I followed SNOA999 for a zero crossing detector using TLV7011 as a comparator. I am not sure about short-circuiting grounds between AC and DC power sources. My concern is creating negative currents on the DC side and thus damaging some more sensitive components.

I tweaked the circuit from the application note to achieve the following:


simulate this circuit – Schematic created using CircuitLab

Where D1 and D2 are schottky diodes.

Vout works as planned in a SPICE simulation. But, my doubt is if by not short-circuiting V- with the inverting input will affect the stability of Vout as zero-crossing reference.

  • \$\begingroup\$ This circuit must be treated as live in all parts because there is no mains isolation. The application note assumes that both sides of the circuit can share a common ground safely. \$\endgroup\$
    – Transistor
    Commented Jun 3, 2020 at 6:14

1 Answer 1


Shorting grounds will result in saftey hazards and most likely burnt components if the voltage source is AC mains (120V or 220V). One reason is both neutral and hot can carry current (especially in the event of a fault). An isolation step down transformer should be used in between the mains source and the opamp. For example a 10:1 step down would give a max voltage of 11V and provide isolation for saftey.

Or if you just need to detect the cycle you can use a circuit like this with an opto-coupler enter image description here

  • \$\begingroup\$ I thought about using a optocoupler. My only concern was power loss on the resistor after the rectifier. I will try with lower currents on the LED. \$\endgroup\$ Commented Jun 3, 2020 at 15:44

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