Is this common practice?

Thread Starter

cspwcspw

Joined Nov 8, 2016
78
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
Peterrectifier.png
 

AlbertHall

Joined Jun 4, 2014
12,345
If a diode fails then the circuit has failed. Whether the repairer has to replace one diode or four diodes on the items which fail would not be a major concern to the designer whereas saving the cost of four more expensive diodes on every product manufactured would be.
 

OBW0549

Joined Mar 2, 2015
3,566
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?
A seasoned designer wouldn't worry too much about it, as there are many MANY situations in which the failure of one component unavoidably causes other components to be overstressed or even destroyed. For example, consider what happens when a step-down voltage regulator supplying +3.3 volts to a complex system from a +12 volt bulk source fails with its input shorted to its output; most likely, almost every IC on the board will get fried. Sad, but it happens.

In this case, R1 and R2 in the above circuit will limit the peak fault current through any of the other components to under a milliampere, which I doubt would bother any of the remaining 1N4148s much.
 
Top