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

[Lux, Jim]

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.