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

[Robert LaJeunesse]

One thing that has helped me build survivable designs is utilizing high impedance inputs, which means adding a buffer before an ADC pin if need be. Most IC inputs clamp to the supply, and can withstand a milliamp or two of current with out damage. ADCs typically want a low impedance source, so a buffer powered from the ADC's supplies guarantees no overvoltage that can cause excess current. A high resistance in series with the buffer input limits its current under transient conditions, providing the needed safety factor for noise spikes. Split that resistance up and add a low-leakage silicon diode (e.g. BAV199) to the supply and now the input is seriously protected with supply on, or off.

One problem with TVS diodes on signal inputs is that they limit to an absolute voltage, not one relative to the supply of the device to be protected. The degree of protection varies whether the device is powered or not. You must design for the unpowered situation, else things go poof when power isn't sequenced right. That will happen sooner or later.

Bob L.

p.s. A note about capacitive coupling: Yes, well proven reliable for carrier current systems BUT... with a sub-octave signal bandwidth far, far removed from the power fundamental. This application is multi-octave and includes the power supply fundamental. Big difference in application.

> Sent: Saturday, January 22, 2022 at 2:15 PM
> From: "willl will" <will at willwhang.com>
> To: "Discussion of precise time and frequency measurement" <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 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.
>
> I am concerned about this even with the transformer, so I've added Ti's TVS
> chips & hookup the transformer after my UPS surge protection plug in the
> previous project.
> Definitely need to step up the protection level using an NTC resistor or
> something like that to handle mains voltage directly..... Or offload the
> concern to a surge protection plug.
>
> Will