Hi Kiss,
I see, so basically you were looking at the instantaneous voltage at X degrees or what degrees a particular voltage occured..
Ok, I have some news that may shed some light on what is happening?
Firstly I tried without the Diac with no difference found, I then tackled it like I used to when my head worked properly & started testing what was happening.
I also tried a new BTA16 Triac & a BT151 with no difference found.
I think it may have something to do with the actual LOAD on the Triac like you mentioned before, let me explain, this will sound a bit stupid but it is really happening so stay with me
.
I returned to the Lamp setup & with the "pot to the Arduino OFF" I get zero Voltage at the Triac output & everything works perfectly.
With the Arduino OFF--No power to it! I have Zero volts of course.
Lamp Resistance = 15Ω
With the Lamp replaced with a 2.3K 50W resistor across the Triac output as a load I get "79V at the Triac output with the Arduino Turned OFF"--No Power to it!, the Triac is Triggering itself.
Total Resistance = 2.3K
I then connected the Transformer which I have a 150Ω 50W resistor in series with one leg of the transformer 240V input, the transformer DC resistance is 39Ω & L=2.7H
With the "Arduino Turned OFF--No power to it!" I get 220V from the Triac output, again it is triggering itself.
XL = 847Ω
Series Resistor = 150Ω
Dc Resistance is 39Ω
Total Resistance is 1036Ω
Don't ask me why it is happening as I have no idea but I am glad I found it.
Cheers