otherwise you'd have a more expensive clock that could run fast or slow and has caps that can go bad.

I prefer the dumb, cheap and reliable option.
 
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Offline David Hess

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #14 on: August 31, 2018, 04:20:59 pm »
Everybody here likely has one or several ubiquitous examples where this was a solved problem going back decades.  The battery backed up clock and CMOS RAM inside most PCs does everything needed with a replaceable CR2032 battery.

Consumer appliances are just cheap.

 

Offline hermit

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #15 on: August 31, 2018, 04:29:36 pm »
As someone who used to work in that industry in the repair field let me give you the 'real answer' as it was given to us.  You ever fix something by unplugging it to clear the corrupted memory?  A battery would prevent this. So, consumers are going to gripe about resetting the time or the 'complex' method of clearing the memory.  Often they would get charged for this by a repair person.
 

Online nfmax

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #16 on: August 31, 2018, 04:43:12 pm »
The OP asked the wrong question. It should have been: "Why do modern ovens require the damn clock to be set before they will cook anything?"
 

Offline mzzj

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #17 on: August 31, 2018, 05:01:32 pm »

How many extra ovens would it sell ?
How often to power failures happen in the countries these appliances typically sell to ?
THIS.

Supercap or battery is probably already dead before the first power outage  ;D
In last 10 years we have had one or two power outages so its not really a selling point for me...on the other hand my parents home on countryside has power outages couple of times each year.
 

Offline David Hess

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #18 on: August 31, 2018, 05:14:45 pm »
The OP asked the wrong question. It should have been: "Why do modern ovens require the damn clock to be set before they will cook anything?"

Mine do not but both of my microwave ovens are on their second high voltage ballast capacitor.
 

Offline factory

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #19 on: August 31, 2018, 05:36:01 pm »
Another unwanted feature to look for if I have to replace either of them, I don't bother with the clock on the Microwave and the oven is gas (I use a 60min clockwork timer for that). Both are switched off at the wall when not in use and not left unattended when in use.

The only kitchen appliance that stays on is the fridge/freezer, everything else would just waste energy.

David

PS, We have had quite a few short power cuts this year.
 

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #20 on: August 31, 2018, 05:49:19 pm »
It is not only the time that is lost. My microwave needs a complete configuration after power failure to set things like language of the display. Without a set language it just displays numbers. It doesn't even default to English. Expected better from such an expensive name brand unit.
Not quite as bad as my Panasonic inverter combi microwave oven.

It has a grill, and three power levels.  It also has a six character display.

So, to display the grill power level, it shows "GRILL " followed by "RILL 1".  That is, it scrolls a seven character message because it will not fit on the six character LCD.  And you need to wait three seconds every time for it to start scrolling,  *every* time you change the power level.

Nothing would have stopped them abbreviating it to GRL 1,  or even just removing the space there...but nope.   It scrolls the message.  It's infuriatingly bad design and I encounter it on an almost weekly basis.  I can guess the power level by pressing it a few times,  but sometimes I'll miscount -- so  I'll have to wait for it to tell me the setting.
 

Online PA0PBZ

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #21 on: August 31, 2018, 06:12:46 pm »
I don't understand why you would even have a time display on a microwave, do you also have one on your coffee maker, fridge, mixer? If I don't set the clock on my microwave the display just stays dark when not in use, the way it should be.
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Offline glarsson

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #22 on: August 31, 2018, 06:19:26 pm »
I don't have any use of the clock on the microwave. Luckily it doesn't flash 00:00 when not set or show any other time, it only shows :. However, it does not display text menus unless the language is set. Ok if you know what menu number 16 of 17 does.
 

Offline steve30

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #23 on: August 31, 2018, 06:22:37 pm »
Not quite as bad as my Panasonic inverter combi microwave oven.

It has a grill, and three power levels.  It also has a six character display.

So, to display the grill power level, it shows "GRILL " followed by "RILL 1".  That is, it scrolls a seven character message because it will not fit on the six character LCD.  And you need to wait three seconds every time for it to start scrolling,  *every* time you change the power level.

Nothing would have stopped them abbreviating it to GRL 1,  or even just removing the space there...but nope.   It scrolls the message.  It's infuriatingly bad design and I encounter it on an almost weekly basis.  I can guess the power level by pressing it a few times,  but sometimes I'll miscount -- so  I'll have to wait for it to tell me the setting.

I've had a Panasonic machine for years and have always noticed this silliness. It never really bothers me though; probably because I tend to count the beeps. one beep for grill 1, two beeps for grill 2, three beeps for grill 3.

