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" <[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.
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