Hello again,
The diac is a bilateral device that has a somewhat high resistance when off, and turns on by going to a lower resistance after a certain terminal voltage is reached. They come in various threshold voltages like 20, 30, etc. What this means to our circuit is that the triac wont be able to turn on until the line voltage reaches maybe 30 volts, and then it will come on because the diac will conduct. However, 30v on a line of 230vac means a delay of only around 250us, so we only have 250us to extract all the energy we need for the entire half cycle. That may not be enough so we might have to use a higher voltage diac or two in series. That's one of the reasons we have to test this.
Side note: Once the diac conducts if the line voltage is 30v and the DC power supply voltage is 15v, that really means we can only extract energy AFTER roughly 125us, so we really only have 125us left to extract all the energy we need for the entire half cycle. That's not much so we'll have to see how it goes and increase if needed. We cant go too high though or the bulb will start to dim.