How to create a zero crossing detector using a full bridge wave rectified circuit

Question

Currently

1) I have a full bridge wave rectified ac waveform of 10Vmax.

2) I also have a potentiometer adjustable DC waveform.

3) My rail voltages are +12V and -12V.

4) I currently supply my full bridge wave rectified wave in to the non inverting terminal and the DC voltage to the inverting terminal.

5) Using an opamp or a comparator, I want to compare the two signals and produce a square wave which varies from 0 to a positive voltage at the output.

6) At every zero crossing , my square wave should go to zero. In all the other times it must stay positive.

I have created an incomplete schematic below and have also tried many different methods but I am having difficulties in achieving my result.

Thank You

^{simulate this circuit – Schematic created using CircuitLab}

EDIT

New schematic

Removed the Buffer, Positive Feedback resistor, Ac source floating and load resistor added.

^{simulate this circuit}

Answer 1

Source link:

The circuit in this Design Idea generates a zero-crossing pulse off the ac mains and provides galvanic isolation. The falling edge of the output pulse happens at approximately 200 μsec before the zero crossing. You can use the circuit to safely stop the triggering of a thyristor gate, giving it time to properly turn off. The circuit generates short pulses only when the mains voltage is approximately 0V, thereby dissipating only 200 mW at 230V and a 50-Hz input.

^{simulate this circuit – Schematic created using CircuitLab}

Rearranging the circuit you can get the same for your XFORMER input:

^{simulate this circuit}

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Modified OP circuit.

Your second circuit has no AC return path. This modification fixes that.

10 V RMS will peak at 14.1 volts which will overload the input to the comparitor. Rearrangement of the two 1k resistors will drop this to 7 V.

Figure 2. The LM311 has the emitter of the open collector output available on a separate pin.

Using the LM311 you can tie the emitter of the output transistor to ground (or anything else) to prevent negative output excursions.

When I simulate this circuit in the circuit lab , it didn't give me any square wave at the output of the comparator? Any ideas on that?

Figure 2. Running the simulation on Figure 1 with the potentiometer K factor set at 0.15 results in the above simulation. (Settings: 0 - 0.1 s, 0.001 s steps.)

You probably had the pot setting too high in your simulation.

Don't forget that most comparitors have open-collector outputs so I've added R3.

Answer 3

This is an old Elektor design. First of all the zero detection is realized on the mains side. The value of the capacitor C1 is important to obtain the desired output pulse. I would strongly discourage the use of transformer since the use of it creates a zero crossing not in sync with mains.