[time-nuts] Line Frequency

[Robert LaJeunesse]

Do you intend to use this wall wart to power the circuit as well? I hope not, as a capacitor input rectifier scheme will result in decidedly non-sinusoidal current on the transformer secondary. Given a low-cost transformer with noticeable inductance and resistance the resulting waveform distortion could have unpleasant consequences for your application.

When using 12VAC into a zero crossing detector be on the lookout for oscillation problems. The dV/dt may not be fast enough to swing the comparator (amplifier?) through its linear range cleanly. It might prove advantageous to use an op-amp to gain up the input, while providing clean clipping to the sine wave, before the comparator. In the past I have used a comparator as a zero-crossing sine to square converter with an AC coupled feedback scheme that gave about 50mV of input overdrive. The overdrive decayed to well less than the comparator offset before the next zero crossing, there was good hysteresis and it was asymmetrical to get a good zero cross detect. Worked pretty nice if I recall correctly.

Bob LaJeunesse



>________________________________
> From: M. Simon <msimon6808 at yahoo.com>
>To: Hal Murray <hmurray at megapathdsl.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Sunday, February 9, 2014 6:11 PM
>Subject: Re: [time-nuts] Line Frequency
> 
>
>... I'm starting with a wall wart. About 12 VAC.  ...
>