[time-nuts] OT: AC voltage standard

Didier Juges didier at cox.net
Tue Nov 6 17:41:21 EST 2007

I thought the question was:

>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? 

There was no reference to a 6 bit scope. 

Obviously, what you need to calibrate a scope's vertical system is quite different from what you need to calibrate, say, a bench AC voltmeter.

If timing is unimportant and RMS value is unimportant and wetted relays are available, then that solution would work. In my shop, CMOS switches are more readily available than wetted relays, and their on-resistance is well low enough for the job, with the advantage of being much faster, but at that point, it's a matter of what you have available and personal preference.

There is still an issue that the relay will switch the voltage ON, and there is no equivalent circuit to switch it off, leaving that to an RC time constant of some sort. I would think it would be of interest to have a symetrical waveform. If you use another relay to switch the voltage off, there will be timing issues, unless you use a PFN of some sort, in which case calibration will be another story.

I thought the more general question about a general purpose AC source that could be built and calibrated using a precise DC voltage source and used to calibrate AC voltmeters for instance is interesting though, because I have that issue.

That will be for another thread :-)

Didier KO4BB

---- Neon John <jgd at johngsbbq.com> wrote: 
> On Tue, 6 Nov 2007 13:52:13 -0500, Didier Juges <didier at cox.net> wrote:
> >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.
> Not at all. The early edition of the Berkeley Nucleonics precision pulser, capable of
> delivering a monotonic amplitude pulse to a 4096 multichannel analyzer, used a pair
> of off-the-shelf Claire MWRs, one to switch the reference voltage to a capacitor and
> another to switch the charge to the pulse forming network.  In later models they
> changed to a complicated solid state circuitry that never was quite as stable.
> None of that is particularly relevant here because he needs a simple circuit to check
> the accuracy of a 6 bit ADC in a scope.  The RMS value doesn't matter, as the output
> is a simple square wave that swings between 0 volts and precisely the value of the DC
> source.  Neither does the frequency.
> The advantage of the reed relay approach, in addition to precision, is that the
> circuit can be thrown together on a bench using jumper clips in 5 minutes, assuming a
> MWR is on hand.  A voltage source (battery even), a good DVM, the relay and a 6 volt
> filament transformer to drive the coil is all that is needed.  More than good enough
> for a 6 bit application.
> --
> 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
> Democracy is three wolves and one sheep voting on what to have for supper.
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