would like to get a feedback on a zero cross circuit as it does not work.

Circuit is based on this. enter image description here 240V AC, 50 Hz

Expect that C2 is 50V. R1,R2, R4 are SMD.

After connecting to mains and measuring voltage across R1, the track between J1(Ac+) and R1 exploded. R1 is alive.

Voltage on C2, R6 and U1 is 100 V

Is this circuit valid ?

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    \$\begingroup\$ If the failure occurred the moment you took the measurement, the original circuit is most likely not related to it. Did you use a voltmeter which was grounded, so that you've shorted J1 to ground? \$\endgroup\$ Commented Aug 17, 2017 at 12:37
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    \$\begingroup\$ Is this circuit valid ? Just like beer, all circuits are valid, some circuit do not work but that is something different. You mean, will this work ? I have my doubts about the connection between D3 and R2, it is not there in the original schematic. The original schematic looks OK to me. \$\endgroup\$ Commented Aug 17, 2017 at 12:44
  • \$\begingroup\$ The way the NPN is used is odd but I think it makes the LED light up on the down going slope of the waveform. Not sure that the LED will light up exactly at the zero crossing though. \$\endgroup\$ Commented Aug 17, 2017 at 12:50
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    \$\begingroup\$ No it is not valid. As Bimpelrekkie points out, you have short circuited one of the diodes in your bridge rectifier. \$\endgroup\$
    – user16324
    Commented Aug 17, 2017 at 13:09
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    \$\begingroup\$ As others have mentioned, pin 4 of the bridge rectifier should not be connected to the DC ground, and the DC ground should not be connected to AC-. You're shorting out the diode between pins 2 and 4. \$\endgroup\$
    – DerStrom8
    Commented Aug 17, 2017 at 14:02

1 Answer 1


I might be going on a side-track but may I suggest you to use this simpler zero crossing detector circuit:

Zero cross detector

This is the output that you will get:


Blue curve is 220 VAC and yellow trace is the output at ZCD_OUTPUT.

  • \$\begingroup\$ not very sharp pulse but can be improved if a spec was given for pulse width before and after true 0 V which affects performance on application. \$\endgroup\$ Commented Aug 17, 2017 at 15:48
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    \$\begingroup\$ Since OP mentioned that he's using SMD components, it might be worth noting that the 300k resistor in this design should be rated for higher voltages than OP's design reference (which drops the mains voltage over two 220k resistors). \$\endgroup\$
    – youtooth
    Commented Aug 17, 2017 at 15:53
  • \$\begingroup\$ @whiskeyjack - Can you described how you arrived at the 300k resistor value? \$\endgroup\$
    – Brad
    Commented Dec 1, 2017 at 15:32

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