[time-nuts] OT: AC voltage standard

[Didier Juges]

The problem with a mercury relay is that the switching delay is significant and not well controlled, so the duty cycle of the resulting waveform is not well controlled, and so would be the RMS value.

I believe CMOS analog switches would provide better control, and with series resistance that is easily below 10 ohm, that would give you negligible error when driving loads in the megohm, such as a voltmeter.

Four switches in a full bridge configuration will give you a true AC square wave output, and if you know the DC voltage feeding the bridge (using your voltmeter calibrated with the Weston cell), you will have an accurate AC source that will not require further calibration, at least good enough for most home lab uses. 

Now, for a sinewave, it's another matter, but Bruce's suggestion of a DAC powered from a precise DC source would work extremely well (limited by the DAC) and provide a low distortion sinewave, which is just as important as controlling the peak voltage. A simple microcontroller is all that's required to drive the DAC. Make sure you understand the delays involved with making software loops. Alternately, a counter driving a suitably programmed EPROM driving the DAC will take software out of the equation, but it sounds like the 70's all over again...

Didier KO4BB


---- Neon John <jgd at johngsbbq.com> wrote: 
> 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
> http://www.johndearmond.com <-- best little blog on the net!
> Tellico Plains, Occupied TN
> Save the whales, collect the whole set!
> 
> 
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