dimmer
Hooked up the scope to a dimmer expecting to see how it altered the waveform, and didn't see it do much of any difference from all the way down to all the way up.
I bought an LED bulb and was told it needs a "special" CL dimmer. How do the different kind of dimmers work, and how do I manage to see what they do with my scope?
There are serial controlled dimmers such as these for various load wattages. There are also some wireless controlled dimmers (WiFi, BlueTooth etc) listed on the same page.
For ballasted fluorescent lights, a different type of dimmer is needed: a Ballast Control Module.
Similar, lower cost products are available on various auction sites, but caution is good when buying mains electrical equipment from overseas. Also, it might be worth verifying certification of these products for use in your country.
If you want to build your own dimmer, please note that playing with live mains can be somewhat fatal, so to speak. Unless you are qualified to work with electrical mains, this is best avoided.
There are even simpler in-series circuits that can achieve the same: diac+triac (for bipolar waveforms) or just a bridge-rectifier+scr (thyristor) [this will only produce unipolar waveforms], or... if don't care about adjustment, just a diode! I'm honestly not sure what is that you don't understand. Reduce the problem to the simplest idea: a diode in series with a load will reduce power on AC because it cuts out half the waveform.
EDIT: Ok, since based on your comment below you're confused about the adjustability part, it helps to look at a simple rectifier+scr circuit. There are dozens on the net, but the one below seems the simplest (courtesy of Kuphaldt's Lessons in Electronic Circuits), so most useful for educational purposes:
Yes, the dimmer does steal some power for its control function/sub-circuit (which in this case is simply the pot and diode on the right side of the figure), but relatively little.
EDIT2: As to your theory that it must "kill its own power" (and thus not work)... again ask yourself: is a diode "killing its own power"? Maybe so, but should you infer from that that it only works for exactly one half-period and then stays off forever? No, because it reacts to change in potential at its inputs caused by the ever-changing AC waveform... which keeps passing through the load.
EDIT3: As for your MOSFET+opamp circuit, I found a good article on EDN explaining how it works: "Use a self-powered op amp to create a low-leakage rectifier". If you understand the basics from the previous simpler circuit, you should have no trouble following that.
Best Answer
Wikipedia has a nice article on phase angle control which is used for most house hold light bulb dimmers. They usually don't work for halogen, LED, TL. This is a typical case where a single image says more than a thousand words:
It works by chopping a (varying) part from the sine wave using a triac. The image shows a rectified wave, but the mechanism works equally well with an unrectified one, the latter being most common in household light bulb dimmers. The dimmer is connected in series with the lamp and therefore needs to see a minimum load (in the order of 10W) to work.
This is the classical dimmer. Not sure how halogen / LED / CFL differs from this, but I know from experience (with CFL) that an unmatched dimmer can make the lamp flash at a really annoying frequency.