Josh Koffman wrote:
> At 12V, current is 600mA. At 24V I read 2.8A. No information is
> available on the LEDs or the internal resistor.

Actually there is, from the measurements you cite.

At low voltages, the LEDs will be off and the string will draw very little
current.  This means all the voltage is accross the LEDs and practically
none accross the resistor.  As you increase the string voltage, the voltage
on each LED will rise.

Eventually the voltage on each LED will rise so that it starts conducting
for real.  At that point the LEDs are acting like conducting diodes, so
their voltage only goes up a little with increased current.  Another way to
say this is that their dynamic impedence goes way down.

Now as the string voltage is increased, most of the increase appears accros=
s
the resistor.  This allows finding the resistor value.  In your case the
dynamic resistance from 600mA to 2.8A is

  Ohms =3D Volts / Amps

  =3D (24V - 12V) / (2.8A - 600mA)

  =3D 12V / 2.2A =3D 5.5 Ohms

That is the series sum of the fixed resistor plus the dynamic impedence of
the LEDs over that current range.  The fixed resistor is probably in the 4.=
5
to 5 Ohm range.  However, what you really want to know is the dynamic
impedence of the whole string, and it doesn't matter much what contribution
comes from the LEDs and what contribution from the resistor for most
purposes of driving the string.

Now that you can see the current rises by 2.8A / 600mA =3D 4.7 times, do yo=
u
really want to do the flash at 24V?  That's not going to be good for the
LEDs at all.

How long is a flash supposed to last?


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