[time-nuts] Building a mains frequency monitor

[Charles Steinmetz]

Jay wrote:

>Q3: The open-ended question: How do I improve on this? I suspect the 
>main place for improvement will be in the trigger, but I'm not sure 
>where to go with that.  Most designs I've seen involve a schmitt 
>trigger, generally with reference voltages set by things like 
>voltage dividers. This seems dubious at best, to me, since that 
>means the reference voltage will be affected by the same effects I'm 
>calling out above. Is there a *specific* design (rather than "make a 
>zero crossing detector!" or something similarly vague) that someone 
>can point me to, that would minimize this kind of trigger noise?

>Q3.1: Is there a better way to get mains voltage down to something I 
>can work more directly with? I saw at least one design that just 
>used a couple of megaohm resistors inline -- does that introduce 
>appreciably less phase noise than random AC/AC power brick?


I posted a very simple circuit that performs very well some time 
ago.  It can be found here:

<http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/4_App_Notes_and_Articles/Simple_AC_mains_zero_cross_detector.pdf>

This circuit was designed specifically to avoid the problems with 
Schmitt triggers and with iffy input circuits.  It is transformer 
isolated, but uses the second primary winding of a small dual-voltage 
power transformer for input (i.e., 120v), which is immediately 
dropped by resistors into a diode clamp.  Why?  Because the higher 
the voltage you start with, the faster it slews through the zero 
cross and, therefore, the more precise the trigger point will 
be.  Using a 120v winding gives you a 10x slew rate advantage 
compared to using a 12v winding.  Free gain, free precision!

There was some on-list discussion starting on 12/16/14, with the 
Subject: "Simple AC mains zero-cross detector".  (I'd include a link 
to the first message in the thread, but febo.com appears to be down 
at the moment.  Use the link at the bottom of this message, choose 
list archive, then choose December 2014 and display the messages by date.)

Best regards,

Charles