AC Lamp Dimmer

"without which dimming is not possible"

Dimming is possible without zero-crossing detection. It's just that not in a design with a triac/quadrac, since when you have a component that you can't turn off on request you instead have to control when it is turned on. So the zero-crossing is important to decide the turn-on time.

If using a component that can be turned both on and off on request, it's possible to ignore the mains frequency and instead turn on/off the power at a much higher frequency. Then it's possible to use PWM and ignore the zero-crossing. The disadvantage is that switch transistors have losses when they turn on/off since the switching isn't instant and in the middle of switching they look like resistors with a significant resistance which means in the middle of the switching they behave like heating elements. Old switch transistors were too inefficient and/or expensive which is why the triac solution was common 20-30 years ago.

The advantage with switching at a much higher frequency is that on average it still produces a sinusoidal load with a bit of low-pass filtering. The triac solution creates very ugly distortion of the sinus wave which affects other customers and the grid - the power company doesn't like consumers who only want to consume the tail part of each half-wave while not loading the network on the leading half of the half-waves. That's not a load the generators are designed to support. And such a load produces huge amounts of overtones that will radiate from the cables.

"without which dimming is not possible"

Dimming is possible without zero-crossing detection. It's just that not in a design with a triac/quadrac, since when you have a component that you can't turn off on request you instead have to control when it is turned on. So the zero-crossing is important to decide the turn-on time.

If using a component that can be turned both on and off on request, it's possible to ignore the mains frequency and instead turn on/off the power at a much higher frequency. Then it's possible to use PWM and ignore the zero-crossing. The disadvantage is that switch transistors have losses when they turn on/off since the switching isn't instant and in the middle of switching they look like resistors with a significant resistance which means in the middle of the switching they behave like heating elements. Old switch transistors were too inefficient and/or expensive which is why the triac solution was common 20-30 years ago.

The advantage with switching at a much higher frequency is that on average it still produces a sinusoidal load with a bit of low-pass filtering. The triac solution creates very ugly distortion of the sinus wave which affects other customers and the grid - the power company doesn't like consumers who only want to consume the tail part of each half-wave while not loading the network on the leading half of the half-waves. That's not a load the generators are designed to support. And such a load produces huge amounts of overtones that will radiate from the cables.

Okay,

I am not that much into the power theory. I am basically a programmer who like to do embedded programming and do some hardware assembly with the help of people like you. After reading your last reply i think i should go with MOC3021 with zero crossing detection for firing the triac. Actually i have one circuit in that i don't see zero crossing circuit. If i have time i will email you the image of that circuit may be you can have a say on how dimming is implemented on it.