I was experimenting the circuit below. In the load, I placed the input of a full bridge rectifier. The issue is that the power triac is never activated. Sounds like the zero cross is not working.

Is that because a bridge rectifier is not a proper load for this circuit?

I simulated trying with an opto with no zero cross, and the simulation worked.

Hot-Line Switching Application Circuit

  • \$\begingroup\$ could be that the optocoupler doesn't like the long deat time you get with the bridge rectifier. \$\endgroup\$ Aug 16, 2018 at 7:12
  • \$\begingroup\$ Could be. Do you think there's a way to solve that? \$\endgroup\$
    – user115094
    Aug 16, 2018 at 11:11
  • \$\begingroup\$ use a non zero-crossing optioisolator. eg: MOC3021 \$\endgroup\$ Aug 17, 2018 at 2:42

1 Answer 1


The load needs to draw more current than the gate trigger current of the triac in the relevant two quadrants for the triac to turn on.

Try putting a substantial resistive load on the output of the simulated bridge rectifier (eg. 200 ohms).

  • \$\begingroup\$ Tried, but still the same. \$\endgroup\$
    – user115094
    Aug 16, 2018 at 4:02
  • \$\begingroup\$ use an incandescent light bulb \$\endgroup\$
    – jsotola
    Aug 16, 2018 at 5:26
  • \$\begingroup\$ Yes, but adding that bulb wouldn't be a practical solution. I think I will get rid of this circuit, and will use a simple electromechanical relay. \$\endgroup\$
    – user115094
    Aug 16, 2018 at 5:39

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