I’m trying to build a small DMX-controlled Xenon strobe light. For prototyping, I’m trying to modify a Velleman kit to trigger it via an external logic level pulse, but I’m struggling a bit with figuring out the triacs. I’m not quite familiar with those, but I’ve tried to read up on them.
The manual for the Velleman kit, with the original schematic, as well as my modified version, is attached. I’ve just omitted R2, RV1, C2 and DI1, and I’m trying to trigger TRI1 (which is actually a TIC225, not BT136 as the schematic says) via a MOC3021 optocoupler/optotriac.
When I apply a brief pulse to the MOC LED, I get one flash from the lamp, then after a few seconds the 470 ohm resistor between rectified mains positive and the MOC (R4 in my schematic) burns. If I immediately disconnect mains, then re-connect, I can trigger one single flash again, ad nauseam. If I don’t disconnect mains, the resistor burns, but nothing else is damaged.
Everything works as expected if I just manually connect the TRI1 gate to DC+, bypassing the MOC (by means of an isolated screwdriver …). That way, I can trigger and re-trigger the strobe as much as I want with no issues.
I’ve tried to google around and read datasheets etc, but I can’t quite grasp what’s going wrong. Well, the resistor burns because the MOC is not turning off, but why is that? I thought that when the triac had triggered, its gate current would drop low enough that the MOC would necessarily turn off? (Its holding current is 100µA by the way)
Does this indicate that the main triac is not turning off? That doesn’t make sense to me either, as everything works when triggering the dodgy way as described above.
Disclaimer: I am aware of the risks involved with this, and am taking safety precautions including using VDE isolated tools, discharging the capacitors, keeping one hand behind my back, not working alone, and so on