Personally I would have thought they'd have named them the other way round, with 1 being the lowest power and 3 being the highest.
 

Offline Halcyon

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #24 on: September 01, 2018, 12:39:02 am »
It's a power-failure notification feature. If you are gone from the house, you will know if a power failure happened. Important information easily displayed for you.

I guess some people look at it that way, but I don't see how it would help the regular consumer?

I've had two brief power failures in two weeks due to wind/weather and other than "oh great, the power went out", I don't know when it occurred or for how long. It's not really all that useful.

Besides, I already have an elaborate power failure notification system: My NAS needs the crypto password before I can access it again. ;-)

My Sharp microwave oven isn't too much of a bother to me. When the clock resets, the LCD just displays "0" (for no time left on the cooking timer). It's logical, I can live with that. It doesn't flash away at some arbitrary time like my wall-oven does.
 

Online xrunner

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #25 on: September 01, 2018, 12:42:04 am »

I guess some people look at it that way, but I don't see how it would help the regular consumer?

I've had two brief power failures in two weeks due to wind/weather and other than "oh great, the power went out", I don't know when it occurred or for how long. It's not really all that useful.

I was mainly joking but actually I have left the house for a couple of hours and come back when a small power failure has occurred. Everything is back on when I come home but I notice the oven clock is flashing, it does tell me that a power failure had occurred. Not very useful I admit but it does tell you something happened.
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Offline Muttley Snickers

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #26 on: September 01, 2018, 01:12:20 am »
It's a power-failure notification feature. If you are gone from the house, you will know if a power failure happened. Important information easily displayed for you.

I agree with this concept, what if you went away for a few days and came home to discover that all of your expensive meat in the freezer had suddenly gone bad because the fucking power was off for a long period. Unless you have a security system which is configured to notify or report an AC failure then you would be none the wiser and perhaps put it down to a dodgy butcher.

If the utility company failed to give advance notification of a prolonged outage due to routine maintenance then the customer might be eligible for compensation. Winner, winner we are now having steak for dinner instead of fish fingers.   :P 
 
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Online BrianHG

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #27 on: September 01, 2018, 01:14:01 am »
IOT appliances, our future:



__________
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Offline Halcyon

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #28 on: September 01, 2018, 01:26:56 am »
It's a power-failure notification feature. If you are gone from the house, you will know if a power failure happened. Important information easily displayed for you.

I agree with this concept, what if you went away for a few days and came home to discover that all of your expensive meat in the freezer had suddenly gone bad because the fucking power was off for a long period. Unless you have a security system which is configured to notify or report an AC failure then you would be none the wiser and perhaps put it down to a dodgy butcher.

Sure, but how would you know if the power went out for 15 seconds or 15 hours? Would you throw out all your food "just in case" ?

A small bag or container of ice with a hole in the bottom would be a far more effective indicator; If the ice has melted, your freezer got too warm.
 

Online xrunner

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #29 on: September 01, 2018, 01:37:02 am »
Sure, but how would you know if the power went out for 15 seconds or 15 hours? Would you throw out all your food "just in case" ?

Very simple (I just tried it to make sure ). The time when the power is restored starts where it left off. It remembers the time when the power went off and starts counting again at that point. So you can very easily see how many minutes or hours have elapsed by comparing the oven time to your correct time. So we know it remembers the time at the point of power failure - at least my oven does.

 :popcorn:
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Offline Muttley Snickers

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #30 on: September 01, 2018, 01:47:23 am »
Sure, but how would you know if the power went out for 15 seconds or 15 hours? Would you throw out all your food "just in case" ?

A small bag or container of ice with a hole in the bottom would be a far more effective indicator; If the ice has melted, your freezer got too warm.

You wouldn't know to check the defrost status of your ice indicator without first being alerted of the AC outage by the flashing 12:00 on your oven. Furthermore, a quick conversation with the neighbour would normally be a good idea after such an event to give a proper indication as to what happened, this is particularly important for those who have a hobby farm or rural weekend retreat and are not there permanently.
 

Offline Halcyon

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #31 on: September 01, 2018, 02:16:45 am »
Sure, but how would you know if the power went out for 15 seconds or 15 hours? Would you throw out all your food "just in case" ?

Very simple (I just tried it to make sure ). The time when the power is restored starts where it left off. It remembers the time when the power went off and starts counting again at that point. So you can very easily see how many minutes or hours have elapsed by comparing the oven time to your correct time. So we know it remembers the time at the point of power failure - at least my oven does.

