@Bruce Brown posted:...
Unless stated otherwise, all further references to VAC are assumed to be RMS voltages.
...
The full waveform represents 18 VAC. To achieve 9 VAC, as an example, the triac is turned on for 50% percent of the time.
This in no way detracts from your above-and-beyond contribution to the hobby. 
But to be precise, if you turn on the triac for 50% of the time as depicted in the dark blue portions of the above waveform, the RMS voltage will be about 13V AC (as opposed to 9V).
Crudely penciling in the original 18V AC waveform, you can see that the 9V AC chopped sinewave is "ON" for less than 50% of the time.
