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

Charles Steinmetz csteinmetz at yandex.com
Sun Dec 21 17:37:07 EST 2014

Ed wrote:

>It seems to me that a low voltage secondary should be OK by using a 
>fast comparator IC rather than a transistor to decide - the gain of 
>the IC allows for much smaller detection levels, so the equivalent 
>zero-crossing velocity could be the same. An IC tripping in a 10 mV 
>band should provide the same effective ZC velocity at 12 V input as 
>a transistor working around 100 mV with 120 V input. Or am I missing something?

When the switching band gets that small, device noise, input offset 
voltage drift, and other errors have a proportionally greater 
effect.  I actually built a similar circuit with a 12v transformer 
and an LT1720 comparator, and it had worse jitter than the 
two-transistor circuit with a 120v feed.  In this case, there is no 
substitute for starting with a higher-slew-rate signal.  (Yes, the 
LT1720 did marginally better than the two-transistor circuit when 
both were fed from 120v -- but the fussiness of working with a fast 
comparator and the small gain over the two-transistor circuit made 
the latter the better choice, particularly in a design being put "out 
there" for others to build.)

Best regards,


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