[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,
Charles
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