The circuit in this Design Ideagenerates a zero-crossing pulseoff the ac mains and provides galvanicisolation. The falling edge of the outputpulse happens at approximately 200μsec before the zero crossing. You canuse the circuit to safely stop the triggeringof a thyristor gate, giving it time toproperly turn off. The circuit generatesshort pulses only when the mains voltageis approximately 0V, thereby dissipatingonly 200 mW at 230V and a50-Hz input.

Testing a physicalcircuit shows good correlationwith the simulation(Figure 3 ). Drivingthe isolated output froma 5V logic supply yieldsa good pulse waveform(Trace 1 ). The mainsinput is fed to the scopewith a 15V isolationtransformer for safety (Trace 2 ).You can use the persistence featureof the oscilloscope to show the zero-crossingpoint when zooming in to thetransition (Figure 4 ). This approachallows you to accurately measure thepulse timing relative to the input zerocrossing.

Figure 4 You can use the oscilloscope’s persistence function to relate the exact zero-crossing point to the output-pulse timing.

Its working even without D2. Not sure why, but its works for me. I used 1N4005 diodes for D1-D4 and a 2N4403 for Q1. I tried an 1N4005, MUR1000, 1N3595 for D2. Still works with or without. Thanks for the circuit. I may use it for a dimmer circuit that us

“I was curious about the 1 minute limit, which I can't find mentioned in a few manufacturers datasheet. Take as an example the Panasonic series ERJ8Gn http://bit.ly/1AzqEnHnwhere “Rated Continuous Working Voltage (RCWV)” is defined as the lower val

“My circuit blew!nnI used (5) IN4007 diodes from the rectifier and D2. I used IN4148 zener diode for D3. I used a 2N3906 transistor, a EL817 optocoupler, and 1/4W resistors. When I applied the 110Vac to the circuit, instantly 3 diode to the bridge re

@MakeIt, I’m too late in replying. However, I wanted to note how the 1N4148 isn’t a Zener diode, that caused the voltage to rise up enough to blow Q1 first. Then probably the optocoupler passed away, and so on…

“I think R3 should be 33K. Otherwise potential over D1-D4 exceeds abs. max limits. “

@nemenem, D3 is limiting voltage after the bridge, and as the current is limited by R1 and R5 before the bridge, even the voltage input to the bridge is limited around Vz(D3) + 2 * Vf(D1,D2), which is safe enough for them.

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