AC zero crossing detection

[tomas632]

Hi Guys,
I am trying to build a zero crossing detector and came across the following two designs...

Zero Crossing Detector - Electronic Circuits-Circuit Diagram-Electronics Projects-Design

Single Supply Zero Crossing Detector | Circuit Diagram

the 1st design looks simpler although a negative supply is required, my question though is if I used a 0V negative reference wouldn't the output just saturate at 0v rather than -Vcc? This is what I am after ideally. Also I am unsure what the diodes are doing, could someone help me out on this one?

with the 2nd design I dont even know where to start, I cannot figure out what sort of configuration the op-amp is in, could someone give me a clue with this?

Thanks for your help

 

[ericgibbs]

hi,
Use circuit #2, its a comparator with hysteresis.

 

[tomas632]

Hi Eric
Thanks for your response, but could you give some details as to why use circuit 2? What is wrong with using circuit 1 and could you explain some of the theory behind circuit 2?
Thanks

 

[cobra1]

I use the H11AA1 in my zero crossing circuits

 

[RCinFLA]

The second circuit has a Schmitt trigger with hysteresis. There is positive feedback to latch the crossing for a small window around zero crossing.

It is usually a good idea to have some amount of positive feedback Schmitt trigger function on comparitors to prevent 'chatter' indecision due to noise at the comparitor threshold crossing.