On Nov 27, 2009, at 2:17 AM, Peter wrote:

> To fire the lamp 20 times per second one needs a trigger circuit  
> without a charge capacitor

I don't see it.  The typical disposable flash trigger cap is something  
like 0.022 uF charged through a 1M resistor, so it ought to charge  
just fine in less than 1/20 second as long as the flash tube doesn't  
completely dump the HV reservoir cap during the first flash.

The technology for repeated flashes in presumably very similar to the  
early "thyristor" automatic flashes that managed to cut off the flash  
after the correct exposure without dumping the whole charge elsewhere  
(earlier automatic flashes would essentially short out the flash tube  
after the correct amount of light was seen, quenching the tube but  
wasting the rest of the charge.)  I can't comment on the early units,  
but like others have said, I believe IGBTs are commonly used in modern  
units.  (flash controller chips frequently include an IGBT driver,  
whatever that means.)


> Current control cannot be used with a plasma arc tube, the rise time  
> is on the order of a few nanoseconds

That doesn't match my observation of cheap flash tubes (discharging  
them through various other components...)  A TRIGGERED flash tube may  
have a fast rise time, but seems to be relatively tolerant of current  
limits and such once it reaches its conductive state.  You can visibly  
WATCH the discharge if you have a low enough current (largish series  
resistor); it slows down and looks like one of the threads in those  
"plasma globe" toys (huh.  That shouldn't be surprising, I guess.)   
Series inductors are a standard mechanism for creating shorter flashes  
from xenon tubes, IIRC.   Or was that longer flashes? :-(

If anyone finds a simple circuit for a flash capable of the "blip blip  
blip FLASH" sort of action, I'd like to see it, too!

BillW

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