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From: cjp@megatek.UUCP (Christopher J. Pikus)
Newsgroups: sci.electronics
Subject: Re: Sidereal Time Clock
Message-ID: <856@valhalla.megatek.uucp>
Date: 14 Dec 90 11:14:21 GMT
References: <1833@umriscc.isc.umr.edu>
Distribution: sci.electronics
Organization: Megatek Corporation, San Diego, Ca.
Lines: 53

From article <1833@umriscc.isc.umr.edu>, by robf@mcs213j.cs.umr.edu (Rob Fugina):
>
> Power supply...how easy is it to make it AC line powered, with a battery
> backup?  This I'm not so worried about, but I'd like advice on whether
> it would be better powered by AC line or by battery...i.e., how long do
> you think the battery would last?
> 
> Rob Fugina
> robf@cs.umr.edu

	It would be preferable for you to run this thing off AC if possible.
It is far more accurate than a crystal. A typical crystal will be accurate to
about 0.1% (1000 ppm) and will drift with temperature. The 60Hz that is derived
from a wall socket is tied into a national power grid and is kept pretty close
in sync. There is a master clock for the power grid and if it starts drifting
away from the 60Hz, then the AC freq is sped up/slowed down to bring it back in
line. short of an atomic clock the 60Hz AC is the most accurate you can get.
(Of course you can spring for one of those Ovenized Temperature Controlled
Crystal Oscillators (OTCXO) with accuracy to 10E-7 but they require 50V to
keep them warm and cost several hundred dollars, the voltage would pretty much
require you to use AC anyway).

	A sample circuit for the AC is as such:

				 Diode	     |\ 74LS14 ? (schmidt trigger buffers)
				  |\ |       | \
			    +-----| >|-------|  >o------------ 60 Hz square wave
			    |     |/ |       | /
			    |                |/
     		            |  _________                  5V regulator
        110VAC              | |		| 6.3VDC             ____           
	________    ________+_|~      + |------------+------|    |------+---------- +5V
		}||{          |		|            |    in|    |out   |
    AC line	}||{          |	bridge	|            |+     |____|      |+
		}||{          |	 rect.	|     Big   ---       |gnd     ---
		}||{          |		|     Cap   ---       |        ---
		}||{          |		|            |        |         | another Big Cap
		}||{          |		|            |        |         | and little caps
	________}||{__________|~      - |------------+--------+---------+
                 ^            |_________|            |
                 |			            _|_ GND
		 |			             =
		 |			             .
		6.3V transformer




-- 
That is all ...
Christopher J. Pikus,               Megatek Corp.
INTERNET:     cjp@megatek.uucp      San Diego, CA
UUCP:         uunet!megatek!cjp or ucsd!megatek!cjp