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

nuts nuts at lazygranch.com
Mon Dec 22 01:04:11 EST 2014

On Sun, 21 Dec 2014 17:37:07 -0500
Charles Steinmetz <csteinmetz at yandex.com> wrote:

> 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,
> Charles

Looking at the data sheet of the LT1720, 1mv would have about 8ns
delay. Call it 10ns. A Vp of 29 volts should be sufficient to put the
delay around 90ns, making 100ns error or target percent of the 1uS

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