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" <[email protected]> > To: [email protected] > 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) = 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. > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
