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

[Lux, Jim]

On 1/21/22 7:00 PM, Robert LaJeunesse wrote:
> 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.


diode clamps or "Tranzorb" (which is basically back to back zeners)

An awful lot of carrier current operated devices use capacitor 
isolation, so it's a "solved problem" (and, of course, carrier current 
means you need to pass higher frequencies)


>> 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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