On Fri, 2007-10-19 at 20:54 +1300, Jinx wrote:
> 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

If what you've got is exactly what's in that schematic I don't see how
it's possible for that resistor to smoke.

So, either you've got a wiring problem, or perhaps your SCR is shot?

TTYL

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