"Justifications" you're funny. Why not talk about prohibitions? Makes as much sense. If you don't know how to do something, or don't want to, why not leave it to someone who does? Please don't strain your brain on my account.

If you haven't already noticed (clearly not).. You haven't gotten any real help yet. (and coming up on page 2 of this thread)
Despite what you think.. these guys may actually help you... And they may actually teach you something along the way... Like why a beginner (like you have already stated) may not want to choose PWM for your intended load..
So just tell them exactly what you are trying to control and why and stop with the games.. And they might actually help..
The more information you give the better the answers will be..
If you don't want to provide sufficient details then.... Just give up and try somewhere else.
While you might not like the attitudes from some individuals.. yours isn't helping to achieve your goal either..

 

Somehow I feel instinctively that detecting voltage level for on/off switching is more straight-forward.

It isn't as straight-forward as you'd like it to be. Firstly, the mains wafeform amplitude is subject to change due to varying load. Secondly, the waveform is far from a pure sine wave and has all sorts of spurious noise superimposed on it.

 

The other thing the mains will do is supply any amount of current you ask for to the point of potentially starting a fire or worse. I'd really hate to see you win a Darwin Award.

http://www.darwinawards.com/

 

I'd be interested to see how you do this.

These would be a good place to start.
(Just skip over the safety nazi fussing parts.)

http://www.electro-tech-online.com/threads/grid-tie-inverter-schematic.86777/
http://www.electro-tech-online.com/threads/grid-tie-inverter-schematic-2-0.105324/

After that reading the other threads I have on that site relating to grid tie inverters would be useful as well.

Mostly it just comes down to setting up a comparator to look at the AC wave formone half at a time and using that to determine when to turn the switching device on and off.

 

Because there is not always one answer to our questions. Without knowing the "justification", it may not be possible to identify which answers apply. Otherwise, simply reverse engineer a household dimmer. I too am interested in this question, so that I may control AC from a micro. So let's not let this get personal.

The dimmer "solution" has been out there a long time. It gives erratic pulse spikes that start abruptly and trail off and is crude to say the least. It's not PWM, at all. I don't understand the need to justify something that's not illegal.

 

These would be a good place to start.
(Just skip over the safety nazi fussing parts.)

http://www.electro-tech-online.com/threads/grid-tie-inverter-schematic.86777/
http://www.electro-tech-online.com/threads/grid-tie-inverter-schematic-2-0.105324/

After that reading the other threads I have on that site relating to grid tie inverters would be useful as well.

Mostly it just comes down to setting up a comparator to look at the AC wave formone half at a time and using that to determine when to turn the switching device on and off.

Hi tcmtech, thanks, I checked and found Jules Theone's schematic, less the comparator. So I gather it's something relatively big, nothing that would fit on the end a wall transformer.

 

Are you looking to do this kind of control?
http://www.vfds.com/blog/what-is-a-vfd

Max.

 

Hi tcmtech, thanks, I checked and found Jules Theone's schematic, less the comparator. So I gather it's something relatively big, nothing that would fit on the end a wall transformer.

You must not have been looking too hard.

The GTI Schematic 2.0 thread has the whole schematics right at the bottom of the first post and the next four posts after that give the whole write upon the concept of operation for every part of the system.

As far as size goes PWM part of the control circuit A could be built onto a double sided 1 inch square circuit board with surface mount components or a 2 inch square single sided board with normal through hole components.

 

tcmtech:

Sorry for missing that, OK I see the Control Circuit A (which seems the only one relevant to what I need); I think I understand how the time delay voltage detection after zero-crossing detection might work, but does it turn off at the same point on the wave's downside? I wouldn't expect that, though that is what I want, a symmetrical pulse, narrower or wider on both sides of the peak, like my original sketch shows. And only at line frequency, doesn't need to cover a wide frequency band.

And for all you other brain-eaters & alien anal probers, it occurs to me that not all devices need a full, long drawn-out sine wave to operate.

 

Yes it shuts off at the same point on the back side of the waveform.

The comparator is just looking at the voltage of the incoming waveform and switching between a high output state and a low output state when the waveforms voltage goes above or below its reference voltage.