Annoying Clock on Microwave

Check your outlet! It sounds like some considerable voltage drop occurs in the wiring and the clock stops during operation. I've seen a microwave gain seconds when in use, too (the TTL chips in the clock circuitry had no bypass capacitors).

A chip would likely be designed to work at either frequency, with a jumper making the selection.

I doubt if it's voltage drop because the clock loses about 6 minutes over a three day (holiday) weekend when no one is there using the microwave.

Also, FWIW, a complete power drop out and recovery causes the clock to start at 12:00.

Yes but some years back our group designed a micro controller project that used AC line zero cross for phase triggering. Making that controller into a clock and using 50 or 60 Hz as timing reference would be as easy for them as it was for us. So THAT isn't a valid excuse.

G=B2

Did it work by looking for a polarity change, or near-zero voltage? If the latter, line noise might cause it to miss some zero crossings, and the clock would run slow.

I don't think this is a 50/60Hz issue. From the error rate you describe, it should run faster not slower, and by a larger margin too.

Regardless, locking your clock to mains is not always a great idea. Some areas are just plain wrong. I have a mains locked clock here that loses a minute every week or so. I have cheap non temperature compensated crystal locked clocks that are by far superior to that.

The answer is worse than that.

The clocks are built to a price, the cheapest possible price.

Consider this: A microwave oven doesn't need a clock at all, just a timekeeper for timed cooking. Accuracy is not a priority here at all. The fact they have clocks at all means the manufacturers wanted to get one leg up on the competitors without expending any money.

We're not talking cameras in cellphones here, there is NO reason for them to make their clocks accurate, because non of the competition are going to bother either.

Probably the clock is line sync'd, and then when cooking, is magnetron supply sync'd - as critically voltage is lost at the connection point where the clock takes its time period sample.

Try not cooking for a day and see if that fixes the time loss, then try using a shorter or heavier cable for the line connection?

On 4/22/2011 10:27 PM John Tserkezis spake thus:

True now, no doubt, but wasn't always so. My Magic Chef, which I got used, lessee, 23 years ago or so and still works perfectly, has a very accurate clock. Crystal-controlled, I'm going to guess.

In the US, line frequency used to be (and probably still is) "tweaked" so that clocks with synchronous motors would have zero net error over a long period of time. This should give better accuracy than a crystal.

Donate it...and buy a new one. I've never had an appliance with a digital clock that kept poor time. Coffee maker, oven/range, MW, or VCR.

Correct. In fact back in the early 70s when they had the big power outage in the NorthEast US my sister was working at a radio station that checked their electric clock against WWV every day. When the blackout occurred their clock lost a bunch of time (I don't remember exactly how much) due to the load slowing down all of the generators. They reset it and then for the next week or more they had to set it back every day as the mains frequency was cranked up a bit until the long term frequency was returned to 60 Hz. In fact it was for a long time known that long term stability of the 60 Hz power was some of the best available. Short term was never as good, it tended to cycle up and down a bit according to various things happening.

Bill

So you live in a place _not_ on the grid?

G=B2

I have a small coffee maker that has never kept good time. My guess is that *any* noise on the AC line affects the pulse counting. I have a weather station whose clock is synced to NIST, so no longer bother to set the clock on the coffee maker.

John

So...a surge suppressor might fix it?

ock

It looked for zero crossings directly into the interrrupt input (via a comparator to clean it up). I had many more problems with extra rather than missing pulses. That was solved easily by using one of the processor timers to disable interrupts until almost time for the next external. I really need to implement the WWVB receiver to REALLY keep it accurate.

G=B2

Maybe. In about 2005, I got a consulting gig to clean up a WWVB receiver design inside a microwave oven. Operation was deemed erratic by the engineers. I had 1 week to pull off a miracle, and predictably failed. The product was released with a ceramic resonator.

I'll spare you the details, but methinks it won't work. The basic problems are:

1. WWVB does not have world wide coverage. You'll need a receiver that scans from 40 to 100Khz looking for time sync signals. Of course, every country has their own protocol. Such chips are available today, but were not when I was dealing with the problem:
2. To get decent field sensitivity, I needed a big fat loopstick like this:

The problem is that the Q is sufficiently high that it won't cover the entire 40 to 100KHz range. Lower Q would work, but at reduced sensitivity. Just finding a place for the loopstick was a challenge as it had to be outside the metal cabinet. Manufacturing went ballistic when they saw the loopstick and announced that the hand labor involved would dramatically raise the price. They were right.

1. Interference from switching supplies and digital rubbish can be fatal. A 20Khz switcher will have enough harmonics to kill the LF receiver. I convinced them to turn off the LED display as a token "energy conservation" measure. However, my switching cell phone charger would kill the receiver up to a distance of about 10 ft.
2. The controller group wanted a WWVB disciplined oscillator at any of the standard CPU clock frequencies (i.e. 3.57Mhz). However, none of the off the shelf chips offered this feature. As a compromise, they asked for 1 pps, which was also not offered. When I offered to feed them encoded BCD character data, they threatened to have me fired for incompetence.
3. Whatever else I forgot...

At this time, I would suspect it can be done with a GPS receiver. High sensitivity GPS receivers will product accurate time sync even indoors.

It may take nearly forever to obtain sync, but for a microwave oven, that only needs to be done once or twice per day. If location is an issue, I can extract the time from some various broadcast and cellular services:

Did you look to see what they actually used? My guess(tm) would be a PZT ceramic resonator.

These have an initial tolerance of about 0.3% to 0.5%, which is a rather large: 24 hrs/day * 3600 sec/hr * 0.0005 = 43.2 seconds/day If you're losing 120 sec/day, then something is wrong with the ceramic resonator, the associated capacitors, or the CPU. Since you're losing time (running slow), the clock oscillator frequency is too low, so it's probably not the capacitor. Find the frequency, identify the CPU, and install a replacement resonator.

Sigh.

I do live on the grid. I, along with everyone else was promised that while short term timing errors on the grid are expected, longer term errors are tweaked and by far less. I could have lived with this if they actually meant it.

However, in real life, it's STILL not good enough for clocks. Like I said, I have many non-temperature crystal locked clocks that are by FAR better.

I have cheap Chinese digital watches (NOT on my wrist) at the whim of the seasonal temperature environment that have minor seasonal shifts, but over the year are better than mains-locked clocks.

I can't even get a straight answer googling. Lots of nice graphs that really don't answer the question too. The BEST was along the lines of "is very precise" and "it shouldn't vary too far".

In engineering terms, that's just plain bullshit.

You mean you had a VCR that *DIDN'T* flash 12:00 all day?

That could only mean one of two things: You're either a Rocket Scientist, or you had a six-year-old around.