 :popcorn:

Ahh well done sir.  :-+
 
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Online Berni

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #32 on: September 01, 2018, 07:01:49 am »

Sure, but how would you know if the power went out for 15 seconds or 15 hours? Would you throw out all your food "just in case" ?

A small bag or container of ice with a hole in the bottom would be a far more effective indicator; If the ice has melted, your freezer got too warm.

There is a trick for that. Take a small plastic bottle, fill it up 1/4 of the way with water and mix in something colorful, put it in the freezer upside down to freeze and then turn it upright again. If the stuff stays in the top of the bottle then your freezer is fine, if you find the liquid in the bottom of the bottle then it defrosted.

But it does take a really long power cut before a proper freezer defrosts itself. It will reach 0°C fairly quickly but once it gets there the ice on the walls (Cause lets face it you been meaning to scrape it off 3 months ago) will start to melt first and hold the temperature at 0°C for as long as there is still ice there and due to how much energy it takes to transition water between states this makes it last a long time. Additionally modern freezers come with sub zero cold packs in the top . These packs are filled with a liquid that freezes a good deal lower than 0°C (Not sure what it is but even just salty water would work) so when the fridge fails that liquid starts to thaw out first and will keep the insides from even reaching 0°C for a very long time (over 12h)
 
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Offline Halcyon

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #33 on: September 01, 2018, 07:16:32 am »

Sure, but how would you know if the power went out for 15 seconds or 15 hours? Would you throw out all your food "just in case" ?

A small bag or container of ice with a hole in the bottom would be a far more effective indicator; If the ice has melted, your freezer got too warm.

There is a trick for that. Take a small plastic bottle, fill it up 1/4 of the way with water and mix in something colorful, put it in the freezer upside down to freeze and then turn it upright again. If the stuff stays in the top of the bottle then your freezer is fine, if you find the liquid in the bottom of the bottle then it defrosted.

The only issue I have with that is that your freezer (or rather the water in the bottle) might defrost just enough for the ice block to slide down the bottle but not actually fully defrost. You would want your "indicator" water to either flow into another container through a hole or escape completely into the freezer cavity, that way there can be no ambiguity.
 

Online Berni

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #34 on: September 01, 2018, 07:50:02 am »

The only issue I have with that is that your freezer (or rather the water in the bottle) might defrost just enough for the ice block to slide down the bottle but not actually fully defrost. You would want your "indicator" water to either flow into another container through a hole or escape completely into the freezer cavity, that way there can be no ambiguity.

That is a feature that gives it "high sensitivity". As soon as the temperatures goes over 0°C it will turn a thin layer on the walls to water and let it drop down. Once its down there it will keep the bottle neck shape, but if continues melting then the bottle shape is lost and you get a varying level of flat liquid at the bottom. So the chunk dropping means it reached above 0°C and how much it melted out of shape on the bottom shows the duration of the fault.

To disable the high sensitivity feature you can throw in some thin rope or twine and let the ends dangle out when putting the cap on so that it grabs them. The twine gets bunched and tangled all trough the liquid before it freezes and keeps it from falling so the only stuff that makes it to the bottom is liquid.
 
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Offline SparkyFX

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #35 on: September 01, 2018, 09:25:55 am »
A small bag or container of ice with a hole in the bottom would be a far more effective indicator; If the ice has melted, your freezer got too warm.
My freezer´s interiors are covered in 1/2 inch thick ice that accumulated over time from humidity that got in when opened. If that melted i´d notice my wet feet before i want to check the clock.
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Online xrunner

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #36 on: September 01, 2018, 12:14:25 pm »
The built-in clock paradigm is fading out. The new Spectrum boxes don't have a clock any more nor a channel display.  :(
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Offline Halcyon

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #37 on: September 01, 2018, 12:50:08 pm »

The only issue I have with that is that your freezer (or rather the water in the bottle) might defrost just enough for the ice block to slide down the bottle but not actually fully defrost. You would want your "indicator" water to either flow into another container through a hole or escape completely into the freezer cavity, that way there can be no ambiguity.

That is a feature that gives it "high sensitivity"... *snip*

To disable the high sensitivity feature...

 :-DD Love it!
 

Offline Eka

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #38 on: September 02, 2018, 07:19:07 pm »
Every appliance I've owned with some form of backup, from a pool filter timer to a standard oil heater has required repair or replacement of the backup source (battery or capacitor) long before the appliance might have reached the end of its useful life. The sort of intervention that had I not been handy with a soldering iron, would have seen it on the kerb and the wife off to buy a new one.

