On Thu, 31 Jul 1997 09:05:35 +0000 Lawrence Lile <lilel@toastmaster.com>
writes:
>John,
>
>
>For my purposes, in AC only powered designs, I'm thinking about
>using the AC line as a frequency standard because it would be more
>precise than an external RC oscillator using 5% and 10% components.
>Hey- it's a cooking appliance.  No AC, no cookie.
>
>I don't need crystal controlled precision, but I need better than
>+/-10%.      even 1% or 2% would be great.
>
> My question has to do with overseas 50 HZ.  Does anyone know if
>they are as stable as the frequency in the US?
>
About the worst would be a place with no electrical "grid", just
independent generators for each house, block, or neighborhood.  The
frequency would be determined by a mechanical governor on the generator's
engine, which could be within 1% with good maintenance but is more likely
in the 3-5% range.  Any place that has a network of plants connected in a
grid is going to pay a lot more attention to frequency.  A cooking
appliance designed to operate from low-grade utility power should have
feedback of the voltage and / or heat developed in addition to just
cooking time.


>I need to design a system that will work on either 50 Hz or 60 Hz.
>In that situation, you can't just count line pulses, you'd need to
>compensate for the different frequencies as well.  I'm thinking about
>a routine that measures the line frequency then decides whether it's
>closer to 50 or 60 hz, then applies a software calibration number to
>the timer.  Haven't thought it through at all.

This could be done.  The RC oscillator has to always be precise enough to
tell the difference between 50 and 60 Hz of course.  This is about +- 5%.
 Otherwise there would be random cookie disasters if the frequency were
chosen wrong.

For this kind of application I'd consider a ceramic resonator, not as
expensive as a crystal but with the 1% or so precision needed and immune
from local variations.