This will IMHO only happen if the 47k's are not 47k (shorted or low) that can keep the current for the MOC 3023 above the hold current and keep it conducting (the 1.8uf cap will not be discharged enough by the flashbulb to reduce current below hold current) thus frying the 22r resistor. Peter ----- Original Message ---- From: Jinx To: pic microcontroller discussion list Sent: Friday, October 19, 2007 3:54:07 AM Subject: [EE] PIC-triggered flash Can someone please suggest a fix to this problem. In the diagram, the 220R 0.5W gate resistor keeps showing me how much smoke it can blow out I plug this in, wait a couple of seconds, then press a pushbutton, which makes a 12F675 send a 10us to the opto-triac. The pulse is OK, checked that on a scope What will happen is that the flash will go off, then the 220R starts slowly burning. The power gets turned off, maybe a second later, before the resistor actually goes open I've tried a large-value resistor pull-down on the SCR gate, but that makes no difference. It's presently 2 x 560k (some resistors in this circuit are series pairs, for the voltage rating) Tried several values for a pull-down, from 1M to 5M, in the position shown, but the 220R burns up just the same. I feel like General Haig sending them over the top, poor little blighters Must be on the wrong track with a pull-down. Can't see where that power is coming from. I figured that for all but a very brief time, the resistor is isolated from any PD across it. Something is obviously not turning off, and making it dissipate a lot more than it should need to. The only candidates, AFAICS, are the tube, the SCR or the opto I don't think anything else is suffering. My logic being that the flash will go off with a new 220R Plan A - replace the 220R after every flash, but I'm pretty sure that's not how the pros do it ;-) Plan B - ask you guys TIA -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist