Jeff,
Auto/TTL flashes add an inductor in series with the tube to greatly
increase duration of
flash discharge. The inductance is choisen to give an approximately fully
damped half sine wave
with the average resistance of the tube and the flash cap.
With the load increased when using the two tubes in series, this would become
more overdamped.
The light output is just controlled by controlling the flash duration with a
series connected
switch to interupt the current to the tube. For many years this was always a
specially optimised
thyristor switch. It is easy to switch on a thyristor but to switch it off is
very hard and
requires the reversal of current through the switch circuit for a brief period
so voltage on
thyristor drops to zero. This is where a second thyristor and a much smaller
"commutation"
capacitor is used to reverse voltage across the thyristor briefly. The energy
in the commutation
capacitor is small, but impedance is kept low to allow reversal of flash
circuit current. The only
significant energy saving occurs because the flash capacitor is only partially
discharged. Nowdays
the main thyristor is replaced by a spcial high current transistor. (IGBJT)
This switch is
relatively easy to switch off, so circuit is greatly simplified.
Early auto flash circuits just dumped the remaining energy in the flash cap
into a seperate
internal flash tube in parallel with main tube, wasting lots of energy. This
was the original auto
flash circuit patented by H. Edgerton of MIT.
Hard to guess whether current is reduced to close to half, because of
non-linear tube resistance
and inductance in circuit. Certainly expect duration to extend and current to
be reduced.
Tim Hughes
>
> My initial thought was that the stored energy is constant whether
> there were two heads or one, the single head would be optimized to
> work with the controller, and that the two flash heads aren't going to
> focus their light on the same area thus two heads aren't likely to
> give the same GN in actual use.
>
> Your comments sparked some interesting ideas though. The main thought
> with the current flow is that it is roughly half for two heads then
> the duration will be twice as long and the light intensity half as
> much. Along with the lower intensity the color temperature would also
> shift (?)
>
> A 10% change in energy would reduce the GN to 28 x sqrt(0.9) = 26.6
>
> Someone posted a link (you?) to a website that described flash
> circuits. One of the things I vaguely remember was that when one of
> the circuits quenched the flash, it dumped energy into a separate
> capacitor. It claimed that the energy was mostly recovered but it
> wasn't clear how they would get it back into the main energy storage.
> This wouldn't appear to make any difference in this case since you
> said they dump the energy into the tube until it quits conducting to
> determine the max GN.
>
> -jeff
>
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