real time clocks & 60hz from power company

Ah, I see. That could be annoying after a while.

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The grid in the US and Canada, at least, is closely controlled over a 24 hour period. The same applies to most of Europe, and other places. However, it can go out (power loss), so that if your system is not mains powered you should have an independant crystal controlled clock. You can resynchronize it at intervals from the mains driven clock, and detect power losses by an excessive change (at which time you resynchronize the mains clock).

The system can easily tell the difference between 50 and 60 hz power. 40 years ago I did it with a 10 hz multivibrator, synchronized by the output of the zero-crossing detector. This gave the same result on either 50 or 60 hz input.

Your biggest problem will be immunity to line noise in the cycle counting mechanism. You need at least a low pass filter together with a Schmidt trigger. The alarm clock mechanisms are basically synchronous motors, which do that filtering very well.

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I checked with my power company about this and was told that their frequency may vary around the 60 Hz nominal, but they strive to make up or lose during each 24 hour period so that the total number of cycles during each 24hr period is 60*86400. Their reference us UTC, which is referenced to the "atomic clock", cesium, and soon an even better one, using mercury atoms. UTC time makes sure the earth is back at its starting point in its orbit every 365.24(whatever it is) UTC days. If needed, the UTC committe can add or subtract a "leap second" once or twice a year to keep things right. It's interesting to compare a digital clock(referenced to 60 Hz, not quartz xtal)to the time signals from WWV. Sometimes you can actually detect a change from the the digital clock gaining to losing during the day. If you really want to hurt your brain over the concept of time, google for sundials or Equation of Time.

Practically, even a regular quartz controlled oscillator is just not too hot for holding time. At 50 ppm, that's 4.3 seconds/day. I built a clock using a Dalis temp-compensated 32KHZ chip together with

2 8-bit conters set to divide by 32768 and wound up with an accuracy of about 0.7 ppm, or 2 seconds/month. With more care given to the supply voltage of the 32KHZ, I think I could have done better by a factor of 3 or so. Could your application use something like this? It's 3 more chips and Does anyone know of any reason to not use the 60hz/110VAC signal coming

If God hadn't intended us to eat animals, He wouldn't have made them out of MEAT! - John Cleese

I have the exact same model! I set it yesterday when I sent my message and it seems to have gained half a second since then.

If you want to see a very interesting photo of the internals of the Mostek MK-5017 clock chip used, then visit my web site at:

There's a little surprise on the chip.

...Tom

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Tom Sheppard
Real-Time Fundamentals instructor-led and web-based training courses
http://www.Surreal-Time.com/

I didn't see it myself, but I heard the story from my astrophysics professor who had been at the observatory in 71 when they noticed that the telescope suddenly sped up to about twice its normal speed and made noises that they had never heard before. Once they determined that the power grid was racing to catch up to the required cycles per day they got a more reliable clock source for their drive. Things may be better today, but it is still an issue of the definition of approximate.

Try the DS32khz oscillator at maxim-ic.com. Worst-case drift should be less than a second a month, and if it switches to battery power when the 60 Hz power goes down, the clock won't need resetting (unless the battery dies) and thus will be MORE accurate than the power line.

If you really want to be anal, use the power line normally for its long-term accuracy, and switch to the oscillator-and-battery when AC power is lost. :)

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That's really surprising, I would think running a large generator at twice standard speed would be dangerous in several ways. I'm sure this was an extremely unusual circumstance. Was power supplied by an on-campus power plant rather than the utility? What school and year was this (exact or approximate date would help)? Is there any way one might track down a news story on this? I (and surely others) want to see documentation, because it's bizarre enough that it sounds like an urban legend.

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That's downright funny.

One of the displays on my clock finally burned out years ago. I swapped the seconds one for the minutes display and ran it for years after that (with no seconds). I'm not sure if it's still in a box around here somewhere or not.

Heathkit must have sold a lot of those particular clock kits.

Casey

[...]

More surprises at the Silicon Zoo. See

Regards,

-=Dave

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Change is inevitable, progress is not.

Are you sure these wall clocks were wired to the regular power net? I remember that in institutions all clocks were controlled from a central source to keep them in sync, change betwn daylight and standard time, etc rw

That's true, but they weren't corrected by changing the frequency of the

60hz supply. It was a signal modulated on top of the line. Might drive some devices crazy I suppose. Still sounds like an urban legend to me :)

Doug

approximate.

"Ben Bradley" schreef in bericht news: snipped-for-privacy@newsgroups.bellsouth.net...

[snip]

You never get an interesting aneckdote without adding a bit of juice ;)

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Frank Bemelman
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School clocks and such worked like that with a local signal over the power line. Hmm... Googling for Western Union and clocks might get you something. (They synced the local time base with a signal over a Western Union wire. Oi!)

--
Ron Sharp.

You might find this thread interesting, particularly the description of how they deal with accumulated drift > 2 secs.

George

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