[time-nuts] Simple AC mains zero-cross detector

nuts nuts at lazygranch.com
Thu Dec 18 02:36:00 EST 2014

On Thu, 18 Dec 2014 01:26:06 -0500
Charles Steinmetz <csteinmetz at yandex.com> wrote:

> There has been some lively debate about how much filtering (if any) 
> is acceptable here.  On the one hand, the AC line is a very noisy 
> source at frequencies above the fundamental, while the fundamental 
> frequency is determined mainly by massive rotating machinery that 
> cannot change frequency very quickly.  On the other hand, if you pass 
> the signal through a narrow filter you could miss the glitches that 
> interest the folks who collect such data (grid switching transients, 
> lightning strikes, etc.), or they could be delayed and smeared out in 
> time so determining when they occurred would be problematic.  The 
> filtering in the circuit I posted (two-pole RC lowpass with a -3dB 
> frequency of ~475 Hz) is a good compromise.  It filters out the worst 
> of the locally-generated hash without masking grid events.  For those 
> who want their data raw, the filter can be omitted as noted in the 
> description sheet that accompanies the schematic.  (You did download 
> and read the material before posting about it, right?)

Of course I looked at the schematic. It is a very basic cascade of
single pole RC filters with components separated by a factor of 10 to
prevent component interaction. Not much of a filter and the corner is
probably a bit soft considering the load impedance is not infinite, but
rather a switch.  

Why not use a lower voltage transformer, preferably not at a lethal
voltage. You only need a couple of volts to drive the rest of the

If you are going to look at glitches, that should be done by sampling
the AC (transformer coupled obviously). Basically the circuit to detect
period is dedicated to that function. Since the frequency won't vary
significantly, a high order filter wouldn't be an issue, as long as
you don't care about delay.  

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