But are there many clocks that run on AC power that use internal oscillators even when they are plugged in?
I don’t have a cite that most AC powered clocks use the line frequency to keep track of time. It was common knowledge, and I assumed it was true, since it made sense. If you already have a very accurate oscillator (the AC line), why bother adding the expense of another one?
That link does show that kanicbird’s statement “no digital clocks use the electrical frequency to gauge time” is incorrect. ETA: Well, except for the citation needed part.
I could not find much on it either except that it does appear that some clocks do use the line frequency - to the point that line frequency is corrected to maintain accuracy.
see Long-term stability and clock synchronization in this link:
I was just reading this thread and found a suggestion that variations in power could cause a clock to run fast. Maybe when our power came back on line it was “dirty” or some other thing, causing the clock to gain time?
Perhaps, before the power went out and/or after the power came back on, there was a lot of noise on the line. “Lots of noise” could mean “lots of zero crossings,” and each clock uses the zero-crossings as a frequency reference.
I was thinking that by time the power came back on, things would have been taken care of and it would be clean, but I like the idea that it was noisy before the power went out.
Yeah. I spent a summer in a remote location in northern Canada. At one time, the forest fire threatened the transmission line, and the local dam was “off the grid”, ie. not syncing to the 60Hz AC power cycle. As a result, that wandered (not power surges) and the typical bedside clock radios would wander by as much as 10 minutes a day compared to official CBC beep time. IIRC so did the big analog office clocks (like the ones we remember from school).
Generally, the 60Hz cycle continent-wide is precisely controlled, so it is a reliable signal to run a clock off. Also, any wander is quickly compensated so there is no long term “drift”. If the device is not continuously powered, obviously, it probably needs a crystal. Remember the first cheap digital watches (push button to see red LED) used cheap crystals, These got progressively better over time so that from the mid 70’s to the mid 90’s watches went from a few minutes a week to a few seconds a month accuracy. Crystals are cheap and reliable. However, they can suffer from drift, so a mains-driven clock is less in need of adjustment. (Unless there’s a power failure.) So odds are a crystal that needs to maintain time occasionally in a clock-radio does not need to be extremely accurate and the makers went even cheaper on the crystal.
My clocks without the battery backup will reset to 12:00 (and flash) in the event of a power failure when power returns. Same with my stove. My microwave will scroll the message to set clock. Since many power failures are of short duration, it best to let you know the time may be off. (It’s also convenient to tell you when the power came on…)
I would tend to agree. But when the power comes back on, lots of wacky things could happen on the power line due to heavy loads coming back on line.
I did some google searching on using the 120 VAC / 60 Hz waveform as a clock timebase. There are many circuits out there, and the few that I looked at simply converted the 60 Hz sine wave to a low-level 60 Hz square wave, divided it down, sent it to a counter, etc. etc.
This works O.K. if you trust the 60 Hz is… 60 Hz. But noise on the line that crosses the zero-axis will increment the counter, thereby making the clock run fast.
A more sophisticated circuit would try and filter out the high frequency garbage and extract the 60 Hz signal. This approach might work if the noise is additive and uncorrelated. But if the frequency of the 60 Hz waveform itself is screwed up, there may not be a clean-and-easy way to do it.
I doubt it. The explanation is probably simpler. Digital clocks controlled by the mains frequency often have an internal oscillator and 9 V battery for backup but the internal oscillator is crap, it is just good for short power loss, so that a micro blackout won’t reset the time, it is not really expected to be accurate. If the battery is not in good shape that could affect the precission as well. In this case I think this is more likely than dirty power line. You can test it easily: just unplug the clock for some time and see what happens.
If it is mains operated and the mains fails then it will make no difference while power is gone but it will set itself to the correct time when power returns.
In Europe they sell them prety cheap too and I have a few but they use DCF77 in Germany.
I really don’t see the pint of a battery backup that lets the clock reset to 12:00 and then keeps time from there. The only advantage I can see at all for this is if I needed to know exactly how long power was out. I’d think most people would rather know the time.
I do not care so much why this is happening, but would like to know how to correct it. We had a seven-hour power failure when a transformer died. When the power came back on, all clocks except two were keeping time perfectly (including clocks on t.v. cable boxes, telephones, computers and individual electric clocks). However, the clock on the Jennair oven and the GE microwave are now running fast. I have reset them multiple times, but to no avail. The transformer has not yet been replaced and our whole block is being run by a generator (now for about 5 days!). Perhaps once the new transformer is installed, the kitchen clocks will slow down. But for now, it is confusing. Isn’t this odd?
It is probably using a different frequency or isn’t a sine wave or something. I know when I used a UPS once for an alarm clock I ran into similar issues. I think some are even marketed as havering “pure sine waves” or something.
I am willing to bet that once the transformer is replaced - you will be fine. I have one of those watt measured devices that shows power usage and it will also show frequency - sometimes it is off a little bit - if memory serves like 59.98. If you think about it - it wouldn’t take much to make a clock go off 60.6 for example is only a 1% variation, but that adds up to 14 minutes a day.
And stuff like your cable box is probably getting the time over the cable line. I know I never have to change mine during DST - other items have their own timing circuit and aren’t using the line frequency as an oscillator. There is nothing you can do until the transformer is replaced - unless you see an engineer/tech by the generator - he might be able to adjust the frequency.
Hi, I’m an electronics engineer and thought I’d throw in some ideas of my own to help out. Ok, not all digital clocks use the line frequency (60hz in US) but most do. The power company is pretty good at keeping the the generator spinning at a constant 60 revolutions per second. That’s why our line frequency is 60 hz. The clock counts out 60 cycles and knows 1 second of time has gone by. Now when the power fails, the clock must produce it’s own signal. Some might use a crystal for this, but the loss expensive clocks will use a ic chip like a lm555. This chip allows you to take a DC voltage (DC = Direct Current = 0 Hz) and have a pulse frequency of whatever you design. You change the frequency (and other things that don’t matter here) by changing the resistors and capacitors at the input. I’m simplifying this as far as I can so let’s pretend it only takes one resistor at the input and is 10000 ohm aka 10k ohm. So at 10k ohm you get a nice 60 hz pulse. Now you build the next one. You put in another 10k ohm resistor but instead of 60hz, you are getting 58 hz. Losing 2 seconds per minute. You read the resistor with a ohm meter and it reads 9923 ohm, not 10000 like it should be. You build another one. Now you get 63hz. You are gaining 3 seconds. Now your resistor reads 10856 ohm. Most resistors have a +/- tolerance of 10%. So just the resistors alone could cause you to gain or lose time. But now we have to realize there are a few resistors in this circuit. Each one adding to the error. Sometimes the high value resistor gets offset by a low value resistor in the same circuit. Other times they are both high or both low so the error is larger. Next you have to take into account that the capacitors have +/- tolerance too. The ic can add a little more. In the end you have a bunch of smaller errors adding together to a big one. If the error was +/- 2 seconds per minute, you’d be looking at +/- 8 minutes in 4 hours. This is looking at it from really high up. I didn’t want to go into electronic theory and do calculations. I’m to tired for that. Lol
Aha: the temporary generator is running fast. That is, it’s generating higher than 60 hz. Clocks using crystals are keeping good time; clocks using line frequency are running fast.
