Disclaimer: I'm a retired hobbyist looking for some wisdom from those with more expertise!
I built a mains zero-crossing detector from something I found on the Internet at https://cdn.hackaday.io/files/1597066832861504/SimpleIsolatedZeroCrossDetector.pdf.
It uses some resistors to voltage divide mains input and then contains a full-bridge rectifier and a single transistor to drive an optic isolator. Diodes in the rectifier are 1n4148 or equivalent - small signal diodes with reverse breakdown voltage of perhaps 75V or 100V. Current demands are very low.
The diodes in the full-bridge rectifier protect each other: reverse voltage doesn't build up on D2 when the AC cycle reverses polarity, because D4 is forward biased and protects D2. So it works. But in the event that D4 goes open circuit or fails, reverse voltages on D2 rise way above spec. So I suspect we'll quickly take out all four diodes if any one fails.
LTSpice seems to confirm what I think - if I look at the reverse voltage across D2 it is modest, until I cut the lead to D4.
Is this a "common" way to do this, or would a seasoned designer be more conservative and use components that didn't depend too much on others around them doing their job properly?
Thanks
Peter
I built a mains zero-crossing detector from something I found on the Internet at https://cdn.hackaday.io/files/1597066832861504/SimpleIsolatedZeroCrossDetector.pdf.
It uses some resistors to voltage divide mains input and then contains a full-bridge rectifier and a single transistor to drive an optic isolator. Diodes in the rectifier are 1n4148 or equivalent - small signal diodes with reverse breakdown voltage of perhaps 75V or 100V. Current demands are very low.
The diodes in the full-bridge rectifier protect each other: reverse voltage doesn't build up on D2 when the AC cycle reverses polarity, because D4 is forward biased and protects D2. So it works. But in the event that D4 goes open circuit or fails, reverse voltages on D2 rise way above spec. So I suspect we'll quickly take out all four diodes if any one fails.
LTSpice seems to confirm what I think - if I look at the reverse voltage across D2 it is modest, until I cut the lead to D4.
Is this a "common" way to do this, or would a seasoned designer be more conservative and use components that didn't depend too much on others around them doing their job properly?
Thanks
Peter