[time-nuts] Simple AC mains zero-cross detector
nuts
nuts at lazygranch.com
Mon Dec 22 06:04:11 UTC 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
target.
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