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I need help modifying the circuit below.

The assumption in this circuit as it stands is that AC Hot would be common to GND and then MT2 of the triac would go through a light source to AC Neutral. P_IRPT pulses high when there is no current flowing between MT1 and MT2 of the triac T1, which occurs at every zero crossing

The modification I need is for my case where AC_HOT is common to the 5V rail rather than GND (my power supply has a negative output).

I know there are other ways to detect zero crossing but this circuit gives the me assurance that I am definitely detecting the zero cross after it has occured so I never run the risk of firing the triac prematurely.

Thanks!

Current Zero Cross Detection

This was taken from page 16 of this TI app note: http://www.ti.com/lit/an/slaa043a/slaa043a.pdf

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  • \$\begingroup\$ The circuit in app. note won't hear the difference if you swap N and L. \$\endgroup\$ – Marko Buršič Jul 20 '17 at 21:11
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    \$\begingroup\$ Craig may be long gone but the App note was only tested with 12Vac not 120Vac nor 240Vac and the grounded button switch may not pass the safety test for ESD, line transients and 250V rated resistors with 1k to 3kV for line transient immunity safety tests. So I would not trust anyone without suitable experience on insulation to try this. There are better "offline supplies" than this. I might consider other approaches for ZVS if I were to redo this App Note.. \$\endgroup\$ – Sunnyskyguy EE75 Sep 29 '18 at 1:39
  • \$\begingroup\$ Although I have made 120V/60Hz offline DC and Triac drivers I relied on grounded Neutral and CMOS logic driving small ZVS pulse transformers to Triacs and used XOR gate to detect the ZVS with a long R divider chain and small 1us delay to other XOR input side to make the ZVS pulse. WHen user touchable parts are electrified with the energy of the grid behind them, you need a good insulated design for safety. 3kV min. \$\endgroup\$ – Sunnyskyguy EE75 Sep 29 '18 at 1:47
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The app note

schematic

simulate this circuit – Schematic created using CircuitLab

App not swapped:

schematic

simulate this circuit

Your intention:

schematic

simulate this circuit

schematic

simulate this circuit

Since the signal is AC it does not need to swap NPN with PNP transistor.

schematic

simulate this circuit Result: enter image description here

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  • \$\begingroup\$ N to N? Eight more characters \$\endgroup\$ – winny Jul 20 '17 at 21:37

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