[time-nuts] OT: AC voltage standard

Neon John jgd at johngsbbq.com
Tue Nov 6 09:08:42 EST 2007


On Tue, 06 Nov 2007 11:57:27 +0000, Joe McElvenney <ximac at btinternet.com> wrote:

>Anyone know of a simple way of producing an AC voltage standard
>suitable for general workshop use without reference to another
>one? About one percent would be good enough, wave shape and
>frequency accuracy not important (wash my mouth out). I have a
>Weston Cell for DC voltage calibration, a Rb one for frequency but
>nothing for AC volts. Perhaps there is a chip out there that
>clocks between accurate limits that I could use as a source?

If a simple square wave will do then one of the simplest and yet most accurate
sources is to switch a known DC reference using a mercury wetted reed relay. A dry
reed will work but will bounce a little.  The small reed relays as were ubiquitous in
data acquisition systems up into the 80s can switch at 150 hz or better.  Driving the
coil of the relay with stepped-down line voltage is a good solution.

You probably know this already but I'll mention it anyway.  You can't draw any
appreciable current from that weston cell and it remain within specs.  Even a 1 meg
scope probe is too much.  I'd use a DC power supply or battery and a quality DVM
(which is probably more accurate than the standard cell) instead.

I started in metrology in the time when the standard cell was the best there was.
Boy, am I glad that era is gone.  I still have one just to stimulate old memories but
my lab standard is a precision 10 volt reference IC.  I don't recall the part number
but both National and Burr-Brown make 'em.  Untrimmed accuracy is something like
0.01%.  That's better than my boat anchor Fluke meter calibrator!

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
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Tellico Plains, Occupied TN
Save the whales, collect the whole set!




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