"Ralph Mowery" wrote in message
ink.net...

"Christopher Tidy" wrote in message
...
Hi all,

I'm thinking of building an electronic clock control circuit which uses
the 50 Hz mains frequency for time keeping. The reason for this is that
the clock dial is rather large, so probably wouldn't run for long on
battery power, and I don't fancy spending £40 buying a programmable
quartz oscillator chip.

Despite doing some research online and in electrical engineering books,
I can't find a figure for the accuracy of the time keeping of the UK
power grid. Perhaps this is because there aren't official bounds set for
the time error - one of my electrical engineering books says it is a
legal requirement that the frequency be kept "as close as possible to 50
Hz" - but even if this is the case it should be possible to determine a
typical error figure.

From my point of view I'd regard an acceptable error as 5 minutes every
6 months. Does anyone know the typical time error seen on the UK grid,
or where I might find this information? Any suggestions would be much
appreciated.

Best wishes,

Chris Tidy


Can't say about the UK but in the US the clocks that run only on the power
grid and depend on the frequency are very accurate over a long period of
time. I would say more like less than a minuit or less over a year period
of time if the clock its self is up to it. During periods of peak loads
the
nominal 60 hz may go down a cycle or so and when the load is taken away
the
power company will raise the frequency long enough to get the clocks back
in
time.


Well, you got the basic idea right. But it never drifts down 1
cycle/second. Very rarely drops even a tenth of a cycle.

In other words the power company keeps the frequency to an average of
exectally 60 hz over a long period of time. Much more stable than any
clock
you could build that depends on a quartz chip.


Indeed. Part of the standard equipment in the old days was a special
'crystal oven' with tightly controlled temperature. By regulating the
temperature of the crystal inside, the accuracy its vibrations was improved.

An old 'urban lengend' was that the first quartz watches were calibrated
assuming the temperature of the crystal was going to be controlled by the
body heat of the wearer. And that leaving your watch on the dresser over
the week-end would cause it to slow down slightly. Don't know if it is
really true, but it's a nice story.

daestrom