I'd rather have an appliance last >30 years and suffer the inconvenience of having to set the clock a couple of times a year.
A simple 0.1F super capacitor would handle most power outages, but it and it's charging circuit costs money. If kept cool it would also last at least as long as the power supply capacitors in the appliance will, unlike a lithium battery.

I've powered the RTC in a NXP ARM chip with a 1F super capacitor for 36 hours. I haven't formally tested longer power outages yet. At 6 to 7 days the RTC clock was still running, but had lost an hour. The exact same super capacitor would power a DS3231 RTC chip for 4+ days with temperature corrections still being applied and a week or longer without them being applied after the voltage drops to low for the temperature correction circuits to run. It did that without loosing any time. My charging circuit is in series a resistor to set a reasonable charging current, and very low leakage current diode to keep the supper capacitor from trying to power the 3.3 VDC rail after the power turns off. It only charges up to 3.0V, but that is enough to keep the RTC clock circuity running after the 3.3 VDC rail fails. My 1F super capacitor cost $5 in single quantity, and the diode cost about a dollar in single quantity. A 0.1F super capacitor would ride through most power outages I see. I get a few short 1 to 5 second long power outages nearly every thunderstorm, and a few 1 to 4 hour power outages every year. I live near the edge of the service area of a rural electric cooperative.
 

Online tom66

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #39 on: September 02, 2018, 08:23:46 pm »
The built-in clock paradigm is fading out. The new Spectrum boxes don't have a clock any more nor a channel display.  :(

Good.  We have on screen displays for channel info, smartphones and watches for the time.  Why do we need yet *another* clock / indicator on a product, adding cost and power consumption for no apparently good reason?

 

Online Mr. Scram

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #40 on: September 02, 2018, 09:53:37 pm »
I just noticed my microwave has a power outage indicator. It resets the clock all the same, so I'm not sure how useful that addition is.
 

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #41 on: September 02, 2018, 11:02:53 pm »
Good.  We have on screen displays for channel info, smartphones and watches for the time.  Why do we need yet *another* clock / indicator on a product, adding cost and power consumption for no apparently good reason?

OK great idea - oh but wait - are you aware that the boxes don't ever truly turn off when the consumer turns them "off" with the remote Tom66? Like the one in the picture? See my thread here -

https://www.eevblog.com/forum/chat/cable-box-rant-related-to-power-consumption/

I've measured the power consumption. They use 13W when on and 12W (probably more like 12.5W) when "off". I put the "off" in quotes because the box isn't off by a long shot. And it's almost too hot to touch at the coax connector when it's "off" because the coax connector and cable is probably being used as a heat sink by design to save money, as one person suggested. The thing is chugging away doing it's thing on the backbone for the sole purpose of being ready to turn on just a touch faster for those impatient consumers. It's only off when the power plug is yanked out of the back.

So you want to reduce cost and power consumption? Why do they let it burn 12 to 13W 24/7/365 when the average consumer thinks they are turned off when they aren't?  Suddenly the clock and or cable channel indicator isn't such a big deal :-//
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Offline mikeselectricstuff

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #42 on: September 03, 2018, 09:50:36 am »
Good.  We have on screen displays for channel info, smartphones and watches for the time.  Why do we need yet *another* clock / indicator on a product, adding cost and power consumption for no apparently good reason?

OK great idea - oh but wait - are you aware that the boxes don't ever truly turn off when the consumer turns them "off" with the remote Tom66? Like the one in the picture? See my thread here -

https://www.eevblog.com/forum/chat/cable-box-rant-related-to-power-consumption/

I've measured the power consumption. They use 13W when on and 12W (probably more like 12.5W) when "off". I put the "off" in quotes because the box isn't off by a long shot. And it's almost too hot to touch at the coax connector when it's "off" because the coax connector and cable is probably being used as a heat sink by design to save money, as one person suggested. The thing is chugging away doing it's thing on the backbone for the sole purpose of being ready to turn on just a touch faster for those impatient consumers. It's only off when the power plug is yanked out of the back.

