Hi Folks, I needed a zero-crossing detector, and decided to use the H11AA1, and bought a couple. The datasheet says maximum input current is 60mA, so I thought that if I drove the diodes with 20mA peak, that would seem "safe". However, I wanted to use the 240VAC (340v peak) as an input, so with 0.02A peak, the resistance required would be 17k, but that would dissipate 4.8W, which seems a bit excessive. So I decided to base the resistance on a dissipation of 0.5W, and calculated a resistor value of 180k, and hooked it up. Max instantaneous current is 1.9mA, so that should be OK.
It works, but the zero crossing pulse that I'm getting is 1mS wide, which is not what I'd hoped for - 10% of the 10mS period of a 50Hz half-wave. I have looked at a number of application circuits that have equal-value resistors on the live and neutral wires. Is there any good sense in that, apart from giving more dissipation from 2 resistors than you can get from one?
Timebase is at 2mS per division.
I have a 22k resistor from the collector to +5V, and the emitter is grounded. I used a 10k at first, but the pulse was 40% wider. When I tried touching another 10k across it to momentarily give 5k, the pulse on the scope got wider still, so I put the 22K resistor in, which improved things marginally.
Should I be driving the diodes harder? Is there any way of getting a pulse that says "This is the moment of zero crossing", rather than "The zero crossing happens round about now"?
The pulse will be used to trigger the interrupt pin of a micro-controller, so I could put 5mS compensation into that, but it would be so much more elegant to have a zero-crossing detector that detects zero crossings.
It works, but the zero crossing pulse that I'm getting is 1mS wide, which is not what I'd hoped for - 10% of the 10mS period of a 50Hz half-wave. I have looked at a number of application circuits that have equal-value resistors on the live and neutral wires. Is there any good sense in that, apart from giving more dissipation from 2 resistors than you can get from one?
Timebase is at 2mS per division.
I have a 22k resistor from the collector to +5V, and the emitter is grounded. I used a 10k at first, but the pulse was 40% wider. When I tried touching another 10k across it to momentarily give 5k, the pulse on the scope got wider still, so I put the 22K resistor in, which improved things marginally.
Should I be driving the diodes harder? Is there any way of getting a pulse that says "This is the moment of zero crossing", rather than "The zero crossing happens round about now"?
The pulse will be used to trigger the interrupt pin of a micro-controller, so I could put 5mS compensation into that, but it would be so much more elegant to have a zero-crossing detector that detects zero crossings.