[time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question

[Robert LaJeunesse]

Stick with the transformer. The use of a capacitive divider is predicated on the line waveform always being a sine wave. Dream on! All it takes is one good spike down the line, maybe only 20-30V amplitude, and your capacitive divider passes it right on to that ADC that has a much lower (3.3V?) limit. Guess what goes poof? 

Bob L.

> Sent: Friday, January 21, 2022 at 8:48 PM
> From: "Lux, Jim" <jim at luxfamily.com>
> To: time-nuts at lists.febo.com
> Subject: [time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question
>
> On 1/21/22 4:43 PM, willl will wrote:
> > Hi everyone,
> >
> > I have an recently finished project that also measuring AC waveform, full
> > description here:
> > https://github.com/will127534/RaspberryPiAtomicNixieClock/wiki
> >
> > Basically using an AC transformer and Ti's ADC8681 @ 50Khz sampling rate.
> >
> > This year I'm working on a earthquake sensor + AC mains monitor system (In
> > an earthquake prone area, AC mains frequency will fluctuate by power
> > generator and machine emergency stop like this one:
> > https://twitter.com/kuriuzu/status/1360602496821911553).
> >
> > I want to improve AC measurement. Apart from the ADC sampling speed upgrade
> > (previously bottlenecked by the SPI connection to FPGA). I'm currently
> > debating about whether or not to bypass the transformer. How does the
> > distortion of an AC transformer impact the accuracy of mains waveform and
> > frequency? I'm not sure if it is worth it to go through the mains voltage
> > safety requirements.
> 
> You can use a capacitorsget your galvanic isolation, and a CR voltage 
> divider with minimal waveform distortion. Pick a burden current (say, 1 
> mA) and for 120V line, you need 120k impedance at line frequency 60Hz
> 
> X = 1/(377*C) --> C = 1/(377 * 120E3)Â  = 22 nF
> 
> Say you want ~100:1 ratio? so 22 nF in series with 1.2k  (or 2.2uF) 
> would do nicely. Then feed your high Z ADC with a couple 0.1 uF 
> capacitors from the ends of the 1.2k.
> 
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