So you want to reduce cost and power consumption? Why do they let it burn 12 to 13W 24/7/365 when the average consumer thinks they are turned off when they aren't?  Suddenly the clock and or cable channel indicator isn't such a big deal :-//
In the case of cable boxes  they're not always doing nothing - updating code and EPG for example. They could in principle have an on/off cycle to do this but synchronising that would get complicated - due to the huge numbers of them, anything that caused a tiny percentage of issues  would affect a significant number of customers.
Even issues like thermal cycling could become significant, so for reliability, leaving them always on apart for the display is probably the best approach.
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Online tooki

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #43 on: September 03, 2018, 10:02:48 am »
Every appliance I've owned with some form of backup, from a pool filter timer to a standard oil heater has required repair or replacement of the backup source (battery or capacitor) long before the appliance might have reached the end of its useful life. The sort of intervention that had I not been handy with a soldering iron, would have seen it on the kerb and the wife off to buy a new one.

I'd rather have an appliance last >30 years and suffer the inconvenience of having to set the clock a couple of times a year.
A simple 0.1F super capacitor would handle most power outages, but it and it's charging circuit costs money. If kept cool it would also last at least as long as the power supply capacitors in the appliance will, unlike a lithium battery.

I've powered the RTC in a NXP ARM chip with a 1F super capacitor for 36 hours. I haven't formally tested longer power outages yet. At 6 to 7 days the RTC clock was still running, but had lost an hour. The exact same super capacitor would power a DS3231 RTC chip for 4+ days [...]
The issue is that it wouldn’t be adding only a supercap. Oven clocks don’t have an RTC, they are mains-clocked. So adding only a supercap would allow it to remember the time, but the time wouldn’t advance! So at the end of the power outage, the clock would be slow, which is arguably worse than being blank. (A clock that’s 10 minutes behind after a 10 minute power outage that happened while you were out, and thus unaware of, could have you arriving at an appointment late.)

(Remember the European line frequency problem earlier this year? That’s when we all learned that ovens don’t have RTCs and are in fact line-frequency-clocked.)
 

Online RoGeorge

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #44 on: September 03, 2018, 10:33:40 am »
  means it's time to check the food in the freezer.  ;)

Online xrunner

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #45 on: September 03, 2018, 12:37:01 pm »
In the case of cable boxes  they're not always doing nothing - updating code and EPG for example. They could in principle have an on/off cycle to do this but synchronising that would get complicated - due to the huge numbers of them, anything that caused a tiny percentage of issues  would affect a significant number of customers.
Even issues like thermal cycling could become significant, so for reliability, leaving them always on apart for the display is probably the best approach.

Well Mike I certainly respect your opinions here. But let's look at it this way - we were talking about saving costs of production, as the other person said
"yea eliminate the display parts". As I found out, the thing is not really turned off unless the plug is pulled. It uses 13W on and 12W "off". My Killawatt is really flickering between 12W and 13W so I'd give it 12.5W when "off". It's probably because of the points you raised.

So why do they even have an "off" function (button) on the box? Why spend money on an "off" hardware button? Why not just design it so the cable installer plugs it in and it's on all the time (which for all practical purposes it is anyway). The old boxes were the same way, they were always really on, except they had a clock at least for the people to use if they wishes.

Good.  We have on screen displays for channel info, smartphones and watches for the time.  Why do we need yet *another* clock / indicator on a product, adding cost and power consumption for no apparently good reason?

I forgot this point last time I responded. When my 85 yr old mother was alive and messed up the settings of her cable box or TV she'd ask me to come over and fix it. She'd always say "Make sure the cable boxes (she had two) are showing the time!". Those were her clocks. Now you can come back and say "Your mother needs to get with it and get some other clocks!"

The point is, even though you and I can get along with technology changes some senior citizens want things the old way. They do not like changes even as small a thing as removing the clock from the cable box. When I asked the cable installer about the missing display he confirmed that many of their older customers did not like the display removed.

Point is, let's let little things be for a while longer until an older generation passes, before we save a few cents and remove little things like clocks they like to use from their lives. Is that really too much to ask?
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Offline NiHaoMike

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Re: Why don't modern ovens keep their time after mains power failure?
« Reply #46 on: September 03, 2018, 12:43:11 pm »
The issue is that it wouldn’t be adding only a supercap. Oven clocks don’t have an RTC, they are mains-clocked. So adding only a supercap would allow it to remember the time, but the time wouldn’t advance! So at the end of the power outage, the clock would be slow, which is arguably worse than being blank. (A clock that’s 10 minutes behind after a 10 minute power outage that happened while you were out, and thus unaware of, could have you arriving at an appointment late.)
I recall Fran Blanche covered how one of the old time clock chips covered that issue - switch to a RC oscillator when running on batteries.
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